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Epidemiology and management of blackleg of canola in response to changing farming practices in Australia
Abstract
The Australian canola industry was established in the 1970s and has expanded since that time, particularly in the last two decades. This review addresses the changes in farming practices since the year 2000 and the epidemiological and management consequences for blackleg, caused by the fungus Leptosphaeria maculans, the main disease impacting Brassica napus production. To help understand the change in production practices, a survey of over 100 growers and agronomists was conducted. Modern management practices include increased crop residue retention, frequency of canola in crop sequences, number of resistance genes in cultivars, use of hybrids and fungicides as well as earlier sowing and flowering times. While some of the changes identified in the survey and in this review increase the risk and severity of disease, including new symptoms like upper canopy infection, others provide novel strategies for control. Keeping these changes in mind, a set of research priorities towards long term sustainable management of blackleg disease are identified.
... Furthermore, phenotypic traits often dictate regional viability as the main influence of yield security are stressors such as biotic (disease and pests) and abiotic (temperature, salinity, pH, heavy metals and hydration) stress. More emphasis has been applied to targeting biotic stress resistance phenotypes as unmanaged biotic stress leads to yield loss. Annually, B. napus yield is affected by 10-20% loss in UK and Canada and up to 90% in Australia, due to L. maculans, the causal agent of blackleg [25][26][27]. As such, to aid in improving Brassica crop yield and yield security, this review will assess current farming and disease management programs; introduce the gap between phenotype and genotype; discuss the underlying genetics of yield; and discuss studies that have adopted molecular technology to develop new and improved Brassica crops. ...
... As such, disease management strategies are critical for yield stability. A survey of over 100 growers and agronomists established that the best approach towards disease management involved the integration of farming techniques, managing genetic diversity, and careful use of fungicides [25]. Integrated strategies for improving yield with respect to farming techniques has been reviewed extensively [6,25,51]. ...
... A survey of over 100 growers and agronomists established that the best approach towards disease management involved the integration of farming techniques, managing genetic diversity, and careful use of fungicides [25]. Integrated strategies for improving yield with respect to farming techniques has been reviewed extensively [6,25,51]. In summary, maximum grain yield from Brassica oilseed crops is dictated by (1) environmental conditions during the critical period, (2) plant density and (3) disease management strategies; grain quality is dictated by: (1) the environmental conditions during the pod-fill stage and (2) genetic diversity. ...
The global demand for oilseeds is increasing along with the human population. The family of Brassicaceae crops are no exception, typically harvested as a valuable source of oil, rich in beneficial molecules important for human health. The global capacity for improving Brassica yield has steadily risen over the last 50 years, with the major crop Brassica napus (rapeseed, canola) production increasing to ~72 Gt in 2020. In contrast, the production of Brassica mustard crops has fluctuated, rarely improving in farming efficiency. The drastic increase in global yield of B. napus is largely due to the demand for a stable source of cooking oil. Furthermore, with the adoption of highly efficient farming techniques, yield enhancement programs, breeding programs, the integration of high-throughput phenotyping technology and establishing the underlying genetics, B. napus yields have increased by >450 fold since 1978. Yield stability has been improved with new management strategies targeting diseases and pests, as well as by understanding the complex interaction of environment, phenotype and genotype. This review assesses the global yield and yield stability of agriculturally important oilseed Brassica species and discusses how contemporary farming and genetic techniques have driven improvements.
... The control of blackleg disease generally involves a three-pronged approach consisting of cultural, chemical, and genetic practices. However, the reliance on cultural and chemical practices changes dramatically in each global region, for example, in Australia 95% of growers apply fungicides, whilst this is much lower in other regions [3,4]. However, breeding for genetic resistance remains a universal approach to minimizing this disease across all canola-growing regions [2]. ...
... maculans, the rotation of single resistance genes is recommended [32]. However, in Australia and Canada, many of the commercial cultivars contain stacks of multiple resistance genes [3,19,35]. The removal of resistance genes to allow the development of single R gene cultivars is extremely laborious using phenotyping alone, and is therefore something in which canola breeders have shown little interest. ...
... Through the validation of the molecular markers, it was found that 47% of cultivars/lines harbored the Rlm4 gene, similar to previous reports that suggested Rlm4 was present in over half of the Australian cultivars [3,35]. The Rlm9 resistance gene was present in 32% of cultivars, whilst LepR3 and Rlm7 was present in 7% and 8%, respectively. ...
Blackleg disease, caused by the fungal pathogen Leptosphaeria maculans, is the most devastating disease of canola (Brassica napus, oilseed rape) worldwide. Breeding for genetic resistance is the most widely used tool for controlling this disease and minimizing the impact on yield. To date, five resistance genes (Rlm2, LepR3, Rlm4, Rlm7, Rlm9) have been cloned from B. napus, representing alleles of two different gene loci, Rlm2-LepR3 and Rlm4-7-9. We report on the development and validation of Kompetitive Allele-Specific PCR (KASP) markers that can discriminate between the resistant and susceptible alleles of each resistance gene. These markers will provide valuable tools for both researchers and industry through the ability to characterize resistance genes without phenotyping.
... Seasonal timing of planting (sowing seeds) can indirectly effect croppest interactions. Early sowing encourages earlier crop growth which may prevent chronic exposure to the disease during the susceptible seedling stage can help reduce spore infection (Kirkegaard et al., 2006;Van de Wouw et al., 2021). In a Canadian study, similar effects were observed when trying to avoid flea beetles (Phyllotreta cruciferae), sowing a winter type canola autumn meant the plants were more mature when the adult flea beetles were able to infest the crops. ...
... Increased use of pesticides with crop intensification allows pests to develop tolerance (Chi et al., 2022). Diamondback moth, Plutella xylostella (Safraz and Keddie, 2005;, fall armyworm (Nguyen et al., 2021) as well as disease-causing pathogens like L. maculans in canola ( Van de Wouw et al., 2021) and Podosphaera xanthii in hull-less pumpkins (Liang et al., 2022) have developed resistance to synthetic pesticide application. Understanding the mechanisms of attack and resistance together with the ability to predict these transformations (Sparks and Nauen 2015;Nguyen et al., 2021) provides an opportunity to simulate resistance development in integrated models of pests and crops. ...
Impacts of pest and diseases on crop productivity comprise one of the greatest existential threats to food security in the 21st century. Despite this, crop models have historically adopted an abiotic lens. Here, we reviewed previous methods aimed at modelling effects of pests on crops and revealed a dearth of integrated approaches that account for pest lifecycles. The few integrated models that do exist tend to be empirical constructs that discount yield, with models of underpinning pest dynamics being extremely rare. Interaction between pests and crops has tended towards pest-induced reductions in plant biomass, leaf area, light interception and/or photosynthetic rates of infected plants, rather than biological modelling of the pest lifecycle per se. The use of process-based models that couple the pest-host interactions and capture the resource competition between the two are more suited to understanding the complexity of the farming system. Given that management interventions – such as crop rotation, intercropping, sowing time, nitrogen fertilisation, planting density and insecticide or fungicide use – underpin host colonisation success, we solicit advances in the modelling of management decisions to mitigate and manage pest and disease populations. Such information will become ever more crucial as global temperatures and extreme weather events increase in frequency and disease infestation proliferates. Harnessing this integrated weather-pest-crop-management continuum within farming systems models will improve farm management decisions. We conceptualise a framework using the lifecycle of blackleg disease (Leptosphaeria maculans) as an example; however, our approach could be generically adapted to other crop-pest interactions.
... Fungicide use in canola crops has increased in Australia over the past 20 years; a survey by Van de Wouw et al. (2021) found that 95% of canola growers currently use fungicides, versus only 52% in the 2000. This shift towards almost universal use of fungicides has been driven by changes such as increased area sown to canola and tighter rotations, which limit the ability for growers to employ cultural practices that can minimise blackleg disease, such as sowing into paddocks that are isolated from previous years' stubble . ...
... These foliar fungicides can be applied at either the 4-10-leaf stage, with the intention of controlling crown canker through the protection of seedlings, or at 30% bloom, with the intention of controlling upper canopy infection. Despite the relatively recent introduction of foliar fungicides, 49% of Australian growers consider applying them when seasonal conditions are favourable for disease, and a further 20% of Australian growers always apply a foliar fungicide ( Van de Wouw et al. 2021). ...
Fungicide use has become a fundamental part of many crop protection systems around the world, including to control blackleg disease on canola (Brassica napus L.). In Australia, most canola growers routinely apply at least one fungicide, and potentially multiple fungicides with different modes of action, in a single growing season. There is evidence for the emergence of fungicide resistance in Leptosphaeria maculans, the causal agent of blackleg disease, to the demethylation inhibitor (DMI) class of fungicides in Australia. However, it is not known whether resistance exists towards other chemical classes such as the succinate dehydrogenase inhibitors (SDHI). In this work, 397 samples were screened for resistance towards seven fungicide treatments in stubble-borne L. maculans populations collected from eight canola-growing agro-ecological regions of Australia from 2018 to 2020, a time frame that bridges the introduction of new chemicals for blackleg control. We confirmed that DMI resistance in L. maculans is pervasive across all of the sampled canola-growing regions, with 15% of fungal populations displaying high levels (resistance scores >0.5) of resistance towards the DMI fungicides. Although resistance to newly introduced SDHI fungicides was low, we found evidence of positive cross-resistance between established DMI-only fungicides and a newly introduced combined DMI and quinone outside inhibitor fungicide, suggesting that the efficacy of the latter may be limited by widespread DMI resistance. Proactive surveillance, as performed here, may provide a means to avoid the rapid loss of fungicide efficacy in the field.
... In Australia, the situation is more 166 complex, with mostly spring types grown for an extended time (5 -7 months from March to 167 September), but also winter-types for dual-purpose used firstly for grazing livestock and then for seed 168 production. These latter types have a long vegetative cycle (11 months; Van de Wouw et al. 2021). 169 ...
Blackleg (also known as Phoma or stem canker) is a major, worldwide disease of Brassica crop species (e.g. oilseed rape, rapeseed, canola) caused by the ascomycete fungus Leptosphaeria maculans. The outbreak and severity of this disease depend on environmental conditions and management practices such as the use of fungicides and crop rotations, tilling, as well as a complex interaction between the pathogen and its hosts. Genetic resistance is a major method to control the disease (and the only control method in some parts of the world, such as continental Europe), but efficient use of genetic resistance is faced with many difficulties: (i) the scarcity of germplasm/genetic resources available, (ii) the different history of use of resistance genes in different parts of the world and the different populations of the fungus the resistance genes are exposed to, (iii) the complexity of the interactions between the plant and the pathogen, that expand beyond typical gene-for-gene interactions, (iv) the incredible evolutionary potential of the pathogen and the importance of knowing the molecular processes set up by the fungus to “breakdown’ resistances, so that we may design high-throughput diagnostic tools for population surveys, and (v) the different strategies and options to build up the best resistances and to manage them so that they are durable. In this paper, we aim to provide a comprehensive overview of these different points, stressing the differences between the different continents and the current prospects to generate new and durable resistances to blackleg disease.
... It is unknown why Cladosporium disease emerged to prominence starting in 2019. One hypothesis is that the species has been present at a low level in canola or other plant species and that relatively recent changes in farming to control diseases caused by L. maculans or S. sclerotiorum, such as fungicide applications during flowering [reviewed by (Van de Wouw et al. 2021)], may have altered the prevalence of fungal species in canola fields. Thus, the UoM19-17 strain was tested for its ability to grow in the presence of two demethylation inhibitor (azole) fungicides, which target ergosterol biosynthesis, and are registered in Australia for use against Sclerotinia stem rot and commonly used to control upper canopy infection caused by L. maculans. ...
New disease symptoms were observed on canola (Brassica napus) crops late in the 2019 and 2020 growing seasons in Western Australia. Cladosporium macrocarpum was isolated from infected material, and the fungus responsible for the symptoms was demonstrated by fulfilling Koch’s postulates. One strain exhibited relatively high tolerance to prothioconazole and tebuconazole fungicides compared to other canola pathogens Sclerotinia sclerotiorum and Leptosphaeria maculans. In a 2019 field trial in a commercial canola crop, symptom incidence caused by Cladosporium was only significantly reduced by a double application of a foliar fungicide (active ingredients prothioconazole and tebuconazole) registered in canola for other diseases, while single applications gave no significant response. This new disease, caused by a ubiquitous fungal species, may be a consequence of changes to farm management strategies to reduce other fungal diseases of canola.
Fungicide resistance is often conferred through the mutation of genes encoding fungicide targets or proteins that remove fungicides from cells, but mechanisms can vary widely between taxa. Discovering the specific resistance alleles present in pathogen populations is essential for monitoring the evolution and movement of resistant genotypes. In this study, we explored the genomic basis of demethylase inhibitor (DMI) resistance in Leptosphaeria maculans , the main pathogen of the canola crop Brassica napus . Using an international collection of over 200 genome‐sequenced isolates, we assayed in vitro sensitivity to the DMI tebuconazole and conducted a genome‐wide association study on a variant set including single‐nucleotide polymorphisms (SNPs), small indels and structural variants. The main resistance allele identified was a 237 bp remnant transposable element insertion in the promoter of the erg11 / CYP51 DMI target gene in a large proportion of isolates from Europe, an allele known to confer DMI resistance in Australia. Several associated loci were identified, none of which are commonly linked to DMI resistance in other phytopathogens. We also found little to no relationship between DMI tolerance and baseline growth rate, suggesting minimal fitness effects of fungicide resistance in these isolates. This study indicates common DMI resistance alleles in L . maculans are shared across continents and erg11 / CYP51 coding mutations, which are near‐ubiquitous in other fungal pathogens, may not underpin DMI resistance in this species. Furthermore, that resistance occurs frequently in numerous canola‐growing regions suggests management is essential for growers.
Phenology is a key adaptive trait of organisms, shaping biotic interactions in response to the environment. It has emerged as a critical topic with implications for societal and economic concerns due to the effects of climate change on species’ phenological patterns. Fungi play essential roles in ecosystems, and plant pathogenic fungi have significant impacts on global food security. However, the phenology of plant pathogenic fungi, which form a huge and diverse clade of organisms, has received limited attention in the literature. This diversity may have limited the use of a common language for comparisons and the integration of phenological data for these taxonomic groups. Here, we delve into the concept of ‘phenology’ as applied to plant pathogenic fungi and explore the potential drivers of their phenology, including environmental factors and the host plant. We present the PhenoFun scale, a phenological scoring system suitable for use with all fungi and fungus-like plant pathogens. It offers a standardised and common tool for scientists studying the presence, absence, or predominance of a particular phase, the speed of phenological phase succession, and the synchronism shift between pathogenic fungi and their host plants, across a wide range of environments and ecosystems. The application of the concept of ‘phenology’ to plant pathogenic fungi and the use of a phenological scoring system involves focusing on the interacting processes between the pathogenic fungi, their hosts, and their biological, physical, and chemical environment, occurring during the life cycle of the pathogen. The goal is to deconstruct the processes involved according to a pattern orchestrated by the fungus’s phenology. Such an approach will improve our understanding of the ecology and evolution of such organisms, help to understand and anticipate plant disease epidemics and their future evolution, and make it possible to optimise management models, and to encourage the adoption of cropping practices designed from this phenological perspective.
The timing of ascospore release is critical in the prediction of Blackleg infection, particularly if the timing of spore release coincides with early development of canola seedlings. Historically, prediction models have used average daily temperature and an environment calibration to estimate a minimum rainfall amount to trigger development. This paper describes a different approach based on hydro-thermal time, where soil evaporation and rainfall are used as a surrogate to estimate when the stubble is wet, and temperature is accumulated on an hourly basis. Furthermore, the stubble orientation due to differing harvest management practices is considered, as stubble knocked down has greater contact with the soil compared to stubble which remains in a standing position. Pseudothecia and ascospore development in the standing and lying treatments was monitored weekly to measure the rate of development in diverse environments. The new modelling approach was used to describe the pseudothecial maturation rate and predict the timing of ascospore release. Subsequently, a range of Australian and international datasets were used to assess the robustness and accuracy of this new model’s predictions. When tested across multiple locations and seasons globally, the hydro-thermal time approach had similar performance (R ² = 0.94, RMSD = 16 days) to the existing Sporacle Ezy model (R ² = 0.90, RMSD = 22 days), but without the need to calibrate in different environments and account for the delayed rate of development in standing stubble. Integrating this new approach into early warning systems for canola growers will help in the management of control measures.
Canola/oilseed rape ( Brassica napus L.) production in Canada has increased to become a foundational crop in the Canadian Prairies and an important economic driver of this region. The increase in seeded area, and by association its reduction in crop rotation frequency, has made it easier for pests to overcome current recommended agronomic management practices. The Canola Council of Canada has been successful in involving the entire commodity value chain in promoting and strengthening the Canadian canola industry, however, because of this production increase it is critically important to understand, evaluate, and mitigate the potential risks of canola yield losses to current and potential pests. This review provides an overview of what are currently the most damaging insects, pathogens and weeds to canola in the Canadian Prairies, potential future threats, and opportunities farmers, agronomists and researchers can take to minimize these risks.
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Continued use of fungicides provides a strong selection pressure towards strains with mutations to render these chemicals less effective. Previous research has shown that resistance to the demethylation inhibitor (DMI) fungicides, which target ergosterol synthesis, in the canola pathogen Leptosphaeria maculans has emerged in Australia and Europe. The change in fungicide sensitivity of individual isolates was found to be due to DNA insertions into the promoter of the erg11/CYP51 DMI target gene. Whether or not these were the only types of mutations and how prevalent they were in Australian populations was explored in the current study. New isolates with reduced DMI sensitivity were obtained from screens on DMI-treated plants, revealing eight independent insertions in the erg11 promoter. A novel deep amplicon sequencing approach applied to populations of ascospores fired from stubble identified an additional undetected insertion allele and quantified the frequencies of all known insertions, suggesting that, at least in the samples processed, the combined frequency of resistant alleles is between 0.0376% and 32.6%. Combined insertion allele frequencies positively correlated with population-level measures of in planta resistance to four different DMI treatments. Additionally, there was no evidence for erg11 coding mutations playing a role in conferring resistance in Australian populations. This research provides a key method for assessing fungicide resistance frequency in stubble-borne populations of plant pathogens and a baseline from which additional surveillance can be conducted in L. maculans. Whether or not the observed resistance allele frequencies are associated with loss of effective disease control in the field remains to be established.
Utilising resistance ( R ) genes, such as LepR1 , against Leptosphaeria maculans , the causal agent of blackleg in canola ( Brassica napus ), could help manage the disease in the field and increase crop yield. Here we present a genome wide association study (GWAS) in B. napus to identify LepR1 candidate genes. Disease phenotyping of 104 B. napus genotypes revealed 30 resistant and 74 susceptible lines. Whole genome re-sequencing of these cultivars yielded over 3 million high quality single nucleotide polymorphisms (SNPs). GWAS in mixed linear model (MLM) revealed a total of 2,166 significant SNPs associated with LepR1 resistance. Of these SNPs, 2108 (97%) were found on chromosome A02 of B. napus cv. Darmor bzh v9 with a delineated LepR1 _mlm1 QTL at 15.11-26.08 Mb. In LepR1 _mlm1, there are 30 resistance gene analogs (RGAs) (13 nucleotide-binding site-leucine rich repeats (NLRs), 12 receptor-like kinases (RLKs), and 5 transmembrane-coiled-coil (TM-CCs)). Sequence analysis of alleles in resistant and susceptible lines was undertaken to identify candidate genes. This research provides insights into blackleg resistance in B. napus and assists identification of the functional LepR1 blackleg resistance gene.
Protection of many crops is achieved through the use of genetic resistance. Leptosphaeria maculans, the causal agent of blackleg disease of Brassica napus, has emerged as a model for understanding gene‐for‐gene interactions that occur between plants and pathogens. Whilst many of the characterized avirulence effector genes interact with a single resistance gene in the host, the AvrLm4‐7 avirulence gene is recognized by two resistance genes, Rlm4 and Rlm7. Here, we report the “breakdown” of the Rlm7 resistance gene in Australia, under two different field conditions. The first, and more typical, breakdown probably resulted from widescale use of Rlm7‐containing cultivars whereby selection has led to an increase of individuals in the L. maculans population that have undergone repeat‐induced point (RIP) mutations at the AvrLm4‐7 locus. This has rendered the AvrLm4‐7 gene ineffective and therefore these isolates have become virulent towards both Rlm4 and Rlm7. The second, more atypical, situation was the widescale use of Rlm4 cultivars. Whilst a single‐nucleotide polymorphism is the more common mechanism of virulence towards Rlm4, in this field situation, RIP mutations have been selected leading to the breakdown of resistance for both Rlm4 and Rlm7. This is an example of a resistance gene being rendered ineffective without having grown cultivars with the corresponding resistance gene due to the dual specificity of the avirulence gene. These findings highlight the value of pathogen surveillance in the context of expanded knowledge about potential complexities for Avr–R interactions for the deployment of appropriate resistance gene strategies. The breakdown of Rlm7 resistance can result from widescale use of cultivars harbouring either Rlm7 or Rlm4 due to the dual specificity of the corresponding avirulence gene, AvrLm4‐7.
Effective crop protection is vital to safeguard food security, but growers are reliant on a limited toolbox in the face of diverse and evolving pathogens. New crop protection methods need to be developed, but it is also important to prolong the effective life of existing products through resistance management. Some core principles in fungicide resistance management, such as using different modes of action and limiting repeat applications of any single mode of action, are long established. However, other aspects have been long debated, such as using higher or lower dose rates, mixtures or alternations, and whether to spray protectively or only if a disease threshold is reached. Continuing research into these questions uses a range of approaches, including modelling, experimental trials, and ongoing monitoring of pathogen populations. Molecular diagnostics allow higher‐throughput monitoring and earlier detection of emerging resistance. Resistance management guidelines must also be continually updated as new fungicides, and other crop protection measures, are introduced. Designing optimal resistance management strategies is only half of the story, however, since a strategy will only be effective if growers follow it in practice. This is more difficult where there are hard trade‐offs between reducing selection for resistance in the future and achieving greater disease control in the present, or where there is a choice between a strategy that would be optimal on average or one that minimises the risk of control failure in any year. Therefore, communication with farmers is an aspect of resistance management that must not be overlooked.
Sustainable canola production is essential to meet growing human demands for vegetable oil, biodiesel, and meal for stock feed markets. Blackleg, caused by the fungal pathogen, Leptosphaeria maculans is a devastating disease that can lead to significant yield loss in many canola production regions worldwide. Breakdown of race-specific resistance to L. maculans in commercial cultivars poses a constant threat to the canola industry. To identify new alleles, especially for quantitative resistance (QR), we analyzed 177 doubled haploid (DH) lines derived from an RP04/Ag-Outback cross. DH lines were evaluated for QR under field conditions in three experiments conducted at Wagga Wagga (2013, 2014) and Lake Green (2015), and under shade house conditions using the ‘ascospore shower’ test. DH lines were also characterized for qualitative R gene-mediated resistance via cotyledon tests with two differential single spore isolates, IBCN17 and IBCN76, under glasshouse conditions. Based on 18,851 DArTseq markers, a linkage map representing 2,019 unique marker bins was constructed and then utilized for QTL detection. Marker regression analysis identified 22 significant marker associations for resistance, allowing identification of two race-specific resistance R genes, Rlm3 and Rlm4, and 21 marker associations for QR loci. At least three SNP associations for QR were repeatedly detected on chromosomes A03, A07 and C04 across phenotyping environments. Physical mapping of markers linked with these consistent QR loci on the B. napus genome assembly revealed their localization in close proximity of the candidate genes of B. napus BnaA03g26760D (A03), BnaA07g20240D (A07) and BnaC04g02040D (C04). Annotation of these candidate genes revealed their association with protein kinase and jumonji proteins implicated in defense resistance. Both Rlm3 and Rlm4 genes identified in this DH population did not show any association with resistance loci detected under either field and/or shade house conditions (ascospore shower) suggesting that both genes are ineffective in conferring resistance to L. maculans in Australian field conditions. Taken together, our study identified sequence-based molecular markers for dissecting R and QR loci to L. maculans in a canola DH population from the RP04/Ag-Outback cross.
Key message: One QTL for resistance against Leptosphaeria maculans growth in leaves of young plants in controlled environments overlapped with one QTL detected in adult plants in field experiments. The fungal pathogen Leptosphaeria maculans initially infects leaves of oilseed rape (Brassica napus) in autumn in Europe and then grows systemically from leaf lesions along the leaf petiole to the stem, where it causes damaging phoma stem canker (blackleg) in summer before harvest. Due to the difficulties of investigating resistance to L. maculans growth in leaves and petioles under field conditions, identification of quantitative resistance typically relies on end of season stem canker assessment on adult plants. To investigate whether quantitative resistance can be detected in young plants, we first selected nine representative DH (doubled haploid) lines from an oilseed rape DY ('Darmor-bzh' × 'Yudal') mapping population segregating for quantitative resistance against L. maculans for controlled environment experiment (CE). We observed a significant correlation between distance grown by L. maculans along the leaf petiole towards the stem (r = 0.91) in CE experiments and the severity of phoma stem canker in field experiments. To further investigate quantitative trait loci (QTL) related to resistance against growth of L. maculans in leaves of young plants in CE experiments, we selected 190 DH lines and compared the QTL detected in CE experiments with QTL related to stem canker severity in stems of adult plants in field experiments. Five QTL for resistance to L. maculans growth along the leaf petiole were detected; collectively they explained 35% of the variance. Two of these were also detected in leaf lesion area assessments and each explained 10-12% of the variance. One QTL on A02 co-localized with a QTL detected in stems of adult plants in field experiments. This suggests that resistance to the growth of L. maculans from leaves along the petioles towards the stems contributes to the quantitative resistance assessed in stems of adult plants in field experiments at the end of the growing season.
Interactions between Leptosphaeria maculans , causal agent of stem canker of oilseed rape, and its Brassica hosts are models of choice to explore the multiplicity of ‘gene‐for‐gene’ complementarities and how they diversified to increased complexity in the course of plant–pathogen co‐evolution. Here, we support this postulate by investigating the AvrLm10 avirulence that induces a resistance response when recognized by the Brassica nigra resistance gene Rlm10 .
Using genome‐assisted map‐based cloning, we identified and cloned two AvrLm10 candidates as two genes in opposite transcriptional orientation located in a subtelomeric repeat‐rich region of the genome. The AvrLm10 genes encode small secreted proteins and show expression profiles in planta similar to those of all L. maculans avirulence genes identified so far.
Complementation and silencing assays indicated that both genes are necessary to trigger Rlm10 resistance. Three assays for protein–protein interactions showed that the two AvrLm10 proteins interact physically in vitro and in planta .
Some avirulence genes are recognized by two distinct resistance genes and some avirulence genes hide the recognition specificities of another. Our L. maculans model illustrates an additional case where two genes located in opposite transcriptional orientation are necessary to induce resistance. Interestingly, orthologues exist for both L. maculans genes in other phytopathogenic species, with a similar genome organization, which may point to an important conserved effector function linked to heterodimerization of the two proteins.
The hemibiotrophic fungus, Leptosphaeria maculans is the most devastating pathogen, causing blackleg disease in canola (Brassica napus L). To study the genomic regions involved in quantitative resistance (QR), 259–276 DH lines from Darmor-bzh/Yudal (DYDH) population were assessed for resistance to blackleg under shade house and field conditions across 3 years. In different experiments, the broad sense heritability varied from 43 to 95%. A total of 27 significant quantitative trait loci (QTL) for QR were detected on 12 chromosomes and explained between 2.14 and 10.13% of the genotypic variance. Of the significant QTL, at least seven were repeatedly detected across different experiments on chromosomes A02, A07, A09, A10, C01, and C09. Resistance alleles were mainly contributed by ‘Darmor-bzh’ but ‘Yudal’ also contributed few of them. Our results suggest that plant maturity and plant height may have a pleiotropic effect on QR in our conditions. We confirmed that Rlm9 which is present in ‘Darmor-bzh’ is not effective to confer resistance in our Australian field conditions. Comparative mapping showed that several R genes coding for nucleotide-binding leucine-rich repeat (LRR) receptors map in close proximity (within 200 Kb) of the significant trait-marker associations on the reference ‘Darmor-bzh’ genome assembly. More importantly, eight significant QTL regions were detected across diverse growing environments: Australia, France, and United Kingdom. These stable QTL identified herein can be utilized for enhancing QR in elite canola germplasm via marker- assisted or genomic selection strategies.
Genetic, physiological and physical homogenization of agricultural landscapes creates ideal environments for plant pathogens to proliferate and rapidly evolve. Thus, a critical challenge in plant pathology and epidemiology is to design durable and effective strategies to protect cropping systems from damage caused by pathogens. Theoretical studies suggest that spatio‐temporal variation in the diversity and distribution of resistant hosts across agricultural landscapes may have strong effects on the epidemiology and evolutionary potential of crop pathogens. However, we lack empirical tests of spatio‐temporal deployment of host resistance to pathogens can be best used to manage disease epidemics and disrupt pathogen evolutionary dynamics in real‐world systems. In a field experiment, we simulated how differences in Brassica napus resistance deployment strategies and landscape connectivity influence epidemic severity and Leptosphaeria maculans pathogen population composition. Host plant resistance, spatio‐temporal connectivity [stubble loads] and genetic connectivity of the inoculum source [composition of canola stubble mixtures] jointly impacted epidemiology (disease severity) and pathogen evolution (population composition). Changes in population composition were consistent with directional selection for the ability to infect the host (infectivity), leading to changes in pathotype (multilocus phenotypes) and infectivity frequencies. We repeatedly observed decreases in the frequency of unnecessary infectivity, suggesting that carrying multiple infectivity genes is costly for the pathogen. From an applied perspective, our results indicate that varying resistance genes in space and time can be used to help control disease, even when resistance has already been overcome. Furthermore, our approach extends our ability to test not only for the efficacy of host varieties in a given year, but also for durability over multiple cropping seasons, given variation in the combination of resistance genes deployed.
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Blackleg disease of Brassica napus caused by Leptosphaeria maculans (Lm) is largely controlled by deploying race-specific resistance (R) genes. However, selection pressure exerted by R genes causes Lm to adapt and give rise to new virulent strains through mutation and deletion of effector genes. Therefore, knowledge of effector gene function is necessary for effective management of the disease. Here we report cloning of Lm effector AvrLm9 that is recognised by the resistance gene Rlm9 in B. napus cultivar Goéland. AvrLm9 was mapped to scaffold 7 of Lm genome, co-segregating with the previously reported AvrLm5 (previously known as AvrLmJ1). Comparison of AvrLm5 alleles among the 37 re-sequenced L. maculans isolates and transgenic complementation identified a single point mutation correlating with the AvrLm9 phenotype. Therefore we renamed this gene as AvrLm5-9 to reflect the dual specificity of this locus. Avrlm5-9-transgenic isolates were avirulent when inoculated on the B. napus cultivar Goéland. Expression of AvrLm5-9 during the infection was monitored by RNA-sequencing. Recognition of AvrLm5-9 by Rlm9 is masked in the presence of AvrLm4-7, another Lm effector. AvrLm5-9 and AvrLm4-7 do not interact, and AvrLm5-9 is expressed in the presence of AvrLm4-7. AvrLm5-9 is the second Lm effector for which host recognition is masked by AvrLm4-7. Understanding this complex interaction will provide new opportunity for engineering broad-spectrum recognition.
Leptosphaeria maculans is the major pathogen of canola (oilseed rape, Brassica napus) worldwide. In Australia, the use of azole fungicides has contributed to the 50-fold increase in canola production in the last 25 years. However, extensive application of fungicides sets the stage for the selection of fungal populations with resistance. A high-throughput in planta assay was developed to allow screening of thousands of isolates from multiple populations. Using this screen, isolates were identified with decreased sensitivity to the fungicide fluquinconazole when applied at field rates as a protective seed dressing: these isolates cause significantly larger lesions on cotyledons and true leaves and increased disease severity at plant maturity. This increased in planta resistance was specific to fluquinconazole, with no cross resistance to flutriafol or tebuconazole/prothioconazole. In a limited set of 22 progeny from a cross between resistant and susceptible parents, resistance segregated in a 1:1 ratio, suggesting a single gene is responsible. A survey of 200 populations from across canola growing regions of Australia revealed fungicide resistance was present in 15% of the populations. Although in vitro analysis of the fungicide resistant isolates showed a significant shift in the average EC50 compared to the sensitive isolates, this was not as evident as the in planta assays. The development of this novel, high-throughput in planta assay has led to the identification of the first fungicide resistant L. maculans isolates, which may pose a threat to the productivity of the Australian canola industry.
Blackleg disease, caused by the ascomycete fungal pathogen Leptosphaeria maculans, is a devastating disease of canola (Brassica napus) in Australia, Canada and Europe. Although cultural strategies such as crop rotation, fungicide application, and tillage are adopted to control the disease, the most promising disease control strategy is the utilisation of resistant canola varieties. However, field populations of L. maculans display a high evolutionary potential and are able to overcome major resistance genes within a few years, making disease control relying on resistant varieties challenging. In the early 1990s, blackleg resistance gene Rlm3 was introduced into Canadian canola varieties and provided good resistance against the fungal populations until the early 2000s, when moderate to severe blackleg outbreaks were observed in some areas across western Canada. However, the breakdown of Rlm3 resistance was not reported until recently, based on studies on R genes present in Canadian canola varieties and the avirulence allele frequency in L. maculans populations in western Canada. The fact that Rlm3 was overcome by the evolution of fungal populations demands canola breeding programs in Canada to be prepared to develop canola varieties with diversified and efficient R genes. In addition, frequent monitoring of fungal populations can provide up-to-date guidance for proper resistance genes deployment. This literature review provides insights into the outbreaks and management of blackleg disease in Canada.
Background
Resistance to the blackleg disease of Brassica napus (canola/oilseed rape), caused by the hemibiotrophic fungal pathogen Leptosphaeria maculans, is determined by both race-specific resistance (R) genes and quantitative resistance loci (QTL), or adult-plant resistance (APR). While the introgression of R genes into breeding material is relatively simple, QTL are often detected sporadically, making them harder to capture in breeding programs. For the effective deployment of APR in crop varieties, resistance QTL need to have a reliable influence on phenotype in multiple environments and be well defined genetically to enable marker-assisted selection (MAS).
Results
Doubled-haploid populations produced from the susceptible B. napus variety Topas and APR varieties AG-Castle and AV-Sapphire were analysed for resistance to blackleg in two locations over 3 and 4 years, respectively. Three stable QTL were detected in each population, with two loci appearing to be common to both APR varieties. Physical delineation of three QTL regions was sufficient to identify candidate defense-related genes, including a cluster of cysteine-rich receptor-like kinases contained within a 49 gene QTL interval on chromosome A01. Individual L. maculans isolates were used to define the physical intervals for the race-specific R genes Rlm3 and Rlm4 and to identify QTL common to both field studies and the cotyledon resistance response.
Conclusion
Through multi-environment QTL analysis we have identified and delineated four significant and stable QTL suitable for MAS of quantitative blackleg resistance in B. napus, and identified candidate genes which potentially play a role in quantitative defense responses to L. maculans.
Blackleg disease, caused by the fungal pathogen Leptosphaeria maculans, is a serious disease of Brassica napus. The disease is mainly controlled by genetic resistance and crop rotation. However, L. maculans has displayed a high evolutionary potential to overcome major resistance genes in B. napus. This study aimed to analyze the major-gene and adult-plant resistance (APR) of Canadian B. napus varieties/lines (accessions) and the avirulence allele frequency in L. maculans populations in western Canada. For resistance identification, a set of L. maculans isolates with known avirulence genes were used to characterize major resistance (R) genes in 104 Canadian B. napus accessions and 102 seed samples collected from growers’ fields; with 104 B. napus accessions further evaluated for APR under controlled conditions. In addition, avirulence genes of 300 L. maculans isolates collected from infected canola stubbles in growers’ fields were determined by cotyledon inoculation and gene-specific PCR assays. The results indicated that R genes were present in the majority of these B. napus accessions, with the Rlm3 gene being predominant while other R genes were rarely detected. APR was identified in more than 50 % of the accessions. Predominance of Rlm3 in 102 seed samples from growers’ fields suggested Rlm3-carrying B. napus varieties were currently widely used in western Canada. Avirulence allele frequency identification of field L. maculans isolates revealed the scarcity of the avirulence allele towards Rlm3, AvrLm3. This indicated the breakdown of Rlm3 resistance, which could be due to the over use of this single resistance gene in Canadian B. napus germplasm.
Wheat crops usually yield more when grown after another species than when grown after wheat. Quantifying the yield increase and explaining the factors that affect the increase will assist farmers to decide on crop sequences. This review quantifies the yield increase, based on >900 comparisons of wheat growing after a break crop with wheat after wheat. The mean increase in wheat yield varied with species of break crop, ranging from 0.5tha-1 after oats to 1.2tha-1 after grain legumes. Based on overlapping experiments, the observed ranking of break-crop species in terms of mean yield response of the following wheat crop was: oats<canola ≈ mustard ≈ flax<field peas ≈ faba beans ≈ chickpeas ≈ lentils ≈ lupins. The mean additional wheat yield after oats or oilseed break crops was independent of the yield level of the following wheat crop. The wheat yield response to legume break crops was not clearly independent of yield level and was relatively greater at high yields. The yield of wheat after two successive break crops was 0.1-0.3tha-1 greater than after a single break crop. The additional yield of a second wheat crop after a single break crop ranged from 20% of the effect on a first wheat crop after canola, to 60% after legumes. The mean yield effect on a third wheat crop was negligible, except in persistently dry conditions. The variability of the break-crop effect on the yield of a second wheat crop was larger than of a first wheat crop, particularly following canola. We discuss the responses in relation to mechanisms by which break crops affect soil and following crops. By quantifying the magnitude and persistence of break-crop effects, we aim to provide a basis for the decision to grow continuous cereal crops, strategic rotations or tactically selected break crops. In many wheat-growing areas, the large potential yield increases due to break crops are not fully exploited. Research into quantifying the net benefits of break crops, determining the situations where the benefits are greatest, and improving the benefits of break crops promises to improve the efficiency of wheat-based cropping systems.
Recent expansion of cropping into Australia’s higher rainfall zone has involved dual-purpose crops suited to long growing seasons and used to produce both forage and grain. Early adoption of dual-purpose cropping has mainly used cereals, however canola (Brassica napus) has potential to provide grazing and grain while providing a break crop for cereals and grass-based pastures. Grain yield and grazing potential were simulated for 4 different canola maturity types at 13 locations across Australia’s high rainfall zone (HRZ) over 50 years using APSIM. The influence of sowing date, nitrogen availability at sowing and crop density on forage and grain production were also explored in a factorial combination of the 4 canola maturity types (winter, winter × spring, slow-spring, fast-spring), fortnightly sowing dates from early March to late June, three nitrogen treatments (50, 150 and 250 kg N/ha at sowing) and four crop densities (20, 40, 60,80 plants/m2). Potential grazing days (DSE.days/ha) were predicted by simulating sheep grazing crops until bud-visible stage. Overall, we found there is significant potential for dual-purpose use of winter and winter × spring maturity types in all regions across Australia’s HRZ. Mean simulated potential yields exceeded 4.0 t/ha at most locations, with highest mean simulated grain yields (4.5 to 5.0 t/ha) in southern Victoria and lower yields (3.3 to 4.0 t/ha) at locations in central and northern NSW. More grazing days were obtained from winter varieties sown early (March to mid-April), which provided > 2000 DSE.days/ha at many locations. However, at locations with Mediterranean climates, low frequency of early sowing opportunities before mid-April (<30% of years) will limit the potential to use winter varieties. The analysis highlights the potential value of a winter × spring maturity type (not yet commercially available) which has a longer, more reliable sowing window, high grazing potential (up to 1800 DSE.days/ha) and high grain yield potential. Spring varieties provided less, but useful grazing opportunities (300-700 DSE.days/ha), and similar yields to early sown cultivars. As for wheat, significant unrealised potential for dual-purpose canola crops was identified in south-west WA and on the northern Tablelands and slopes of NSW and southern Qld. The simulations also demonstrated the importance of early sowing, sufficient N supply and sowing densities in order to maximise grazing potential from dual-purpose crops.
Dual-purpose crops can provide valuable winter forage in livestock production systems and increase subsequent pasture availability. Using experimental measurements of sheep grazing on pasture only or dual-purpose crops of wheat, canola, and wheat and canola in combination, and their associated effects on subsequent pasture grazing, we estimated for two different years the whole-farm changes in whole-farm sheep grazing days (SGD), relative farm production and farm economic impact. The increased winter feed supply and higher grazing intensity on dual-purpose crops allowed 2–3 times the area of pasture to be spelled, which together enabled increases in potential year-round pasture stocking rate. Up to 20% of farm area could be allocated to dual-purpose crops while still obtaining the same number of SGD per farm ha with additional grain production (5.0–5.4 t wheat ha–1 and 1.9–3.6 t canola ha–1) adding significantly to farm profitability and production. Allocating 10–20% of the farm to a combination of dual-purpose wheat and canola grazed in sequence could increase whole-farm SGD by 10–15%, increase farm output by >25% and increase estimated farm profit margin by >AU$150/farm ha compared with pasture-only livestock systems. The long crop-grazing period from wheat and canola in combination providing a large pasture-spelling benefit was a key factor enabling these economic and productivity increases. Introducing wheat or canola alone on up to 30% of the farm is likely to reduce SGD per farm ha, but still significantly increase whole-farm productivity (10–20%) and estimated profit margin ($50–100 ha–1). Over the two very different experimental growing seasons, the estimated relative changes in whole-farm productivity and estimated
profit margin were similar, indicating that these benefits are likely to be consistent over a range of years. Together, these findings suggest that once whole-farm livestock feed-base effects are considered, large economic and productivity benefits can be attributed to dual-purpose crops when integrated into livestock production systems in Australia’s southern high-rainfall zone.
Canola (Brassica napus L.) production has been steadily increasing in western Canada. Here we determine the effect of canola rotation frequency on canola seed yield, quality and associated pest species. From 2008 to 2013, direct-seeded experiments involving continuous canola and all rotation phases of wheat (Triticum aestivum L.) and canola or field pea (Pisum sativum L.), barley (Hordeum vulgare L.) and canola were conducted at five western Canada locations. Fertilizers, herbicides, and insecticides were applied as required for optimal production of all crops. Canola rotation frequency did not influence canola oil or protein concentration or the level of major (composition > 1%) seed oil fatty acids. High canola yields were associated with sites that experienced cooler temperatures with adequate and relatively uniform precipitation events. For each annual increase in the number of crops between canola, canola yield increased from 0.20 to 0.36 Mg ha-1. Although total weed density was not strongly associated with canola yield, decreased blackleg disease and root maggot damage were associated with greater canola yields as rotational diversity increased. Long-term sustainable canola production will increase with cropping system diversity.
Leucine-rich repeat receptor-like proteins (LRR-RLPs) are highly adaptable parts of the signalling apparatus for extracellular detection of plant pathogens. Resistance to blackleg disease of Brassica spp. caused by Leptosphaeria maculans is largely governed by host race-specific R-genes, including the LRR-RLP gene LepR3. The blackleg resistance gene Rlm2 was previously mapped to the same genetic interval as LepR3. In this study, the LepR3 locus of the Rlm2 Brassica napus line ‘Glacier DH24287’ was cloned, and B. napus transformants were analysed for recovery of the Rlm2 phenotype. Multiple B. napus, B. rapa and B. juncea lines were assessed for sequence variation at the locus. Rlm2 was found to be an allelic variant of the LepR3 LRR-RLP locus, conveying race-specific resistance to L. maculans isolates harbouring AvrLm2. Several defence-related LRR-RLPs have previously been shown to associate with the RLK SOBIR1 to facilitate defence signalling. Bimolecular fluorescence complementation (BiFC) and co-immunoprecipitation of RLM2-SOBIR1 studies revealed that RLM2 interacts with SOBIR1 of Arabidopsis thaliana when co-expressed in Nicotiana benthamiana. The interaction of RLM2 with AtSOBIR1 is suggestive of a conserved defence signalling pathway between B. napus and its close relative A. thaliana.
Dual-purpose canola (Brassica napus) describes the use of a canola crop for grazed winter forage before seed production, a practice that has only recently been developed in southern Australia. Long-season winter canola has been grazed without yield penalty in higher rainfall zones of Australia (>650 mm) and the USA, but the potential areas are small. The feasibility to graze spring canola varieties across wider areas of the medium-rainfall (450-650 mm), mixed-farming zone in Australia is therefore of interest. We conducted a series of six field experiments involving a range of canola cultivars and grazing management and agronomy systems from 2007 to 2009 at Young in southern New South Wales, Australia, to determine the feasibility of and refine the principles for grazing dual-purpose spring canola without significant yield penalty. Mid-season, Australian spring canola cultivars including conventional and hybrid varieties representing a range of herbicide tolerance (triazine-tolerant, Clearfield (R), and Roundup Ready (R)) were sown from 16 April to 12 May and grazed with sheep at a range of growth stages from early vegetative (June) to mid-flowering (September). In general, early-sown crops (sown mid-April) provided significant grazing (similar to 800 dry sheep equivalent grazing days/ha) in winter before bud elongation, and recovered with no impact on grain yield or oil content. As previously reported, yield was significantly reduced (by up to 1 t/ha) when grazing occurred after buds had elongated (late July), due to the delayed flowering associated with bud removal by sheep and insufficient time for biomass and yield recovery. However, yield was also reduced in crops grazed before bud elongation if insufficient residual biomass remained (<1.0 t/ha for late July lock-up) to facilitate crop recovery even when there was little delay in crop development. We suggest that refinements to the existing 'phenology-based' grazing recommendations would assist to avoid yield loss in grazed spring varieties, and propose three grazing stages (safe, sensitive, and unsafe) that integrate the impacts of time, crop growth stage, residual biomass, and seasonal conditions to avoid yield loss under different circumstances. Such refinements to reduce the likelihood of grazing-induced yield loss would provide more confidence for mixed farmers to maximise the benefits from dual-purpose canola in different environments. Based on the outcomes of these experiments, dual-purpose spring canola is likely to have significant potential for wider application in other mixed farming zones, with similar region-specific refinements based on the principles reported here.
Botrytis cinerea isolates (n = 122) were collected from strawberry fields located in northern Greece during a 3-year period (2008-10) and tested for their sensitivity to the succinate dehydrogenase inhibitor boscalid. Sensitivity measurements showed three distinct phenotypes consisting of isolates highly sensitive (fungicide concentration causing inhibition of germ tube growth by 50% [EC50 values] of 0.05 to 0.21 mu g ml(-1)), moderately resistant (EC50 values of 1.37 to 7.79 mu g ml(-1)), or highly resistant (EC50 values of >50 mu g ml(-1)) to boscalid. Sequence analysis of the sdhB gene revealed five mutations leading to amino acid substitutions in the SdhB subunit in isolates moderately resistant and highly resistant to boscalid. Three moderately resistant isolates showed a nucleotide change from A to T at codon 230, resulting in an asparagine to isoleucine (N230I) substitution. Several moderately resistant isolates showed a nucleotide change from C to T at codon 272, resulting in a substitution from histidine to arginine (H272R) whereas, in another set of isolates, a nucleotide change from A to G was found at the same codon, leading to a substitution from histidine to tyrosine (H272Y). One highly resistant isolate had a nucleotide change from A to T at codon 272, leading to a substitution from histidine to leucine (H272L), whereas, in three other highly resistant isolates, a double nucleotide change from CC to IT was observed at codon 225, resulting in a substitution from proline to phenylalanine (P225F). To facilitate rapid detection of these mutations associated with resistance to boscalid, a primer-introduced restriction analysis polymerase chain reaction was developed. The method was successfully applied to the moderately and highly resistant subpopulations and showed that the H272R mutation was predominant with relative frequencies of 28.5, 37.5, and 30% during 2008, 2009, and 2010, respectively. In contrast, the H272L mutation was detected at a frequency of 2.5% only in the 2009 population, whereas the P225F mutation was detected at a frequency of 7.5% only in the 2010 population.
Southeast Australia (SEA) experienced a protracted drought during the
mid-1990s until early 2010 (known as the Big Dry or Millennium Drought)
that resulted in serious environmental, social and economic effects.
This paper analyses a range of historical climate data sets to place the
recent drought into context in terms of Southern Hemisphere inter-annual
to multi-decadal hydroclimatic variability. The findings indicate that
the recent Big Dry in SEA is in fact linked to the widespread Southern
Hemisphere climate shift towards drier conditions that began in the
mid-1970s. However, it is shown that this link is masked because the
large-scale climate drivers responsible for drying in other regions of
the mid-latitudes since the mid-1970s, did not have the same effect on
SEA during the mid to late-1980s and early-1990s. More specifically,
smaller-scale synoptic processes resulted in elevated autumn and winter
rainfall (a crucial period for SEA hydrology) during the mid to
late-1980s and early-1990s, which punctuated the longer term drying.
From the mid-1990s to 2010 the frequency of the synoptic processes
associated with elevated autumn/winter rainfall decreased, resulting in
a return to drier than average conditions and the onset of the Big Dry.
The findings presented in this paper have marked implications for water
management and climate attribution studies in SEA, in particular for
understanding and dealing with "baseline" (i.e. current) hydroclimatic
risks.
In China, the incidence of phoma stem canker observed in pre-harvest surveys from 2005 to 2012 was greater on winter oilseed rape in provinces in central China (in May) than on spring oilseed rape in north China (in August). In all 742 cases when the causal pathogen was isolated from stem cankers, it was identified as Leptosphaeria biglobosa by morphology in culture and/or by species-specific polymerase chain reaction. Both L. biglobosa and Leptosphaeria maculans were detected on crop debris and seed in shipments of oilseed rape seed imported into China through Shanghai or Wuhan ports in 2009–2011. Descriptions of the observed spread of L. maculans into areas previously colonized by L. biglobosa across a spring oilseed rape growing region (Alberta, Canada, westwards, 1984–1998) and across a winter oilseed rape growing region (Poland, eastwards, 1984–2004) were used to estimate the potential westward spread of L. maculans in China across spring oilseed rape growing regions (north China) and winter oilseed rape growing regions (central China, generally provinces along the Yangtze River), respectively. The rates of spread were estimated as 47 km per year across spring oilseed rape in north China and 70 km per year across winter oilseed rape in central China. Dispersal modelling suggested that the rate of spread of L. maculans across Alberta, Canada (c. 17 km per year) could be explained by windborne dispersal of ascospores.
Blackleg, caused by Leptosphaeria maculans, is a serious disease on canola or rapeseed in Canada, Australia and Europe. Disease levels have increased in western Canada recently and additional tools are needed to improve blackleg management. Seed treatment can be a cost-effective approach against blackleg, but there is currently no registered seed-treatment product targeting aboveground blackleg infection. In this study, a range of fungicides was evaluated for control of cotyledon infection by L. maculans and blackleg of canola. Under controlled environmental conditions, fluopyram showed greater efficacy than fluquinconazole (used against blackleg in Australia) in limiting cotyledon infection, while Prosper® EverGol and Helix® Vibrance (industry standards) were ineffective. Fluopyram at a rate of 30 to 150 g/100 kg seed generally inhibited cotyledon infection and this efficacy was not affected by delayed soil moisture for 3 weeks after planting. This indicates that the efficacy will unlikely be affected by a slight delay in germination due to a soil moisture deficit after seeding. In addition to typical mechanisms of succinate dehydrogenase inhibitor fungicides, fluopyram also induced expression of the indicator gene coding PR-1 proteins substantially, indicating the potential for inducing disease resistance. In field trials at 75 and 150 g/100 kg seed, fluopyram reduced the infection on cotyledons resulting from pricking inoculation on both susceptible and resistant canola cultivars, potentially limiting the infection into the stem. These seed treatment rates reduced the disease severity index (DSI) substantially on the susceptible cultivar, while DSI was low on a resistant cultivar with or without fluopyram treatment.
Blackleg is a worldwide disease of canola (Brassica napus), caused by a complex of fungal species in the genus Leptosphaeria, that impacts canola production and seed quality. Demethylation inhibitor (DMI) fungicides that target sterol 14α-demethylase are an integral part of disease control. Here, we report six DMI-resistant isolates of Leptosphaeria maculans and two different types of genetic modification related to the resistance. Analysis of the regulatory region of the DMI target gene ERG11 (also known as CYP51) revealed a 275-bp insertion in two of the isolates and three long terminal repeat retrotransposons (5,263, 5,267, and 5,248 bp) inserted in the promoter region of three resistant isolates. Genetic approaches confirmed that these elements are responsible for DMI resistance in L. maculans and crosses show segregation consistent with a single locus. Reverse-transcription quantitative PCR assays demonstrated that the 275-bp insertion increases ERG11 gene expression, conferring DMI fungicide resistance both in vitro and in planta. Moreover, transformation of a susceptible isolate of L. maculans with ERG11 driven by a promoter containing the 275-bp insertion increased resistance to tebuconazole. A minimal shift of the values of concentration whereby 50% of the mycelial growth is inhibited in vitro was observed in resistant isolates containing long terminal repeat retrotransposons; nevertheless, these isolates were able to develop significant lesions on cotyledons from fungicide-treated seedlings. This is the first report of genetic modifications in L. maculans relating to DMI fungicide resistance.
The most common and effective way to control phoma stem canker (blackleg) caused by Leptosphaeria maculans in oilseed rape (Brassica napus) is through the breeding of resistant cultivars. Race specific major genes that mediate resistance from the seedling stage have been identified in B. napus or have been int-rogressed from related species. Many race specific major genes have been described and some of them are probably identical in B. napus (allotetraploid AACC) and the parental species B. rapa (diploid AA). More work is needed using a set of well-characterised isolates to determine the number of different major resistance genes available. In some B. napus cultivars, there is resistance which is polygenic (mediated by Quantitative Trait Loci) and postulated to be race non-specific. Many of these major genes and Quantitative Trait Loci for resistance to L. maculans have been located on B. napus genetic maps. Genes involved in race specific and polygenic resistance are generally distinct.
Recent advances in genomics have led to a greater understanding of blackleg disease of canola, caused by the fungus Leptosphaeria maculans. Genome sequences are available for several L. maculans isolates and different Brassica species, and several resistance and avirulence genes, which have a ‘gene for gene’ relationship, have been characterised. Although several pairs of complementary resistance and avirulence genes have been sequenced, the molecular basis of their recognition is unknown. Mechanisms responsible for conferring virulence include amino acid substitutions, gene deletion and Repeat Induced Point mutations of avirulence genes. In some cases, a single avirulence gene is recognised by two resistance genes, and virulence towards each resistance gene is conferred by separate mutations. Furthermore, some avirulence-resistance gene interactions are epistatic over others; for instance, the presence of AvrLm7 (avirulent towards Rlm7) masks the AvrLm3-Rlm3 interaction resulting in isolates phenotypically appearing virulent towards Rlm3 despite having the avirulent form of the AvrLm3 allele. Several high throughput methods, including the ascospore shower technique, are used to characterise cultivar specificity, frequency of avirulence genes, and to detect fungicide sensitivity of fungal populations from stubble. The frequency of virulence in field populations can change rapidly under selection pressure when cultivars with the same resistance genes are sown in subsequent seasons. If there is a high risk of breakdown of a particular source of resistance, canola growers can be advised to sow a different cultivar, thus avoiding an epidemic.
Increasingly variable rainfall and temperatures and larger farm size in Australia have prompted earlier sowing of canola (Brassica napus L.). Improved agronomy and the release of cultivars suited to a wider range of environments have facilitated the expansion of canola production into new environments. To maximise grain yield at earlier sowing dates and in new environments, growers need to know the most suitable development pattern for their environment and sowing date to reduce production losses associated with frost, heat and drought at flowering. A simulation analysis was conducted at 77 locations across the Australian cropping zone to determine the optimal period for flowering to start in order to maximise long-term yield. The simulation accounted for effects of frost, heat and water stress on yield as well as duration of biomass accumulation. The optimal start of flowering period (OSF) was a function of the environment rather than the cultivar, and the relative importance of frost, heat and water stress varied with location. We found that the OSF was earlier (mid to late July), in most Mediterranean environments of Western Australia, South Australia and Victoria and later (August to September) in the southern temperate environments of Tasmania, Victoria and New South Wales (NSW). In medium and low rainfall areas of NSW, temperature extremes and rainfall interacted so that the OSF occurred in mid-July to mid-August. In low rainfall environments the duration of the optimal period was shorter (19–35 days) in most cases than in high rainfall environments (30–52 days). Knowledge of the OSF for a location, will allow breeders to develop cultivars with appropriate phenological characteristics for target environments. For growers, an understanding of the OSF, combined with an appropriate phenology classification of commercial cultivars, will allow selection of cultivar and sowing date combinations which decrease the risk of stress and maximise productivity.
From 1990, the Agricultural Production Systems sIMulator (APSIM) has grown from a field-focused
farming systems framework used by a small number of people, into a large collection of models used
by thousands of modellers internationally. The software grew to consist of several hundred thousand
lines of code in multiple programming languages. This has led to a large, complex software ecosystem
that is difficult to maintain. In addition, systems modellers increasingly require software systems that
integrate multiple disciplines, can represent evermore complex farming systems, can run on multiple
operating systems (desktop, web, mobile), can operate at or be adjusted to multiple temporal and spatial
scales (field, farm, region, continent, global) and run faster for larger simulation analyses. This is difficult
to achieve in an aging framework.
For these reasons, the APSIM Initiative is building the next generation of APSIM. This manuscript
outlines the approach taken and lessons learnt.
Despite its global significance as an edible oil and biofuel, the critical period for yield determination in canola (edible oilseed rape – Brassica napus L.) has not been determined in the field. Field experiments were conducted at two contrasting sites in southern Australia where 100 °Cd shading periods (15% PAR transmitted) were applied from early vegetative growth until maturity to identify the developmental period when the crop was most sensitive to stress. Despite the significant difference between the two sites for yield in the unshaded control (450 g m⁻² in New South Wales, and 340 g m⁻² in South Australia), the critical period was consistent at both sites extending from 100 to 500 °Cd after the start of flowering (BBCH60), and centred 300 °Cd after BBCH60. Seed number (seed m⁻²) was reduced by an average of 48% in the critical period, generated in equal parts by reduced pod m⁻² in the early part of the period, and reduced seed pod⁻¹ in the latter part. Reduced seed number was partially compensated by an increase in seed size of 29%. These trends were similar on the branches and main stem. On the main stem, the timing of the critical period moved from earlier to later from lower to upper pods linked to the timing of their development. Seed oil content declined and protein content increased under shading in the critical period, while both oil and protein yield (kg m⁻²) were reduced by 40–50% and 30–40% respectively. The critical period is coincident with the greatest number of near-open buds and newly opened flowers, which are highly sensitive to assimilate supply for ovule development. Both pod abortion and restricted capacity for compensatory growth of surviving pods are consequences of assimilate restriction on developing ovules. Identification of the critical period provides a useful target for breeding and management strategies to maximize productivity.
Blackleg, caused by Leptosphaeria maculans, is the main disease constraint for canola production in Australia. The fungus infects all aboveground and belowground parts of the canola plant. Yield loss in Australia and worldwide is generally associated with cankers at the crown, which arise from leaf infections during the early stages of seedling growth. Infection of flowers, peduncles, siliques, main stems and branches, with resultant lesions and canker formation, are typically uncommon symptoms. We propose the term ‘upper canopy infection’ to encompass symptoms on all of these plant parts because they generally occur together on the same plant and appear after the plant has undergone elongation. Branch and stem lesions observed in a commercial crop in 2010 were confirmed as L. maculans. Since then, assessment of upper canopy symptoms at 25 sites across the canola-producing regions of Australia between 2011 and 2016 show that symptoms are more prevalent, although they differed between sites and seasons. In 2011, symptoms were present at a single site, and this increased to seven sites in 2013 and 23 sites in 2016. Preliminary data indicate that infection arises from both ascospore and pycnidiospore inoculum, and that earlier onset of flowering is a key risk factor for more severe upper canopy infection. Evidence suggests that host genetic resistance may be an effective control for upper canopy infection.
Leptosphaeria maculans, the causal agent of blackleg disease, interacts with Brassica napus (oilseed rape, canola) and other Brassica hosts, in a gene-for-gene manner. The avirulence gene AvrLmJ1 was previously cloned and shown to interact with an unidentified B. juncea resistance gene. In this study, we show that the AvrLmJ1 gene maps to the same position as the AvrLm5 locus. Furthermore, isolates complemented with the AvrLmJ1 locus confer avirulence towards B. juncea genotypes harbouring Rlm5. These findings demonstrate AvrLmJ1 is AvrLm5 and highlight the need for shared resources to characterise accurately avirulence and/or resistance genes. This article is protected by copyright. All rights reserved.
Blackleg disease is caused by the stubble-borne pathogen Leptosphaeria maculans and results in significant yield losses in canola (Brassica napus) worldwide. Control of this disease includes breeding for resistance, fungicides and cultural practices including stubble management. In recent years, cropping systems have changed with the introduction of no-till farming and inter-row sowing, and it is unknown what impact these changes have had on stubble retention. The aim of this study is to investigate the impact of inter-row sowing on stubble retention and spore release. The use of inter-row sowing resulted in 25–48% of stubble remaining standing (vertical) in fields after 1 year. Furthermore, spore release was significantly (P < 0.05) delayed in stubble that remained vertical in the field compared with stubble lying down, with total spore release from vertical stubble 66% less than from horizontal stubble. The impact these changes have on the epidemiology of blackleg disease remains unknown.
The short-term economic benefit has in recent years prompted farmers to grow oilseed rape (Brassica napus L.) and thus the frequency of this crop increased in German crop rotations. Here, we investigate the impact of high intensity OSR crop rotations on yield, yield formation and blackleg disease (Leptosphaeria maculans) in a rotation experiment in the Hercynian dry region of Central Germany over two seasons (2014/15 – 2015/16). The preceding crop combinations compared were winter wheat (Triticum aestivum L.)-winter wheat (WW), WW-oilseed rape (OSR), OSR-OSR and an OSR monoculture. Furthermore, the fertilizer treatments 120 kg N ha⁻¹ and 180 kg N ha⁻¹ were analyzed.
Higher OSR cropping intensity decreased seed yields, however, with a variation among years and oil yield was highest when OSR was following WW-WW over both years. Minor differences were observed among the yield components, but significantly less pods per m² were developed in a long-term OSR monoculture. The disease assessment clearly showed an increased blackleg incidence and severity when OSR was grown successively.
Results of our study emphasize that high intensity OSR production will very likely be unsustainable over the long term associated with yield losses and increased infestation levels of blackleg disease.
In Australia, canola was initially grown in more reliable rainfall areas (G>400mm annual rainfall) due to its greater sensitivity to heat and drought than cereals, and the higher production costs increasing risk in more marginal environments (Colton and Potter, 1999). Improved varieties and agronomy, along with the overall farming systems benefits of weed and disease control in cereals, have expanded the area cultivated under canola, and it is now grown in all but the driest margins of the wheat belt.
Australian canola growers have new technology options including hybrid and herbicide technologies, which have offered yield and profitability advantages in other canola-growing regions of the world. This study compared the yield and gross margins of hybrid and open-pollinated (OP) canola from different herbicide tolerance groups: triazine-tolerant, Roundup Ready, Clearfield and conventional across a wide range of environments in south-Western Australia, and in the National Variety Trial network in southern Australia to investigate the relative advantages of these technologies. There were significant differences in yield responsiveness between hybrid and OP canola, the magnitude of which was determined by the growing-season rainfall/available water to the crop. Hybrid out-yielded OP canola in favourable environments where rainfall was high and the growing season was long. However, in areas of low rainfall where yield potential was low, hybrids showed little yield advantage over OP. In contrast, there were no differences in yield response between the four herbicide tolerance groups across the rainfall zones. The economic analysis showed that the break-even yield for hybrids versus OP canola was 1.25t/ha for triazine-tolerant canola, 0.7t/ha for Roundup Ready canola, and 1.7t/ha for hybrid Clearfield canola. The gross margin analysis suggested that hybrid triazine-tolerant, Clearfield and Roundup Ready canola was more profitable than the OP system in the medium (growing-season rainfall of 265-330mm) and high (330mm) rainfall environments, but not profitable in the lower (<265mm) rainfall area because the cost associated with hybrid seed outweighed the small yield benefit. The sensitivity analysis indicated that±10% changes in canola price and seed cost shifted the break-even yield by±0.1t/ha. Our study makes a case for Australian canola breeders to maintain OP canola varieties, rather than shifting their focus entirely to hybrids, to underpin continued productivity and profitability in lower rainfall areas.
Optimising the sowing date of canola (Brassica napus L.) in specific environments is an important determinant of yield worldwide. In eastern Australia, late April to early May has traditionally been considered the optimum sowing window for spring canola, with significant reduction in yield and oil in later sown crops. Recent and projected changes in climate, new vigorous hybrids, and improved fallow management and seeding equipment have stimulated a re-evaluation of early-April sowing to capture physiological advantages of greater biomass production and earlier flowering under contemporary conditions. Early-mid-April sowing generated the highest or equal highest yield and oil content in eight of nine field experiments conducted from 2002 to 2012 in south-eastern Australia. Declines in seed yield (-6.0% to -6.5%), oil content (-0.5% to -1.5%) and water-use efficiency (-3.8% to -5.5%) per week delay in sowing after early April reflected levels reported in previous studies with sowings from late April. Interactions with cultivar phenology were evident at some sites depending on seasonal conditions. There was no consistent difference in performance between hybrid and non-hybrid cultivars at the earliest sowing dates. Despite low temperatures thought to damage early pods at some sites (<-2°C), frost damage did not significantly compromise the yield of the early-sown crops, presumably because of greater impact of heat and water-stress in the later sown crops. A validated APSIM-Canola simulation study using 50 years of weather data at selected sites predicted highest potential yields from early-April sowing. However, the application of a frost-heat sensitivity index to account for impacts of temperature stress during the reproductive phase predicted lower yields and higher yield variability from early-April sowing. The frost-heat-limited yields predicted optimum sowing times of mid-April at southern sites, and late April to early May at the northern sites with lower median yield and higher yield variability in crops sown in early April. The experimental and simulation data are potentially compatible given that the experiments occurred during the decade of the Millennium drought in south-eastern Australia (2002-10), with dry and hot spring conditions favouring earlier sowing. However, the study reveals the need for more accurate and validated prediction of the frost and heat impacts on field-grown canola if simulation models are to provide more accurate prediction of attainable yield as new combinations of cultivar and sowing dates are explored.
Background:
Succinate dehydrogenase inhibitor fungicides are important in the management of Zymoseptoria tritici in wheat. New active ingredients from this group of fungicides have been introduced recently and are widely used. Because the fungicides act at a single enzyme site, resistance development in Z. tritici is classified as medium-to-high risk.
Results:
Isolates from Irish experimental plots in 2015 were tested against the SDHI penthiopyrad during routine monitoring. The median of the population was approximately 2 x less sensitive than the median of the baseline population. Two of the 93 isolates were much less sensitive to penthiopyrad than the least sensitive of the baseline isolates. These isolates were also insensitive to most commercially available SDHIs. Analysis of the succinate dehydrogenase coding genes confirmed the presence of the substitutions SdhC-H152R and SdhD-R47W in the very insensitive isolates.
Conclusion:
This is the first report showing that the SdhC-H152R mutation detected in laboratory mutagenesis studies also exist in the field. The function and relevance of this mutation, combined with SdhD-R47W, still needs to be determined.
Blackleg disease caused by the fungus Leptosphaeria maculans is the most important disease of canola worldwide. The impact of this disease on the development of the Australian canola industry, particularly over the last 20 years, is discussed. Deployment of a range of disease control measures has resulted in a thriving canola industry with production now approaching 4million tonnes annually. Discoveries about disease mechanisms and key plant and fungal genes are described. Analysis of the L. maculans genome sequence has enabled an understanding of how fungal populations can evolve rapidly to overcome disease resistance bred into canola cultivars.
The blackleg fungus, Leptosphaeria maculans, interacts with canola (Brassica napus) in a gene-for-gene manner. These major resistance genes are well characterized in the seedling stage of development, but not in other plant organs. Cotyledons, leaves, pods and stems of plants of two cultivars of B. napus, each harbouring a different major resistance gene (Rlm1 and Rlm4), were inoculated with two individual L. maculans isolates with different alleles of the corresponding avirulence genes (AvrLm1, avrLm4 and avrLm1, AvrLm4), and the disease phenotype in terms of lesion development was determined. Major gene resistance was expressed in cotyledons, all leaves and during pod set, but not in the stems of the adult plant. This is the first time major gene resistance has been shown to be effective in B. napus pods.
Extending the durability of plant resistance genes towards fungal pathogens is a major challenge. We identified and investigated the relationship between two avirulence genes of Leptosphaeria maculans , AvrLm3 and AvrLm4‐7 . When an isolate possesses both genes, the Rlm3 ‐mediated resistance of oilseed rape ( Brassica napus ) is not expressed due to the presence of AvrLm4‐7 but virulent isolates toward Rlm7 recover the AvrLm3 phenotype.
Combining genetic and genomic approaches (genetic mapping, RNA ‐seq, BAC (bacterial artificial chromosome) clone sequencing and de novo assembly) we cloned AvrLm3 , a telomeric avirulence gene of L. maculans . AvrLm3 is located in a gap of the L. maculans reference genome assembly, is surrounded by repeated elements, encodes for a small secreted cysteine‐rich protein and is highly expressed at early infection stages.
Complementation and silencing assays validated the masking effect of AvrLm4‐7 on AvrLm3 recognition by Rlm3 and we showed that the presence of AvrLm4‐7 does not impede AvrLm3 expression in planta . Y2H assays suggest the absence of physical interaction between the two avirulence proteins.
This unusual interaction is the basis for field experiments aiming to evaluate strategies that increase Rlm7 durability.
Cropping has recently expanded into arable areas of the high rainfall zone (HRZ) of Australia. We assessed the suitability of canola varieties of winter, winter×spring and spring-maturity at six sites across the south-eastern, northern and western HRZ of Australia for their suitability for dual-purpose production. Experiments measured potential forage production and the effect of defoliation or grazing on grain yield of crops sown from mid-March to mid-May. Overall, these experiments demonstrated the potential for dual-purpose canola across a wide area of the HRZ. In the south-eastern HRZ where winter conditions were sufficient for vernalisation and spring conditions were mild, winter and winter×spring types outperformed spring types as they provided an extended vegetative period for 'safe' grazing (prior to stem elongation), producing 3.0-6.8tdrymatter(DM)ha-1 of forage and recovered to produce 2.5-4.9tha-1 of grain yield. In the south-eastern region, early-sown winter types produced more forage than other canola types for grazing in late autumn and winter. In one experiment with four sowing times, consecutive delays in sowing of 2 weeks reduced forage available for grazing by 58%, 72% and 95% compared with the earliest sowing time of 10 March (6.1tDMha-1). Although spring types in this region provided some potential for grazing, the phenology was unsuitable for early sowing as the rapid onset of flowering reduced the period of safe grazing. Winter types were not suited to the western region, but the winter×spring and spring types produced >1.0tDMha-1 of forage and grain yield of 2.3tha-1. In the northern region, spring types produced the highest grain yield (>3.0tha-1) but suffered significant yield penalties associated with grazing. In other regions there was generally little or no effect of grazing on grain yield when crops were grazed or defoliated before stem elongation. These experimental studies confirm the potential for dual-purpose canola across all regions of the HRZ when suitable maturity types are sown, managed and grazed appropriately.
New canola cultivars have much higher yield potential than conventional canola cultivars and changes in production practices may be needed to achieve optimum yield from these cultivars. Studies were conducted to investigate the influence of seeding rates (2.8, 5.6 and 8.4 kg ha-1), fertilizer level (67% - low, 100% - medium, 133% - high of the commercial recommendation) and fungicide application on growth, dry matter accumulation, seed yield and seed quality using two high-yielding canola cultivars [cv. Quantum, open pollinated (OP), and cv. Invigor™, hybrid (HYB)]. The studies were conducted from 1999 to 2001 at three sites in the Parkland region of the Canadian prairies. The two cultivars did not differ in their responses to seeding and fertilizer rates, or fungicide application. Plant density was lower for the HYB than the OP because the HYB had larger seeds, with fewer seeds per kilogram. Emergence declined slightly at high fertilizer levels in some site-years due to fertilizer induced seedling damage, but increased with seeding rate. Time to onset of flowering and maturity showed no consistent influence of cultivar, and tended to be somewhat prolonged with an increase in fertilizer levels, but reduced by an increase in seeding rate. On average, the HYB produced 822 kg ha-1 more biomass and 200 kg ha-1 more seed yield than the OP. Sclerotinia stem rot incidence was very low, except at Melfort in 2000, and fungicide application generally failed to benefit the crop. Seed yield was generally increased when seeding rate was increased, with the largest increase occurring from 2.8 to 5.8 kg ha-1. Seed yield responses to the high rates of seeding or fertilizer only occurred where both inputs were at the highest level, indicating that the optimum level of one was dependent upon the level of the other input. Oil and protein concentration in the seed was higher for the HYB than the OP. High fertilizer levels generally increased the total yield of biomass and seed, and protein concentration in the seed, but usually reduced oil concentration in the seed. Increased seeding rate resulted in a small increase in oil concentration and a small reduction in protein concentration in the seed. Net returns were greatest and least variable for the HYB cultivar. Overall, the HYB performed better than the OP, and the full economic value of high-yielding canola cultivars was only realized when fertilizer and seeding rates were at or above the current recommended rates.
Five avirulence genes from Leptosphaeria maculans, the causal agent of blackleg of canola (Brassica napus), have previously been identified through map-based cloning. In this study a comparative genomic approach was used to clone the previously-mapped AvrLm2. Given the lack of a presence-absence gene polymorphism coincident with the AvrLm2 phenotype, 36 L. maculans isolates were resequenced and analyzed for single-nucleotide polymorphisms (SNPs) in predicted small secreted protein-encoding genes present within the map interval. Three SNPs coincident with the AvrLm2 phenotype were identified within LmCys1, previously identified as a putative effector-coding gene. Complementation of a virulent isolate with LmCys1, as the candidate AvrLm2 allele, restored the avirulent phenotype on Rlm2-containing B. napus lines. AvrLm2 encodes a small cysteine-rich protein with low similarity to other proteins in the public databases. Unlike other avirulence genes, AvrLm2 resides in a small GC-island within an AT-rich isochore of the genome, and was never found completely deleted in virulent isolates.
Blackleg disease (phoma stem canker) caused by the fungus Leptosphaeria maculans is a major disease of canola (oilseed rape, Brassica napus) worldwide. Canola plants in pots were exposed to blackleg‐infested stubble of canola with different complements of resistance genes and then assessed for disease. Plant mortality was reduced when plants were exposed to stubble from a cultivar with a different complement of resistance genes compared to stubble of a cultivar with the same resistance gene. These findings were consistent with 7 years of field surveys, which showed that changes in selection pressure as a result of extensive sowing of cultivars with major‐gene resistance, termed ‘sylvestris resistance’, dramatically influenced the frequency of virulent isolates in the population towards particular resistance genes, and therefore disease severity. All these data were supported by PCR‐genotyping surveys of fungal populations whereby the frequency of virulence alleles of avirulence genes AvrLm1 and AvrLm4 changed significantly depending on the resistance gene present in the cultivar from which the isolates were cultured. This is the first example of a study showing that sowing of canola cultivars with different complements of resistance genes in subsequent years, i.e. rotation of resistance genes, minimizes disease pressure by manipulating fungal populations. This approach provides a valuable disease management strategy for canola growers and is likely to be applicable to other plant diseases.
The efficacy of the fungicide Impact® (a.i. flutriafol at 250 g/L) was
tested for control of blackleg (Leptosphaeria maculans),
and for improved yield and oil content in canola
(Brassica napus) cultivars with varying levels of
blackleg resistance. Field trials were conducted in 1996 in Western Australia
at 3 locations (Merredin, Wongan Hills, Mt Barker) in paddocks containing
1–4-year-old blackleg-infested residues. The fungicide (400 mL
product/ha) was coated on a double superphosphate fertiliser and applied
at seeding. Blackleg was substantially reduced and the seed yield improved
following the application of Impact® in most treatments at all locations
except Mt Barker, where the fungicide had no effect on reducing the blackleg
severity. The percentage reduction in blackleg severity with Impact®
ranged between 18 and 59% and 1 and 43% at Merredin and Wongan
Hills, respectively, in cultivars with different levels of resistance and
exposed to infected residues of various ages. Likewise, the application of
Impact® increased the seed yield by 40–322, 186–357, and
71–426 kg/ha at Merredin, Wongan Hills, and Mt Barker, respectively,
on residue of various ages. Seed oil content was also improved following the
application of Impact® in most treatments at all locations. The
improvement in seed yield when using Impact® was variable for different
ages of the residue, and was greater under severe to moderate disease
conditions caused by exposure to more recent residues than under the milder
disease conditions resulting from older residues. In general, susceptible to
moderately resistant cultivars showed greater improvement in yield than
resistant cultivars. The rates of Impact® were further evaluated in
paddocks containing 3-year-old residue in field trials at the same 3 locations
during 1997. The fungicide was applied at 200, 400, and 800 mL product/ha.
Although blackleg severity was substantially reduced following application of
Impact® at 400 and 800 mL/ha compared with 0 and 200 mL/ha, yield
was improved only in some cultivars and at some locations.
The fungus Leptosphaeria maculans causes blackleg of Brassica species. Here we report the mapping and subsequent cloning of an avirulence gene from L. maculans. This gene, termed AvrLmJ1, confers avirulence towards all three Brassica juncea cultivars that were tested. Analysis of RNA-seq data showed that AvrLmJ1 is housed in a region of the L. maculans genome that contains only one gene that is highly expressed in planta, as shown by RNA seq data. The closest genes are 57 and 33 kb away, and like other avirulence genes of L. maculans, AvrLmJ1 is located within an AT-rich, gene-poor region of the genome. The encoded protein is 141 amino acids, has a predicted signal peptide and is cysteine-rich. Two virulent isolates contain a premature stop codon in AvrLmJ1. Complementation of an isolate that forms cotyledonary lesions on B. juncea with the wild-type allele of AvrLmJ1 confers avirulence towards all three B. juncea cultivars tested, suggesting that the gene may confer species-specific avirulence activity.
Due to the large increase of canola production in Australia, current blackleg cultural control recommendations (extended rotation length and isolation distance from canola stubble) are not adhered to by farmers in many canola-producing regions. Canola crops are increasingly being sown in short rotation and, in many instances, adjacent to paddocks containing canola stubble. In this study, the level of disease in commercial canola crops was determined for different rotations and distances from canola stubble. There was a strong relationship between the presence of canola stubble from the previous year (6-month-old stubble) and distance to current canola crops, but no relationship between the presence of older (18–42 month old) stubble and distance to current canola crops. Blackleg severity was highest where canola crops had been sown adjacent to 6-month-old canola stubble, with the level of blackleg severity decreasing markedly in the first 100 m. Disease severity then generally declined up to 500 m. Plants 500–1000 m from 6-month-old stubble had similar levels of blackleg infection. Blackleg severity was similar between canola crops sown into 18-month-old canola stubble (short rotation) and crops sown into paddocks that had no history of canola for at least the previous 3 years (long rotation). Based on these findings, we recommend that canola crops should be sown at distances greater than 100 m and preferably 500 m from last season's canola stubble, rather than extending rotation length between crops.
The infection by Leptosphaeria maculans of Brassica napus cultivars with major gene resistance derived from Brassica rapa subsp. sylvestris was studied in southeastern Australia. Following the commercial release of these cultivars in Australia in 2000, plants with stem cankers were first reported in 2002 at two geographically isolated regions in South Australia and New South Wales. In 2003, this study showed that the major gene resistance had been overcome in an area of approximately 50,000 ha in South Australia and in two fields in New South Wales (0.5 and 30 ha). There was no relationship between disease severity and incidence in 2003 and the proximity to the sites where resistance breakdown occurred in 2002. At some locations, the frequency of isolates able to overcome the B. rapa subsp. sylvestris-derived resistance had increased between 2002 and 2003. Isolates cultured from canola cultivars with either B. rapa subsp. sylvestris-derived resistance or polygenic resistance showed host specificity when inoculated onto cultivars with B. rapa subsp. sylvestris-derived or polygenic resistance, respectively. The most likely cause of the resistance breakdown was the rapid increase in frequency of L. maculans isolates virulent on this particular resistance source. The selection pressure leading to this increased frequency was probably mediated by the planting of cultivars harboring the major resistance gene in the same locations for a 3-year period, and the ability of the pathogen to produce large numbers of asexual and sexual spores.