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Advances in Downy Mildew Research

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Abstract

P. T. N. SPENCER-PHILLIPS Co-ordinator, Downy Mildew Working Group of the International Society for Plant Pathology University of the West of England, Coldharbour Lane, Bristol BS16 1QY, UK Email: peter.spencer-phillips@uwe.ac.uk It is a very great privilege to write the preface to the first specialist book on downy mildews since the major work edited by D. M. Spencer in 1981. The idea for the present publication arose from the Downy Mildew Workshop at the International Congress of Plant Pathology (ICPP) held in Edinburgh in August 1998. Our intention was to invite reviews on selected aspects of downy mildew biology from international authorities, and link these to a series of related short contributions reporting new data. No attempt has been made to cover the breadth of downy mildew research, but we hope that further topics will be included in future volumes, so that this becomes the first of a series following the five year ICPP cycle.

Chapters (11)

The non-protein amino acid BABA (DL-3-amino-n-butanoic acid, β-aminobutyric acid) is reported here to induce local and systemic resistance against downy mildew in grape leaves. Leaf discs of susceptible cultivars placed on BABA solutions and inoculated with Plasmopara viticola on the counter surface produced brownish restricted lesions below the inoculation site (Hypersensitive-like response, HR) which failed to support fungal sporulation. Histochemical analyses of such HR lesions revealed the accumulation of lignin-like deposits in the host cells. In contrast, water-treated inoculated discs produced expanded chlorotic lesions with profuse sporulation in which no lignin accumulation was observed. Mock-inoculated BABA-treated leaf discs showed no HR or lignin accumulation. Concentrations as low as 25 µg/ml (0.25 mM) of BABA sufficed to prevent tissue colonization with the fungus. Five other isomers of aminobutyric acid, namely L-2 aminobutyric acid, 2-amino isobutyric acid, DL-2-aminobutyric acid (AABA), DL-3-amino isobutyric acid, and 4-aminobutyric acid (GABA) gave no protection against the downy mildew fungus. Of the two (R and S) enantiomers of BABA only the R form was active in producing HR, suggesting a specific stereostructure requirement for activity. BABA could stop fungal colonization even when applied post-infectionally to leaf discs. Resistance of BABA-pulse-loaded leaf discs persisted for more than 14 days. BABA provided systemic protection against the disease when applied via the root system or via the lower leaves of grape plants. Application of 14C-BABA to a single leaf of intact plants showed the accumulation of the 14C label in upper leaves (and root tips), suggesting sink-oriented transport.
... In the present study, using fungicide Amistar Top as dipping treatment in combinations with different concentrations of chemical inducers used as spraying treatment were significantly effective for reducing diseases severity of downy mildew and purple blotch diseases under naturally conditions in field trials compared with untreated control (without any treatment). This is in agreement with Spencer et al. (2003) whom reported that, treated the transplants for many crops i.e. brassicas, soybean, peas and sunflower with fungicide led to more control of downy mildew diseases. That might be due to the infection with downy mildew fungi happens by two ways: first way onion transplant infected by the mycelium or oospores which left on plant remains in the field after harvesting (systemic infected) and the second way the plant leaf infected by sporangium spores (Local infection). ...
... That might be due to the infection with downy mildew fungi happens by two ways: first way onion transplant infected by the mycelium or oospores which left on plant remains in the field after harvesting (systemic infected) and the second way the plant leaf infected by sporangium spores (Local infection). In this respect, the first ways affected by using application Amistar Top as dipping treatment and give more effect for controlling downy mildew (Spencer et al., 2003). ...
... Pierre Marie Alexis Millardet, a French botanist, observed that copper sulfate, which had been effective for many years in preventing passersby from eating grapes along the road, was also efficient in combating Downy Mildew (McBride et al., 1981;Moolenaar, 1998). It is now known that Cu creates complexes in pathogens, destroying cell proteins and enzyme functions (Spencer-Phillips et al., 2002). Although the original application rate is not known, it can be speculated from recent restrictions that >8 kg ha À1 y À1 was used. ...
... Instead, copper hydroxide (Cu(OH) 2 ) and copper oxychloride (Cu 3 Cl 2 (OH) 4 ), both less concentrated, are extensively sold and applied by grape cultivators. In 1995, copper solutions represented 20% of global fungicide sales (Spencer-Phillips et al., 2002). Anthropogenic sources have increased the soil copper load. ...
Article
Viticulturists use copper fungicide to combat Downy Mildew. Copper, a non-degradable heavy metal, can accumulate in soil or leach into water sources. Its accumulation in topsoil has impacted micro and macro organisms, spurring scientists to research in situ copper removal methods. Recent publications suggest that microorganism assisted phytoextraction, using plants and bacteria to actively extract copper, is most promising. As vineyards represent moderately polluted sites this technique has great potential. Active plant extraction and chelate assisted remediation extract too little copper or risk leaching, respectively. However, despite interesting pot experiment results using microorganism assisted phytoextraction, it remains a challenge to find plants that primarily accumulate copper in their shoots, a necessity in vineyards where whole plant removal would be time consuming and financially cumbersome. Vineyard remediation requires a holistic approach including sustainable soil management, proper plant selection, increasing biodiversity and microorganisms.
... ci. All of the studied strobilurin-insensitive Z. tritici isolates show a modification of the target gene, the G143A point mutation within the cytochrome b gene (Gisi et al., 2002). Nevertheless in the resistant isolates, MgMfs1, a strong strobilurin transporter, was transcriptionaly upregulated after application of a sublethal dose of strobilurin. ...
... sensitivity profile of the PA active ingredient kiralaxyl is presented inTable 2. Using the methodology adopted by Clerjeaux et al. (1985), 100% of P. viticola populations of were classified as extremely resistant to the PA (R+++) (Table 2), indicating that they contained at least 50% of strains resistant to PA. In France, the resistance in P. viticola populations varied between 15 and 75% for the seasons 1987 and 1988, respectively (Gisi, 2002). Fourie (2004) classified the grape downy mildew populations according to their MIC values: populations exhibiting a MIC up to 200 mg/l were considered as highly resistant. ...
... Expression level changes marked with "*" were significantly different (one-sample-t-test, SPSS version 16) from 1. ci. All of the studied strobilurin-insensitive Z. tritici isolates show a modification of the target gene, the G143A point mutation within the cytochrome b gene (Gisi et al., 2002). Nevertheless in the resistant isolates, MgMfs1, a strong strobilurin transporter, was transcriptionaly upregulated after application of a sublethal dose of strobilurin. ...
... The sensitivity profile of the PA active ingredient kiralaxyl is presented in Table 2. Using the methodology adopted by Clerjeaux et al. (1985), 100% of P. viticola populations of were classified as extremely resistant to the PA (R+++) (Table 2), indicating that they contained at least 50% of strains resistant to PA. In France, the resistance in P. viticola populations varied between 15 and 75% for the seasons 1987 and 1988, respectively (Gisi, 2002). ...
... Plant diseases caused by oomycetes are a primary problem for agriculture in the world [1]. Late blight disease is one of the most important and dangerous potato and tomato diseases with the potential for practical annihilation of potato plants [2,3]. ...
Article
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We report the inhibitory effect of peptide extracts obtained from seven medicinal plants against a causative agent of late blight disease Phytophthora infestans. We find that all the extracts possess inhibitory activity toward the zoospores output, zoosporangium germination, and the development of P. infestans on potato disc tubers at different quantitative levels. Based on the biological effects detected, an extract of common horsetail (Equisetum arvense) biomass is recognized as the most effective and is selected for further structural analysis. We perform a combination of amino acid analysis and MALDI-TOF mass spectrometry, which reveal the presence of Asn/Asp-and Gln/Glu-rich short peptides with molecular masses in the range of 500-900 Da and not exceeding 1500 Da as the maximum. Analytical anion-exchange HPLC is successfully applied for separation of the peptide extract from common horsetail (E. arvense). We collect nine dominant components that are combined in two groups with differences in retention times. The N-terminal amino acid sequence of the prevalent compounds after analytical ion-exchange HPLC allows us to identify them as peptide fragments of functionally active proteins associated with photosynthesis, aquatic transport, and chitin binding. The anti-oomycete effects may be associated with the conversion of ribulose-1,5-bisphosphate carboxylase/oxygenase to produce a number of biologically active anionic peptides with possible regulatory functions. These data inform our knowledge regarding biologically active peptide fragments; they are the components of programmed or induced proteolysis of plant proteins and can realize secondary antimicrobial functions.
... The mycotoxins are the product of secondary metabolites, of low molecular weight, produced by fungi and are present in several products of human and animal food and provoking numerous diseases and death ( Bennett and Klich 2003); for example, aflatoxins, fumonisines, moniliformines and ochratoxines. The fungi that produce these toxins are, for example, Aspergillus, Penicillium, Claviceps, Alternaria and Fusarium but certain other fungi cannot produce them; for example, those causing downy mildews ( Spencer-Phillips et al. 2002), powdery mildews ( Belanger et al. 2002), smuts and rusts (Nedelnik and Repkova 1998). A very important group of Fusarium mycotoxins are the trichothecenes. ...
Chapter
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Date palm (Phoenix dactylifera L.) is the most economically important food crop in Moroccan oasean agricultural areas, contributing to preserving an arid ecosystem threatened by desertification. The bayoud disease, caused by the fungus Fusarium oxysporum f. sp. albedinis (Foa), is incontestably the most serious disease affecting date palm in North Africa. The selection for resistance among date palm cultivars was the preferred way to control the disease. New performing cultivars were selected, mass propagated and distributed to farmers. The use of pathogen toxins in in vitro selection is an innovative approach for rapid screening for resistance to bayoud disease. This chapter gives an overview of recent knowledge about toxins and other substances produced by plant pathogenic fungi and their applications in in vitro and in vivo selection for resistance. Foa toxins contain fusaric acid and other toxic fractions. These fractions of toxins have some chemical and biological characteristics that differ from among other fractions isolated from other pathogenic and nonpathogenic strains of F. oxysporum. These toxins could be exploited for pre-selection of plants for resistance to bayoud among populations of plants originating from either irradiated tissue culture or conventional breeding programs. KeywordsDate palm-Breeding for resistance- In vitro selection-HPLC analysis-Toxins
... The sensitivity profile of the PA active ingredient kiralaxyl is presented in Table 2. Using the methodology adopted by Clerjeaux et al. (1985), 100% of P. viticola populations of were classified as extremely resistant to the PA (R+++) ( Table 2), indicating that they contained at least 50% of strains resistant to PA. In France, the resistance in P. viticola populations varied between 15 and 75% for the seasons 1987 and 1988, respectively (Gisi, 2002). Fourie (2004) classified the grape downy mildew populations according to their MIC values: populations exhibiting a MIC up to 200 mg/l were considered as highly resistant. ...
Article
Full-text available
Resistance of grape downy mildew caused by Plasmopara viticola against different fungicide classes has been reported in Europe. The main objective of the present study was to measure the sensitivity status of P. viticola populations to four fungicide active ingredients used in the Luxembourgian vine-growing region and to investigate the presence of resistant populations. Thus, leaf samples with downy mildew symptoms were collected in 19 vineyards along the river Moselle in 2010 and 2011. Samples were analysed in both years for carboxylic acid amides (CAA) and the quinone outside inhibitors (QoI), using a microplate conidia growth inhibition assay. In 2011, a leaf disc assay was used in addition to determine the sensitivity profile of the sampled populations to phenylamides (PA) and to cymoxanil. Results showed that in average, 64% (2010) or 81% (2011) of the zoospores were resistant to QoI fungicides, and 91% (2010) or 85% (2011) to the CAA fungicide dimethomorph. Furthermore, in 2011, all populations were classified as extremely resistant to the PA fungicide kiralaxyl, indicating that the sampled Luxembourgian populations contained at least 50% of resistant strains. No population was controlled by cymoxanil at the rate of 100 mg/l. In average, 84% of the populations showed a minimum inhibitory concentration (MIC) between 300-800 mg/l, and 16% a MIC reaching 800 mg/l. The presented results indicate that fungicide resistance against CAAs, QoIs, PAs and cymoxanil is currently widespread in the Luxembourgian wine-growing area.
... The first commonly applied Cu solution, " Bordeaux mixture " (CuSO 4 .5H 2 O and lime mixture), was first used in France in 1885, and has since been used extensively as a fungicide/bactericide (McBride et al., 1981). Copper forms complexes within pathogens, which destroy cell proteins and disrupt enzyme functions (Spencer-Phillips et al., 2002). In much of the world CuSO 4 is no longer recommended for use as it is highly soluble and toxic to the spray applicators and the environment (Mackie et al., 2012). ...
Conference Paper
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The spraying of copper-based bactericides is one of the most effective practices in protecting against Psa. Although their use has yet to be optimised, copper-based bactericides play a major role in reducing the production of spores from the cankers. In New Zealand, copper compounds have been recommended for spraying immediately after winter pruning, at bud break, two and four weeks after bud break, and in high risk situations like after a major wind, rain or hail event. Some studies have reported that the application of Cu-based bactericides significantly affects the kiwifruit yield. However, studies on the most effective system for application of these Cu-based agrichemicals have been limited. In addition, there have been no studies published on the occurrence of Cu resistant Psa in Kiwifruit vines. This review of literature has identified the research gap that is needed to be addressed immediately for the effective control of Psa and the occurrence of Cu resistant Psa in Kiwifruit vines.
Article
Simultaneously with the progress in plant biotechno-logy since the 1980s, new methods in plant pathology have been developed. This review summarizes papers that cover basic research on the effects of selective agents on in vitro cultures of host plants, as well as applications of agents in regeneration systems that result in lines with increased variability in resistance or susceptibility. The first part of the study deals with theoretical aspects of the interactions between plants and toxin-producing pathogens, mode of phytotoxic action, and host-and non-host-selective toxins. The second part lists and describes various agents used for selections in vitro. In the last two decades more than 100 publications focused on these selections for the improvement of resistance to plant pathogens. Over 30 plant species were examined to utilise various selec-tion agents extracted from about 40 plant pathogens. The review covers basic research studies and methods that elucidate the relationships between in vitro and in vivo mechanisms of resistance, but also try to develop practical applications to obtain resistant breeding lines. Such methods often utilise some type of explant cul-tures of the host plants that are treated with various selective agents (culture filtrates, toxins, elicitors), which then elicit typical reactions that parallel those by the pathogens. Their application successfully resul-ted in resistant lines in banana, carnation, grapevine, strawberry and wheat. Nowadays, these techniques are an important complement to classical breeding methods.
Chapter
Peronosporomycetes are “fungi” on both physiological and morphological criteria. They are eukaryotic, uninucleate or coenocytic protoplasts bounded by cell walls in their assimilatory states and are thus obligately osmotrophic heterotrophs (Dick 1997a). Phylogenetic relationship with fungi of the kingdom Eumycota (i.e., Zygomycota, Ascomycota, Basidiomycota) is nonexistent.
Article
Hyaloperonospora arabidopsisdis (Hpa) is a naturally occurring pathogen of the reference plant Arabidopsis thaliana, upon which it causes downy mildew disease. Hpa has been utilized since the late 1980s as a model pathogen of Arabidopsis to elucidate fundamental aspects of plant-oomycete interactions. Hpa is also a model for obligate biotrophic pathogens that cannot exist apart from their hosts, and for downy mildew pathogens that cause destructive diseases of crops. The recently sequenced genome of Hpa isolate Emoy2 has opened the door to deeper understanding of many aspects of plant-oomycete interactions, most notably the first genome-level insights into the molecular basis and evolution of the obligate biotrophy. Additionally, bioinformatic screens of the Hpa genome have identified a large collection of virulence genes (e.g., RxLR effectors). Functional analyses of these genes, along with the experimental tools of Arabidopsis, is already providing new insights into the mechanisms through which oomycetes manipulate host cells. These and other applications of data from the Hpa genome will ultimately translate into new strategies to control downy mildew and other oomycete diseases. © 2014 Springer-Verlag Berlin Heidelberg (outside the USA). All rights reserved.
Chapter
Sorghum is a vital life-sustaining food crop for humans and livestock. Sorghum is also a biofuel crop of growing importance. Moreover sorghum is drought and heat tolerant, and is an important crop in arid and semi-arid regions where major cereals fail to grow successfully. Sorghum thus constitutes a major staple foods for the world’s poorest people. Sorghum is an important food crop within traditional low input, cereal-based farming systems in Africa, where 41 % of the world-wide area of this crop's production is located. However, the crop suffers from low yields due to a number of biotic stresses.
Chapter
Oomycetes were originally classified among the fungi, as Oomycota. It is now known that oomycetes are allied with certain algae and thus they were excluded from true fungi and placed in the kingdom Chromista. Oomycetes produce filamentous hyphae that resemble those of the true fungi, but without cross septa (coenocytic hyphae). The cell wall in oomycetes is composed primarily of ß-glucans and cellulose, while those of true fungi are mostly chitin. Their asexual reproduction is by means of sporangia and biflagellate zoospore, and their sexual spore is an oospore. Oospores are thick-walled, resistant structures capable of surviving under unfavorable environmental conditions. Major oomycete pathogens of cucurbits are Phytophthora capsici, Pseudoperonospora cubensis, and Pythium spp. Phytophthora capsici is the most important Phytophthora species infecting cucurbits and it can infect cucurbit plants at any growth stage causing damping-off, crown rot, foliar blight, and fruit rot. Phytophthora capsici was first described in 1922 in New Mexico, USA. Since then, this pathogen has been reported from many cucurbit-growing areas in the world. It can infect more than 50 plant species in more than 15 plant families. Main hosts of P. capsici are cucurbits and peppers. Phytophthora capsici is a soil-borne pathogen and survives between crops as oospores in soil or mycelium in plant debris. Oospores can survive in the soil, in the absence of a host plant, up to 4 years. Pseudoperonospora cubensis incites downy mildew, a significant disease of cucurbits. Downy mildew of cucurbits was first reported in 1868 in Cuba. Now, this disease occurs in most of the cucurbit-growing areas in the world. Pseudoperonospora cubensis can infect cucurbit plants at all ages. Infection is limited to leaves. Symptoms of downy mildew are small, slightly chlorotic to bright yellow areas on the upper leaf surface and mold (sporangiophores and sporangia) of P. cubensis on the corresponding lower leaf surface. Pseudoperonospora cubensis is an obligate parasite and thus survives only on living host tissues. Host range of P. cubensis is limited to only members of the Cucurbitaceae family. Temperatures of 15-25°C and moist conditions are favorable for downy mildew development. Pythium spp. are soil inhabitants, existing as nonspecialized parasites. Pythium anandrum, P. aphanidermatum, P. irregular, P. myriotylum, oligandrum, P. periplocum, and P. ultimum cause damping-off, root rot, and Pythium cottony leak. Pythium spp. cause infection in a wide range of species in more than 24 pant families. Cooler and moist conditions favor infection by Pythium spp. All of the oomycete pathogens of cucurbits have been reported from cucurbit-growing areas throughout the world. Under favorable environmental conditions, these pathogens can cause up to 100% crop losses. Strategies for effective management of diseases caused by P. capsici, P. cubensis, and Pythium spp. In cucurbits are different from each other, but no single method is adequate for successful control of any of these pathogens. Successful, long-term management of each of the diseases caused by oomycete pathogens in cucurbits requires integrated strategies with measures that eradicate or reduce initial inoculum, reduce effectiveness of primary inoculum, increase the resistance of the plant, delay the onset of disease, and slow spread of disease.
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Potassium phosphite (KPhi) is widely used as a resistance inducer to protect plants against fungal pathogens. In the present study, the effect of KPhi on the activation of defence-related enzymes and biochemicals in Pseudoperonospora cubensis-challenged cucumber plants was investigated. Cucumber plants were treated with KPhi before or after inoculation with P. cubensis and leaf samples were collected at different time courses for physiological and biochemical assessments. Results revealed that the activity of reactive oxygen species (ROS)-scavenging enzymes like catalase, guaiacol peroxidase, superoxide dismutase and ascorbate peroxidase as well as proline and total carbohydrates contents were significantly increased by KPhi application, while hydrogen peroxide (H2O2) concentration, as a disease damage indicator was reduced. The maximum activity of ROS-scavenging system was achieved 3–4 days after KPhi application. These findings suggest that KPhi application prior to pathogen infection efficiently triggers plant defence responses which may reduce the disease severity.
Chapter
The anteriorally uniflagellate Hyphochytriomycota and biflagellate Oomycota are in the Kingdom Straminipila (commonly referred to as stramenopiles) which are part of the SAR superkingdom. Both appear to be basal to the large assemblage of golden-brown algae, the Ochrophyta. Both feature osmotrophic nutrition and have traditionally been considered as zoosporic “fungi,” but are unrelated to organisms in the monophyletic kingdom Mycota. The Hyphochytriomycota is a small group encompassing around half a dozen genera, which have simple nonmycelial, holocarpic thalli, traditionally encompassing three families: the endobiotic Anisolpidiaceae, the polycentric Hyphochytriaceae, and the monocentric Rhizidiomycetaceae. Recently the former have been shown to be placed among the early diverging Oomycota, leaving just the latter two families in the monophyletic Hyphochytriomycota clade. Hyphochytriomycota are widespread in occurrence, and most are saprotrophs or parasites, infecting the resting spores of Oomycota and Glomeromycota. In contrast, the Oomycota are a large and diverse assemblage, consisting of two major (class level) clades, the Saprolegniomycetes and Peronosporomycetes, and several early diverging classes most of which are simple holocarpic organisms that lack mycelial organisation. Many of these early-diverging clades are as yet poorly resolved because of sparse taxon sampling. The early-diverging orders include the Eurychasmales and Olpidiopsidales, both of which are marine seaweed parasites, the nematode infecting Haptoglossales and crustacean infecting Haliphthorales. The Saprolegniomycetes mostly have fungal-like mycelial thalli and include the orders Atkinsiellales s.lat., Leptomitales, and Saprolegniales, which are mostly saprophytes or parasites of invertebrates and, occasionally, vertebrates such as fish and amphibians. A few species in the Saprolegniales are root infecting parasites of plants. The Peronosporomycetes are the second major fungal-like class, and include the largely saprotrophic Rhipidiales, the facultively parasitic Pythiales s.lat., which can infect both animals and plants and the predominantly plant pathogenic Albuginales and Peronosporales sensu lato. Indeed, the Oomycota are significant parasites of both animals and plants, impacting both natural ecosystems and causing significant economic losses in both aquacultural and agricultural systems. The molecular systematics of the Oomycota is still in a state of flux, and in this account a relatively conservative approach has been taken. It is apparent that most of the early-diverging genera are almost exclusively marine and that the Peronosporales represents the main terrestrial and plant pathogenic lineage. Most early-diverging genera lack the oogamous sexual reproduction that characterizes this group and suggests that the oogenesis evolved around the time of emmergence from the sea to the land and freshwater ecosystems. It is also clear that obligate biotrophy in the white blister rusts (Albuginales) and downy mildews (Peronosporales s.str.) has evolved independently.
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The effect of potassium phosphite (KPhi) on the photosynthetic parameters of cucumber plants inoculated with Pseudoperonospora cubensis was investigated in the present study. Cucumber plants were treated with KPhi before or after inoculation with P. cubensis and leaf samples were collected at different time courses for assessments. Results showed that in pathogen-inoculated plants Fv/Fm was decreased up to 3%. The rate of quantum photosynthetic was also decreased significantly in inoculated plants. Downy mildew led to a decrease in chlorophyll amount which in turn reduced the efficiency of photosystem II. In the KPhi-treated leaves, chlorophyll a and b decreased by 72% and 68%, respectively. Remarkable reduction in the efficiency of photosystem II as well as increased lipid membrane disruption, led to increased lipid peroxidation rate of the membranes up to 52%. The results of this study indicate the mitigating role of potassium phosphite in reducing the adverse effects of pathogen and maintaining the photosynthetic apparatus efficiency in cucumber plants.
Article
Studies were undertaken on the effects of temperature (14/10 °C and 22/17 °C day/night) and plant age (15, 23, 31 and 40 day-old-plants) on the severity of downy mildew (Hyaloperonospora parasitica) on oilseed Brassica cultivars (temperature: Brassica juncea Montara, B. napus Atomic, ATR-Hyden, Hyola 432, Hyola 450 TT, Thunder TT; plant age: B. juncea Dune, B. napus Surpass 402 and Hyola 450 TT). For temperature studies, there were significant (P < 0.001) effects of temperature, cultivar, and cultivar x temperature interaction. On cotyledons of susceptible cultivars (B. napus Hyola 450 TT and Thunder TT), plants were symptomatic at 22/17 °C by 48 h post inoculation (hpi) and with abundant sporulation evident by 72 hpi, and with all cotyledons of B. napus Thunder TT collapsed by 7 days post inoculation (dpi). However, at 14/10 °C, there were no symptoms on the same cultivars until 5 dpi, and sporulation only observed at 7 dpi. Percent disease index values (DI%) at 22/17 °C of B. juncea Montara and B. napus ATR-Hyden, Hyola 432, Atomic, Hyola 450 TT and Thunder TT were 4.5, 49.0, 51.4, 65.8, 86.3 and 96.0, respectively, with all except B. juncea Montara having significantly lower (P < 0.001) disease at 14/10 °C with DI% values of 2.8, 30.4, 27.9, 31.1, 44.4 and 76.4, respectively. For plant age studies, there were significant (P < 0.001) effects of plant age, cultivar, and cultivar x plant age interaction. DI% was significantly higher at 15 compared to 40 day-old-plants (dop) across all cultivars. B. juncea Dune showed greatest resistance, particularly on 40 dop, with DI% values of 25.8, 24.6, 22.9 and 7.5, for 15, 23, 31 and 40 dop, respectively. B. napus Surpass 402 showed high susceptibility on cotyledons of 15 dop but moderate resistance on leaves of other ages, with DI% values of 59.0, 31.2, 27.1 and 26.2 for 15, 23, 31 and 40 dop, respectively. B. napus Hyola 450 TT showed very high susceptibility at the cotyledon stage on 15 dop, but some resistance on 23 dop and more so on 31 and 40 dop, with DI% values of 84.0, 41.2, 35.4 and 32.9 for 15, 23, 31 and 40 dop, respectively. Together, these findings explain for the first time why development of downy mildew epidemics on susceptible cultivars occurs early in the growing season when warmer seasonal temperatures in autumn coincide with presence of seedlings; in contrast to later in the growing season on less susceptible older plants coinciding with cooler and less favourable winter temperatures. Increasing maximum and minimum temperatures associated with climate change have likely fostered the increased severity of downy mildew over the past 15 years.
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A total of 235 Phytophthora infestans isolates were collected from five regions of the Czech Republic during the growing seasons 2012–2014 and 2016 and examined using the in vitro amended agar method for their sensitivity to metalaxyl-M (MFX), propamocarb-HCl (PCH), and dimethomorph (DMM). A majority of the isolates (50%) were sensitive to MFX. Resistant isolates were found in all four years of the survey; they represented 30% of the samples. The EC50 values of PCH in inhibiting mycelial growth of 65% of the overall isolates were higher than 100 µg ml–1, which indicates the occurrence of insensitivity to PCH in the Czech P. infestans populations. DMM was very effective, and the mycelial growth of all isolates tested was completely suppressed at the concentration of 0.1 µg ml–1. Furthermore, the efficacy of 12 plant essential oils was tested against 20 isolates of P. infestans using the in vitro amended agar method. Essential oils of Cymbopogon winterianus, Litsea cubeba, Mentha spicata, Pelargonium graveolens, Syzygium aromaticum, and Thymus vulgaris were observed to have the highest antifungal activity against P. infestans, with minimal inhibitory concentrations less than or equal to 1 µl ml–1.
Article
Some 154 Brassicaceae genotypes (78 Brassica napus, 38 B. carinata, 25 B. juncea, three Raphanus sativus, two each of Rapistrum rugosum and B. incana and one each of Crambe abyssinica, B. fruticulosa, Hirschfeldia incana, B. insularis, B. oleracea and Sinapis arvensis), were inoculated with a mixture of seven isolates of Hyaloperonospora brassicae to identify effective host resistances. Many highly resistant genotypes were identified, particularly R. sativus Krasnodar Market B (%Disease index 6.6) and Pegletta (%DI 9.0); B. carinata Tamn Rex-sel Green (%DI 7.6), BRA926/18 (%DI 9.7) and PI360884 (%DI 9.7); and B. juncea, Ringot1 (%DI 9.7). A further 13 B. carinata, seven B. juncea and single R. sativus (Boss) and B. incana (UPM6563) genotypes were also highly resistant (%DI 11.1), as were B. oleracea CPI106844 (%DI 14.6) and Crambe abyssinica (%DI 17.4). Almost all B. carinata and B. juncea genotypes showed high resistance (%DI 7.6–22.2). In contrast, B. napus genotypes showed wide ranging responses, from high resistance in SN-8 (%DI 22.2%) to extreme susceptibility in Hyola 450TT and Thunder TT (%DI 83.7, 95.5, respectively). R. rugosum, B. fruticulosa, H. incana and B. insularis genotypes ranged from moderately to highly susceptible (%DI 55.2–78.8). This study highlights the ready availability of very high levels of pathotype-independent resistance across diverse Brassicaceae to H. brassicae, particularly R. sativus, B. carinata, B. juncea, B. oleracea and C. abyssinica. Resistances identified can be utilized as sources of resistance in oilseed and vegetable Brassicaceae breeding programs and/or directly deployed as new varieties where downy mildew is prevalent.
Chapter
The mode of action of carboxylic acid amide (CAA) fungicides has in the past been associated with an inhibition of phospholipid biosynthesis, but it has now been confirmed to be an interference with cell wall deposition and cellulose biosynthesis. Pyrimorph is an oomycete‐selective fungicide, because it is described to be active against Phytophthora infestans, Phytophthora capsici, and Pseudoperonospora cubensis. Dimethomorph has a good preventive activity, as well as pronounced curative and antisporulant activities. Following a soil drench or foliar application, iprovalicarb is reported to be more systemic than both dimethomorph and mandipropamid. The results of cytological studies have implied that dimethomorph, iprovalicarb, and benthiavalicarb inhibit processes involved in cell‐wall biosynthesis and assembly. Although cross‐resistance was identified among all CAAs for the vast majority of resistant isolates, as might have been expected, no cross‐resistance was found between CAAs and other modes of action, such as phenylamides, quinone outside inhibitor (QoI) fungicides, and zoxamide.
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The control of diseases caused by plant pathogens of the Oomycetes has been a major target since the beginning of modern chemical crop protection. To date, metalaxyl‐M, also called mefenoxam, is an indispensable product in the control of plant pathogenic Peronosporales and Pythiales. The origin of the phenylamide fungicides is linked to the cross‐indication screening of analogs belonging to the chloroacetanilide herbicide class, such as metolachlor. The phenylamide fungicides, including metalaxyl and metalaxyl‐M, inhibit ribosomal RNA synthesis, specifically RNA polymerization (polymerases). The phenylamide fungicides affect especially hyphal growth and the formation of haustoria and spores in Oomycetes. Although metalaxyl, metalaxyl‐M, oxadixyl, benalaxyl, and ofurace exhibit different levels of intrinsic activity and rRNA polymerase inhibition, cross‐resistance was observed between all phenylamide fungicides. The metabolic pathways of metalaxyl‐M in soil, plants, and animals are very similar to metalaxyl. The main metabolite of metalaxyl‐M in soil is, like for metalaxyl, the corresponding acid of metalaxyl‐M.
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Thirty-six novel threoninamide carbamate derivatives were designed and synthesised using active fragment-based pharmacophore model. Antifungal activities of these compounds were tested against Oomycete fungi Phytophthora capsici in vitro and in vivo. Interestingly, compound I-1, I-2, I-3, I-6 and I-7 exhibited moderate control effect (>50%) against Pseudoperonospora cubensis in greenhouse at 6.25 μg/mL, which is better than that of control. Meanwhile most of these compounds exhibited significant inhibitory against P. capsici. The other nine fungi were also tested. More importantly, some compounds exhibited remarkably high activities against Sclerotinia sclerotiorum, P. piricola and R. solan in vitro with EC50 values of 3.74–9.76 μg/mL. It is possible that the model is reliabile and this method can be used to discover lead compounds for the development of fungicides.
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