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Susceptibility of cover crop plants to Fusarium virguliforme , causal agent of soybean sudden death syndrome, and Heterodera glycines , the soybean cyst nematode
Abstract
Greenhouse studies were conducted to evaluate the susceptibility of cover crop species to infection by Fusarium virguliforme (Fv), a soilborne fungus that causes sudden death syndrome (SDS) of soybean (Glycine max [L.] Merr.), and the soybean cyst nematode (SCN; Heterodera glycines), both important soybean pathogens. In the SDS experiments, cover crops were planted in Fv-infested soil, and plants were assessed for fresh biomass, root rot severity, foliar symptoms, and amount of Fv DNA in roots. Inoculated alfalfa (Medicago sativa L.), crimson clover (Trifolium incarnatum L.), red clover (Trifolium pratense L.), and pea (Pisum sativum L.) had more root necrosis than the noninoculated controls, and Fv DNA quantities in roots did not differ from those found in soybean roots. Inoculated alfalfa, corn (Zea mays L.), crimson clover, oat (Avena sativa L.), red clover, sorghum (Sorghum bicolor L.), and turnip (Brassica rapa L.) plants had lower biomass compared to noninoculated controls, although corn, oat, and turnip had no root necrosis. Biomass reduction and root necrosis were not observed in inoculated hairy vetch (Vicia villosa Roth), false flax (Camelina sativa [L.] Crantz), millet (Pennisetum glaucum [L.] R.Br.), mustard (Brassica juncea L.), rye (Secale cereale L.), ryegrass (Lolium multiflorum Lam.), triticale (Triticale hexaploide Lart.), and wheat (Triticum aestivum L.), and Fv DNA quantity in the roots of these species was lower than in soybean. These results suggest that the legume species tested are hosts of Fv whereas grasses and Brassicas spp. are nonhosts or poor hosts. In the SCN experiment, select leguminous and nonleguminous cover crop plants were grown in soil naturally infested with SCN, and the number of females formed per root after 30 days was determined. There were very few (zero to five) SCN females on the roots of multiple varieties of leguminous cover crop species studied. No females were recovered from the roots of any of the nonleguminous species studied, except for a single female on four plants from three different species. None of the cover crop plants studied were susceptible hosts for SCN. With the increasing interest in using cover crops as a soil conservation practice in corn-soybean production systems, it is important to understand how this practice would impact major soybean diseases. Knowing the impact that cover crops may have on SDS and SCN is important to help farmers make better decisions when planting cover crops in areas with history of these diseases. © 2017 Soil and Water Conservation Society. All rights reserved.
... Recently, two field populations of SCN were tested for reproduction in brown mustard cv. Kodiak and other cover crops in greenhouse conditions in Iowa, and both SCN populations were not able to reproduce or had very limited reproduction on those crops (Kobayashi-Leonel et al., 2017). However, the effect of those crops on SCN population reduction has not been reported in the northern Great Plains. ...
... Similar to the present study, brown mustard cv. Kodiak was reported to be a non-host of SCN in Iowa (Kobayashi-Leonel et al., 2017). Additionally, Wen et al. (2017) tested a different brown mustard cv. ...
Industrial oilseeds have a great potential in the northern Great Plains both as oilseeds and as cover crops sown following wheat (Triticum aestivum L.) harvest and before soybean [Glycine max (L.) Merr.] sowing in the following spring. One of the most important biotic stresses in soybean production is soybean cyst nematode (Heterodera glycines Ichinohe, SCN), a serious pest that affects 90% of the soybean producing areas in the U.S. The objective of this study was to evaluate the host status of and the SCN population reduction by, winter camelina [Camelina sativa (L.) Crantz, cv. Joelle], crambe (Crambe abyssinica Hochst. Ex R.E.Fr., cv. BelAnn), and brown mustard (Brassica juncea L. cv. Kodiak). The experiments were performed in a growth chamber at 27°C for 35 days by planting the crops in soil naturally infested with SCN and autoclaved sandy soil artificially inoculated with two SCN populations from two fields in North Dakota. Soybean cyst nematode did not reproduce on brown mustard or camelina with a female index (FI) of 0, suggesting these are non-hosts, while it reproduced on crambe. The numbers of white females on crambe ranged from 1 to 13 per plant with FI of 0.2 to 1.1 in naturally infested soils, and 1 to 4 per plant with FI of 1.2 to 2.5 in artificially infested soils, thus crambe would be classified as a poor-host (FI < 10). Brown mustard and winter camelina reduced the SCN populations by an average of 51% and 48%, respectively, while crambe only reduced the populations by an average of 24%, across all the experiments with naturally infested soils when compared with the initial population levels. Both brown mustard and camelina consistently reduced the SCN populations but crambe did not steadily reduce the SCN populations when compared with the non-planted control (fallow). Further understanding the effects of these crops on SCN populations under natural field conditions is needed to determine if cover crops can be used for sustainable SCN management in SCN-infested soybean fields.
... However, controlling diseases like S. sclerotiorum is challenging due to their wide host range. Caution is needed when selecting cover crops to avoid hosting soilborne diseases (Kobayashi-Leonel et al. 2017). Long-term field evaluations are required to measure the residual impacts of cover crops under different conditions (Nordblom et al. 2023). ...
A healthy soil is considered a system that involves many interacting components like substrate, microbiome, and environmental factors such as temperature, pH, humidity, and in some cases cultivated crops. All these components are continually interacting with each other, and striving towards equilibrium. Depending on the composition of soil as a system, it may be stable with a certain level of equilibrium that could lead to the production of quality yields under environmentally friendly conditions. Conversely, a defective soil system can be a source of different types of soilborne diseases that imply a reduction in crop performance, decreased yield, higher production costs, and environmental insecurity. Before the implementation of the Montreal Protocol, methyl bromide was extensively used to control soilborne pathogens; however, the high cost of chemical fungicides, the appearance of fungicide-resistant strains, the support of (micro and micro) environmental change, new disease outbreaks, and increasing concerns about chemical side effects on the environment as well as soil health are increasingly evident. Therefore, the use of integrated soilborne disease regulation strategies for crop production such as tillage, crop rotations, and biological control is necessary. This approach seeks to optimize disease management while preserving soil health and ecosystem integrity. With a focus on the Global South, this chapter summarizes existing methods for the regulation of soilborne diseases from a systems perspective to enhance crop production through soil sanitation, disinfestation, steam sterilization, solarization, and cultivation of disease-resistant cultivars, as well as the use of biofumigants, soil amendments, soilless cultures, and biochemical controls. Also, the different methods with their strengths and weaknesses, modes of action, and interactions are discussed in this chapter. In addition, future perspectives that might lead to the implementation of soilborne disease regulatory and management strategies from a system-based approach are highlighted.
... Still, they may also introduce associated problems, such as adding additional weed seed potential into the soil in subsequent arable crops through seeding these species. Furthermore, the potential that incorporation of more susceptible host plants into cropping systems results in increased propagation of soilborne pathogens like Plasmodiophora brassicae, Fusarium virguliforme, or Heterodera glycines, which could lead to disadvantages in future crop rotations, cannot be ruled out(Kobayashi-Leonel et al. 2017;Zamani-Noor et al. 2022). ...
Cover crops and flower strips are used in agricultural fields as part of integrated pest management strategies. However, their potential as secondary hosts of soilborne pathogens such as Sclerotinia sclerotiorum in oilseed rape cultivation is not fully comprehended. In the current study, we evaluated the effect of pathogen virulence on the development of Sclerotinia stem/leaf rot and sclerotia production in 33 plant species from 11 botanical families using two S. sclerotiorum isolates. Furthermore, the effect of sclerotial size on carpogenic germination was studied. Results showed that the pathogen's virulence significantly affected the occurrence and development of Sclerotinia stem/leaf rot and the subsequent production of sclerotia. Among all plant species tested, 26 were more susceptible to the aggressive S. sclerotiorum isolate, which produced more and bigger sclerotia in 17 species than the less aggressive isolate. Moreover, a stronger positive correlation was found between the relative lesion length of plants inoculated with the aggressive isolate and the number of sclerotia produced by this isolate (rs = 0.572; p = 0.004). Additionally, we found that larger and heavier sclerotia produced stipes and apothecia earlier and at a greater rate than smaller ones. The sclerotia in the heavy-weight class had the highest carpogenic germination rate (82.4%), followed by the average (67.2%) and lightweight classes (59.5%). Our findings highlight the need for further investigation into the potential risks associated with cover crops, weeds, and flower strips as secondary hosts of soilborne pathogens in agricultural fields.
... Rotation as a management tool is challenging since 2-year corn-soybean rotation systems have been shown to increase F. virguliforme abundance in soil (Roy et al. 1997;Rupe et al. 1999;van Eck 1978). Longer, more diverse crop rotations have been found to reduce soil inoculum levels, SDS incidence, and symptom severity, but more research is needed (Kobayashi-Leonel et al. 2017;Leandro et al. 2018). Because F. virguliforme can infect and colonize a broad variety of common field crop and weed species, soil inoculum levels can be maintained or increased in the absence of soybean, as has been reported regarding other phytopathogenic Fusarium species (Baetsen-Young et al. 2021;Blok and Bollen 1996;Hennessy et al. 2005;Henry et al. 2019;Kolander et al. 2012;MacDonald and Leach 1976;Navi and Yang 2016;Smith and Snyder 1975). ...
Sudden death syndrome (SDS), caused by Fusarium virguliforme, is an important yield-limiting disease of soybean (Glycine max). From 1996 to 2022, cumulative yield losses attributed to SDS in North America totaled over 25 million metric tons, valued at over $7.8 billion USD. Seed treatments are widely used to manage SDS by reducing early season soybean root infection by F. virguliforme. Fluopyram (SDHI - FRAC 7), a fungicide seed treatment for SDS management, has been registered for use on soybean in the U.S. since 2014. A baseline sensitivity study conducted in 2014 evaluated 130 F. virguliforme isolates collected from five U.S. states to fluopyram in a mycelial growth inhibition assay and reported a mean EC 50 of 3.35 mg/L. This baseline study provided the foundation for the objectives of this research: to detect any statistically significant change in fluopyram sensitivity over time and geographical regions within the U.S. and to investigate sensitivity to the fungicide pydiflumetofen. We repeated fluopyram sensitivity testing on a panel of 80 historical F. virguliforme isolates collected from 2006-2013 (76 of which were used in the baseline study) and conducted testing on 123 contemporary isolates collected from 2016-2022 from eleven U.S. states. This study estimated a mean absolute EC 50 of 3.95 mg/L in isolates collected from 2006-2013 and a mean absolute EC 50 of 4.19 mg/L in those collected in 2016-2022. There was no significant change in fluopyram sensitivity (P = 0.1) identified between the historical and contemporary isolates. A subset of 23 isolates, tested against pydiflumetofen under the same conditions, estimated an mean absolute EC 50 of 0.11 mg/L. Moderate correlation was detected between fluopyram and pydiflumetofen sensitivity estimates (R = 0.53, P < 0.001). These findings enable future fluopyram and pydiflumetofen resistance monitoring and inform current soybean SDS management strategies in a regional and national context.
... Large numbers of lesion nematodes in the roots of grain sorghum raise the possibility that these pests are responsible for yield losses in speci c environments. Grain sorghum is neither a host for the reniform nematode (Rotylenchulus reniformis) nor the soybean cyst nematode (Heterodera glycines) (Kobayashi-Leonel et al., 2017). However, grain sorghum is prone to M. incognita, even though hybrids differ in their ability to maintain root-knot populations. ...
Resistance and cultural control strategies are usually sufåcient for managing sorghum diseases.
The majority of the issues associated with sorghum diseases can be prevented by taking one or
more of the following practical measures:
• It is recommended to use locally adapted, disease-resistant germplasm if it is available.
• Grow healthy plants from fungicide-treated (high-quality) seed.
• Grow plants in well-drained, fertile (neutral to alkaline) soils with a pH of 6.0 to 6.5.
• Sow seeds when the soil is at least 65°F (averaged daily).
• To avoid damage from water, it is important to drain åelds properly.
• Eliminate grassy weeds (especially Johnsongrass and related Sorghum spp.) that could
serve as reservoir hosts near production åelds.
• Plant hybrids with improved stalk strength and other structural properties.
• Weed, insect, and plant population control can help reduce plant stress. Plant populations
that are too dense in dryland production systems are often best avoided.
• To prevent the spread of soil-borne plant pathogens, sorghum should be rotated with
noncereal crops regularly.
Sorghum disease can be managed somewhat through the aforementioned general practices,
but difåculties persist. Knowledge of etiological agents and their interaction with the sorghum host and the environment is still lacking in many cases, including viral diseases, bacterial leaf diseases, nematode pathogens, and many minor fungal foliar diseases. It is possible
to learn quite a bit about the life cycle of certain pathogens, such as anthracnose, ergot, and
grain mold.
Most agricultural regions will experience signiåcant global climate change in the coming century, with warmer and drier conditions expected. The frequency or severity of plant pathogen–
caused epidemics may change due to these shifting environmental conditions. However, the
directionality of these alterations may differ depending on the pathosystem and agricultural
ecosystem, and this will be determined as more empirical evidence is collected. Due to the
ever-changing climate brought on by global warming, germplasm must be constantly evaluated
to pinpoint the origins of resistance to both major and minor diseases and their integration with
abiotic stress tolerance.
... However, while camalexin production was induced by S. sclerotiorum inoculation, the relative degree of disease resistance was not correlated with levels of camalexin production, suggesting that other antimicrobial activities might be responsible for the observed disease resistance [142]. C. sativa also shows resistance to the soil-born fungus Fusarium virguliforme [144]. However, C. sativa is susceptible to other fungal diseases common to the Brassicaceae, such as damping-off (caused by Rhizoctonia solani), clubroot (Plasmodiophora brassicae Woronin.), and white rust (Albugo candida Pers. ...
Camelina sativa (L.) Crantz. is an annual oilseed crop within the Brassicaceae family. C. sativa has been grown since as early as 4000 BCE. In recent years, C. sativa received increased attention as a climate-resilient oilseed, seed meal, and biofuel (biodiesel and renewable or green diesel) crop. This renewed interest is reflected in the rapid rise in the number of peer-reviewed publications (>2300) containing “camelina” from 1997 to 2021. An overview of the origins of this ancient crop and its genetic diversity and its yield potential under hot and dry growing conditions is provided. The major biotic barriers that limit C. sativa production are summarized, including weed control, insect pests, and fungal, bacterial, and viral pathogens. Ecosystem services provided by C. sativa are also discussed. The profiles of seed oil and fatty acid composition and the many uses of seed meal and oil are discussed, including food, fodder, fuel, industrial, and medical benefits. Lastly, we outline strategies for improving this important and versatile crop to enhance its production globally in the face of a rapidly changing climate using molecular breeding, rhizosphere microbiota, genetic engineering, and genome editing approaches.
... However, while several species of legumes used as cover crops, such as Trifolium spp, Medicago sativa, and Pisum sativum, are considered hosts of F.v., other legumes, grasses, cereals, and Brassicaceae cover crops, such as Vicia villosa, Camelina sativa, Brassica juncea, Pennisetum glaucum, Secale cereale, Lolium multiflorum, Triticale hexaploide, and Triticum aestivum, are nonhosts or poor hosts. 85 Therefore, knowledge of the susceptibility of cover crops to SDS-causing Fusarium species can help farmers choose which species should be planted in field plots with a history of SDS epidemics. Similarly, recent research reported promising results when green manure amendments were tested for SDS suppression in experimental plots in the greenhouse. ...
The increase in food production requires reduction of the damage caused by plant pathogens, minimizing the environmental impact of management practices. Soil‐borne pathogens are among the most relevant pathogens that affect soybean crop yield. Soybean sudden death syndrome (SDS), caused by several distinct species of Fusarium, produces significant yield losses in the leading soybean‐producing countries in North and South America. Current management strategies for SDS are scarce since there are no highly resistant cultivars and only a few fungicide seed treatments are available. Because of this, innovative approaches for SDS management need to be developed. Here, we summarize recently explored strategies based on plant nutrition, biological control, priming of plant defenses, host‐induced gene silencing, and the development of new SDS‐resistance cultivars using precision breeding techniques. Finally, sustainable management of SDS should also consider cultural control practices with minimal environmental impact. © 2021 Society of Chemical Industry.
... In fact, higher levels of F. virguliforme DNA were detected in soil from the 2-year system compared with the 4-year system (Leandro et al., 2018). In a different study, the susceptibility of 15 cover crop species belonging to 13 different genera together with two soybean cultivars and one maize hybrid planted in F. virguliforme-infested soil was evaluated (Kobayashi-Leonel et al., 2017). Four inoculated cover crops (alfalfa, crimson clover, red clover, and pea) had more root symptoms than the uninoculated control, seven inoculated crops (alfalfa, maize, crimson clover, oat, red clover, sorghum, and turnip) had a lower biomass than the respective Canada (Ontario) showed no correlation between early planting date and SDS severity (Kandel et al., 2016b). ...
Soybean sudden death syndrome (SDS) is a fungal disease caused by members of clade 2 of the Fusarium solani species complex (FSSC). These fungi are soilborne pathogens that infect soybean plants through the roots and produce toxins that translocate to aerial parts of the plant, inducing foliar chlorosis and necrosis followed by premature defoliation. Here, we first give the current state of knowledge of early pathogen detection and infection establishment for the SDS pathosystem. Subsequently, we discuss the nature and activity of secreted toxins, followed by an overview of changes in plant metabolism and factors that influence fungus–soybean interaction. Finally, we summarize the advances in plant disease resistance, symptom evaluation, and treatment. The current state of knowledge of soybean sudden death syndrome‐causing Fusarium species pathogenesis, fungi–soybean interaction and plant disease resistance, symptom evaluation, and treatment is reviewed.
... A study by Kobayashi-Leonel et al. (2017) showed that alfalfa, Austrian winter pea, berseem clover (Trifolium alexandrinum L.), cowpea, crimson clover, field pea, hairy vetch (two different cultivars), red clover (three different cultivars), and white clover (two different cultivars) supported SCN reproduction at very low level, suggesting poor hosts. Our results support their findings that Austrian winter pea, crimson clover, field pea (Aragorn and Cooper), and hairy vetch were poor host/suitable hosts but alfalfa, cowpea, and berseem clover were non-hosts for both SCN populations in our study. ...
Soybean cyst nematode (SCN; Heterodera glycines) reproduces on a wide range of plants, including some cover crops. However, reproduction of SCN populations on a number of cover crops has not been investigated in the northern Great Plains. Thirty-five cover crop species/cultivars from four plant families were evaluated as hosts for SCN. Greenhouse evaluations were done with two common SCN populations, SCN103 (HG type 0) and SCN2W (HG type 7) under controlled conditions. The sources of two SCN populations were two soybean fields of North Dakota. After 35 days of growth, white SCN females were extracted from individual plants and counted to determine a female index (FI = average number of females on a tested crop/average number of females in a susceptible check x 100) for each crop. Out of the 35 cover crop species/cultivars tested, at least one of the SCN populations reproduced on seven crops/cultivars but did not reproduce on the other 28 crops/cultivars. Out of these seven crops, only white lupine (Lupinus albus L.) was a suitable host (FI ≥ 10) for both SCN populations in all the experiments, while others showed varied responses from poor host to suitable host for the SCN populations. The host crops were from the family Brassicaceae or Fabaceae, while all the crops in the Linaceae or Poaceae family were non-hosts. The non-host crops can be planted in SCN-infested fields without the concern of increasing SCN populations, while poor hosts with low female index should be evaluated for effects on reduction of SCN numbers in the fields before they are used as cover crops in a soybean cropping system.
... By contrast, Kobayashi-Leonel and her colleagues reported that the cover crops like alfalfa, red clover and pea can harbor soilborne pathogen like F. virguiliforme (soybean sudden death). Thus, careful selection of crops for rotation is recommended [55]. Moreover, Hiddink and his collegues reported the null effect of mixed cropping of soil with brussels sprouts and barley or with triticale and white clover [56]. ...
The significant problems caused by soilborne pathogens in crop production worldwide include reduced crop performance, decreased yield, and higher production costs. In many parts of the world, methyl bromide was extensively used to control these pathogens before the implementation of the Montreal Protocol—a global agreement to protect the ozone layer. The threats of soilborne disease epidemics in crop production, high cost of chemical fungicides and development of fungicide resistance, climate change, new disease outbreaks and increasing concerns regarding environmental as well as soil health are becoming increasingly evident. These necessitate the use of integrated soilborne disease management strategies for crop production. This article summarizes methods for management of soilborne diseases in crop production which includes the use of sanitation, legal methods, resistant cultivars/varieties and grafting, cropping system, soil solarization, biofumigants, soil amendments, anaerobic soil disinfestation, soil steam sterilization, soil fertility and plant nutrients, soilless culture, chemical control and biological control in a system-based approach. Different methods with their strengths and weaknesses, mode of action and interactions are discussed, concluding with a brief outline of future directions which might lead to the integration of described methods in a system-based approach for more effective management of soilborne diseases.
Soybean cyst nematode (SCN; Heterodera glycines ) is a major yield-reducing pathogen of soybean worldwide. Microplot experiments were conducted to evaluate ten cover crops for their effects on two SCN populations (SCN103 and SCN2W) collected from North Dakota, USA, soybean fields in 2019 and 2020. Experiments were conducted in a randomised complete block design using naturally infested field soil. A susceptible soybean ‘Barnes’ and non-planted natural soil (fallow) were used as controls. Plants were grown in outdoor conditions for 75 days before soil samples were collected. SCN eggs and juveniles were extracted from the soil samples to determine final population, population reduction and suppression. Soybean had significantly greater final population densities than all the cover crops and fallow. All cover crops and fallow reduced the initial densities of both SCN populations. All the cover crops, except chickling vetch ‘Pulse’, had lower final population densities than fallow and suppressed the SCN populations throughout the experiments. Sunnhemp (cultivar not stated; CNS), oilseed radish ‘Concorde’ and ‘Control’, and daikon radish ‘Eco-Till’ significantly reduced the SCN103 population compared to fallow. Sunnhemp, alfalfa ‘Bullseye’, and daikon radish had significant population reductions of SCN2W than fallow. Sunnhemp was found to have the greatest reductions in SCN populations ranging from 55 to 86% compared to the initial densities. This study demonstrated cover crop species/cultivars with the ability to reduce SCN populations in outdoor conditions, and the findings indicate that they could be utilised in infested fields to manage SCN.
Greenhouse experiments were conducted to determine if cover crops directly decrease population densities of the soybean cyst nematode (SCN), Heterodera glycines, and/or have residual effects on reproduction of the nematode on soybean (Glycine max). Population densities of SCN were not significantly decreased by nine cover crop plants or three cover crop mixes compared to a non-planted soil control in a repeated 60-day-long greenhouse experiment. When susceptible soybeans were grown in the soils after cover crop growth, fewer SCN females formed following three annual ryegrass (Lolium multiflorum) cultivars (Bounty, King, and RootMax), the Daikon radish (Raphanus sativus var. longipinnatus) cultivar CCS779, Kodiak mustard (Brassica juncea), and a mix containing cereal rye, crimson clover (Trifolium incarnatum), plus Daikon radish (cultivars not stated) compared to following the non-planted control. In another repeated experiment, cover crops were grown for 56 days in SCN-infested soil in the greenhouse then exposed to Iowa winter conditions for 28 days to simulate winter termination of the plants. One treatment, a cover crop mix containing Bounty annual ryegrass plus Enricher Daikon radish, had a decrease in SCN population density greater than the non-planted control at the end of the experiment. Significantly fewer SCN females formed on soybeans grown following several cover crops, including the three annual ryegrass cultivars that had the suppressive residual effects in the first experiment. In summary, there were no cover crop treatments that consistently decreased SCN population densities across experiments, and only one cover crop treatment in one experiment significantly reduced SCN population densities more than a non-planted soil control. However, there was a somewhat consistent, adverse, residual effect of cover crops on reproduction of SCN on susceptible soybeans following growth of multiple cover crops.
The asymptomatic host range of Fusarium virguliforme includes corn, a common crop rotated with soybean that we hypothesize may alter F. virguliforme population dynamics and disease management. A field-based approach explored the temporal dynamics of F. virguliforme colonization of corn and soybean roots under different tillage and residue managements. Experiments were conducted in IA, IN, MI, WI, and Ontario, Canada from 2016 to 2018. Corn and soybean roots were sampled at consecutive time points between 1 and 16 weeks after planting (WAP). DNA was extracted from all roots and analyzed by real-time qPCR for F. virguliforme quantification. Trials were rotated between corn and soybean, containing a two x two factorial of tillage (no-tilled or tilled) and corn residue (with or without) in several experimental designs. In 2016, low (ca. 100 fg/10 mg root tissue) F. virguliforme was detected in the inoculated IA, IN and MI locations, and non-inoculated WI corn fields. However, in 2017 greater levels of F. virguliforme DNA were detected in IA, IN and MI across sampling time points. Tillage practices showed inconsistent effects on F. virguliforme root colonization and SDS foliar symptoms among trials and locations. Yet, residue management did not alter root colonization of corn or soybean by F. virguliforme. Plots with corn residue had greater SDS foliar disease index in Iowa in 2016. However, this trend was not observed across the site-years, indicating corn residue may occasionally increase SDS foliar symptoms depending on the disease level, soil and weather factors.
The effects of cover crops on the biology of the soybean cyst nematode (SCN), Heterodera glycines, are not well established. It is possible that cover crops may reduce SCN population densities by acting as trap crops. Cover crops with potential to serve as trap crops may stimulate hatching and/or attract hatched SCN juveniles and also may be penetrated by large numbers of nematodes that cannot feed. Experiments were conducted to determine if root exudates and soil leachates (RE+SL) from various cover crop plants affected SCN hatching and chemotaxis and if there were significant differences in SCN juvenile root penetration among different cover crop plant types. In fourteen-day-long hatching experiments, there was greater SCN hatching in crimson clover (Trifolium incarnatum) RE+SL than in RE+SL from all other cover crop treatments in the experiments. No other cover crop RE+SL significantly affected hatching. In chemotaxis experiments, SCN juveniles were attracted to RE+SL from annual ryegrass (Lolium multiflorum) and cereal rye (Secale cereal) after 24 hours. In greenhouse experiments, significantly more SCN juveniles penetrated the roots of single cultivars of crimson clover, mustard (Brassica juncea), and rapeseed (Brassica napus) than 11 other cover crop species/cultivars evaluated in the experiment over the course of 20 days. Few SCN juveniles penetrated the roots of annual ryegrass and cereal rye. The results suggest crimson clover, grown as a cover crop, has the most potential to act as a trap crop for SCN. Cover crop plants may affect SCN biology in ways other than the mechanisms investigated in these experiments.
We exploited the broad host range of Fusarium virguliforme to identify differential fungal responses leading to either an endophytic or a pathogenic lifestyle during colonization of corn (Zea mays) and soybean (Glycine max), respectively. To provide a foundation to survey the transcriptomic landscape, we produced an improved de novo genome assembly and annotation of F. virguliforme using PacBio sequencing. Next, we conducted a high-resolution time course of F. virguliforme colonization and infection of both soybean, a symptomatic host, and corn, an asymptomatic host. Comparative transcriptomic analyses uncovered a nearly complete network rewiring, with less than 8% average gene coexpression module overlap upon colonizing the different plant hosts. Divergence of transcriptomes originating from host specific temporal induction genes is central to infection and colonization, including carbohydrate-active enzymes (CAZymes) and necrosis inducing effectors. Upregulation of Zn(II)-Cys6 transcription factors were uniquely induced in soybean at 2 days post-inoculation, suggestive of enhanced pathogen virulence on soybean. In total, the data described herein suggest that F. virguliforme modulates divergent infection profiles through transcriptional plasticity.
Annual decreases in soybean (Glycine max L. Merrill) yield caused by diseases were estimated by surveying university-affiliated plant pathologists in 28 soybean-producing states in theUnitedStates and in Ontario, Canada, from 2010 through 2014. Estimated yield losses from each disease varied greatly by state or province and year. Over the duration of this survey, soybean cyst nematode (SCN) (Heterodera glycines Ichinohe) was estimated to have caused more than twice as much yield loss than any other disease. Seedling diseases (caused by various pathogens), charcoal rot (caused by Macrophomina phaseolina (Tassi) Goid), and sudden death syndrome (SDS) (caused by Fusarium virguliforme O'Donnell & T. Aoki) caused the next greatest estimated yield losses, in descending order. The estimated mean economic loss due to all soybean diseases, averaged across U.S. states and Ontario from 2010 to 2014, was $60.66 USD per acre. Results from this survey will provide scientists, breeders, governments, and educators with soybean yield-loss estimates to help inform and prioritize research, policy, and educational efforts in soybean pathology and disease management.
The effect of fungicides on severity of sudden death syndrome (SDS; caused by Fusarium virgulifonne), plant establishment, and soybean yield was evaluated in 12 field experiments conducted in Illinois, Indiana, Iowa, Michigan, and Ontario in 2013 and 2014. Two soybean cultivars that differed in susceptibility to SDS were planted in fields with a history of SDS or with artificial augmentation of F. virguliforme. Efficacy of seed, in-furrow, and foliar-applied fungicides was assessed. SDS levels varied across locations and years. Fluopyram applied on the seed or in-furrow reduced foliar disease index maximum up to 95% in 5 of the 12 experiments. In three experiments with significant (P < 0.10) treatment effect, fluopyram seed treatment improved yields up to 11% compared with the base seed treatment comprising prothioconazole + penflufen + metalaxyl and clothianidin + Bacillus firmus. Meta-analysis also indicated that the fluopyram seed treatment and in-furrow application were effective at reducing SDS and increasing yield relative to the control; however, the baseline disease influenced the yield and disease response to fungicide treatments. Treatment effect was not significant when disease pressure was low. The concentration of F. virgulifonne DNA in soybean roots, measured by a specific real-time quantitative polymerase chain reaction assay, was not different among fungicide treatments in 9 of 10 experiments. Moderately resistant cultivars had less disease than susceptible cultivars, indicating that resistant cultivars in combination with fluopyram seed treatment or in-furrow application could provide effective management of SDS.
Subsurface tile drainage is a significant pathway for nitrogen (N) and phosphorus (P) transport from agricultural fields. The objective of this study was to evaluate N and P loss through tile drainage under corn (Zea mays L.) and soybean (Glycine max L.) production in a corn-soybean rotation typical of agricultural management across the eastern Corn Belt of the US Midwest. Differences in nutrient concentrations and loadings between crop type and between growing (GS) and nongrowing seasons (NGS) were assessed. From 2005 through 2012, discharge and water quality were monitored at three end-of-tile locations that had estimated contributing areas ranging from 7.7 to 14.9 ha (19.0 to 36.8 ac) in a headwater watershed in central Ohio, United States. Nitrate-N (NO3-N) and dissolved reactive P (DRP) were the primary (> 75%) forms of N and P in drainage water. DRP concentration and loading was not significantly different between crop types, but differed significantly by season. Mean weekly DRP concentration (0.22 mg L-1 [0.22 ppm]) was greater during the GS, while mean weekly DRP load (0.010 kg ha(-1) [0.009 lb ac(-1)]) was greater in the NGS. In comparison, NO3-N concentration and load was dependent on the interaction between crop type and season, with the greatest NO3-N concentration (17.1 mg L-1) observed during the GS under corn production. Differences in N and P loss to tile drains were attributed to the timing of nutrient application and differences in seasonal discharge. Practices such as cover crops and drainage water management that target nutrient transport in the NGS should be explored as a means to decrease annual N and P loads. Adherence to recommended 4R nutrient stewardship (right fertilizer source, right rate, right time, and right placement) practices should also help minimize nutrient leaching to tile drains under a corn-soybean rotation.
Cover crops are widely viewed by the soil and water conservation community to be an effective means for reducing soil erosion and nutrient loss and increasing soil health, yet relatively few farmers have adopted the practice. Despite the widespread recognition of cover crops' benefits and increased promotional efforts, there have been very few peer-reviewed studies focused on farmer perspectives on or adoption of cover crops. This study, which analyzed data from a survey and in-depth interviews with Iowa farmers, examined the roles that perceived practice characteristics, perspectives on potential facilitating factors, and crop and livestock diversity play in cover crop adoption among Iowa farmers. As expected, perceived benefits were strongly associated with cover crop use. Measures of crop and livestock diversity were also positive predictors of adoption. In addition, farmers who endorsed strengthening of facilitating factors such as educational and technological infrastructure to support cover crop use were more likely to have adopted cover crops. Farmers who perceived higher levels of risks associated with cover crop use, on the other hand, were less likely to use them. Results suggest that research and promotional efforts should focus on both raising awareness of potential benefits and quantifying and communicating potential risks and risk abatement strategies. Helping farmers to better understand (1) the benefits of cover crops and how they can be enhanced, and (2) the potential risks and ways that they can be minimized might allow farmers to more effectively weigh the probable benefits and costs of cover crop use. The findings further suggest that farmers believe that better facilitating infrastructure, in the form of technical assistance (e.g., agricultural retailers and custom operators) and education, is needed to support the widespread adoption of cover crops.
Asian soybean rust caused by Phakopsora pachyrhizi, occurs in all soybean production regions of the world. Rust is the most destructive foliar disease of soybean and can cause yield losses of over 80%. To date, six race-specific resistance genes have been identified in plant introductions. However, races of P. pachyrhizi able to overcome the resistance conferred by these genes have evolved. Due to the limited availability of resistant varieties, fungicide application is the only management tool available for farmers, which significantly raises the production cost and the risk of environmental and human contamination. Thus, the transfer of resistance genes through classical breeding followed by marker-assisted selection allows the development of resistant varieties and their use as an efficient and cost-effective method for controlling soybean rust. The objective of this review is to provide a broad overview of the Asian soybean rust resistance, and a useful tool to guide future researches as well.
The ability to accurately detect and quantify Fusarium virguliforme, the cause of sudden death syndrome (SDS) in soybean, in samples such as plant root tissue and soil is extremely valuable for accurate disease diagnoses and to address research questions. Numerous quantitative real-time PCR (qPCR) assays have been developed for this pathogen, but their sensitivity and specificity for F. virguliforme have not been compared. In this study, six qPCR assays were compared in five independent laboratories using the same set of DNA samples from fungi, plants, and soil. Multi-copy gene based assays targeting the ribosomal DNA intergenic spacer (IGS) or the mitochondrial small subunit (mMtSSU) showed relatively high sensitivity (lLimit of detection (LOD) = 0.05 to 5 pg) compared to a single copy gene (FvTox1) based assay (LOD = 5 to 50 pg). Specificity varied greatly among assays, with the FvTox1 assay ranking the highest (100 %) and two IGS assays being slightly less specific (95 to 96%). Another IGS assay targeting four SDS-causing fusaria showed lower specificity (70%), while the two mMtSSU assays were lowest (41% and 47%). This study has identified strengths and weaknesses of six qPCR assays for detection and quantification of F. virguliforme in root and soil samples. An IGS based assay showed consistently highest sensitivity (LOD = 0.05 pg), and specificity and inclusivity above 94%, and thus is suggested as the most useful qPCR assay for F. virguliforme diagnosis and quantification. However, specificity was also above 94% in two other assays and their selection for diagnostics and research choice of assay for diagnosis or research will also depend on objectives, samples,s and materials used. These results will facilitate both fundamental and disease management research pertinent to SDS.
Disease resistance is an important crop trait that protects yield and farmers' incomes. The objective of the soybean breeding program for disease resistance at Iowa State University (ISU) is to develop germplasm lines for Iowa. AR10SDS (Reg. No. GP-391, PI 669816) is the first sudden death syndrome (SDS) partially resistant line released by ISU, Project nos. 4403 and 0159 (Agronomy and Plant Pathology departments), ISU Research Foundation (ISURF) Docket #03264. AR10SDS is partially resistant to Fusarium virguliforme which causes SDS, and resistant to soybean cyst nematode (SCN) phenotypic race 3 (HG types 0 and 7) caused by Heterodera glycines. AR10SDS is a bulk of 60 F-3:8 plants uniform in plant and seed traits. The parentage is 'Ripley' x 'IA1008'. Ripley is resistant to SDS. IA1008 is resistant to SCN. The cross and generations advances were made in Puerto Rico beginning in 2000, ISU research site at the Isabela Substation, University of Puerto Rico, Isabela, PR. Yield was evaluated from 2003 to 2006 in Iowa and in the northern U.S. regions. From 2003 to 2011, SDS resistance was evaluated under greenhouse conditions and in SDS-infested field soil. Soybean cyst nematode screening was conducted in 2005 to 2006 under greenhouse conditions. Three SDS resistance quantitative trait loci (QTLs) were inherited from Ripley, and two SCN resistance QTLs from IA1008. Seed yield is similar to IA1008 and significantly (P = 0.05) better than 'Parker' and 'Freeborn'. It is of early- to mid-maturity group I and adapted from 40 to 42 degrees N latitude. AR10SDS may serve breeding programs for its dual disease resistance.
Soil health has been defined as the capacity of soil to function as a vital living system to sustain biological productivity, maintain environmental quality, and promote plant, animal, and human health. Building and maintaining soil health is essential to agricultural sustainability and ecosystem function. Management practices that promote soil health, including the use of crop rotations, cover crops and green manures, organic amendments, and conservation tillage, also have generally positive effects on the management of soilborne diseases through a number of potential mechanisms, including increasing soil microbial biomass, activity, and diversity, resulting in greater biological suppression of pathogens and diseases. However, there also may be particular disease issues associated with some soil health management practices. In this review, research and progress made over the past twenty years regarding soil health, sustainability, and soil health management practices, with an emphasis on their implications and effects on plant disease and disease management strategies are summarized. Expected final online publication date for the Annual Review of Phytopathology Volume 53 is August 04, 2015. Please see http://www.annualreviews.org/catalog/pubdates.aspx for revised estimates.
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In 1972 President Nixon signed the Clean Water Act (CWA) into law, making clean water a public right and establishing a goal that the nation's waters should be both “fishable and swimmable.” It is considered by many to be the most important and effective environmental law ever passed. Before the CWA, two-thirds of US waterways were considered unsafe for fishing and swimming, and waste from households, municipalities, factories and power plants, including sewage, livestock processing, waste oil, and chemicals, flowed untreated into rivers, streams, and lakes. The law reduced the discharge of sewage and other industrial point source pollution into waterways, but most agricultural nonpoint source pollution, the greatest source of water pollution today, was exempted. The agriculture exemption, called “one of the last, great intractable problems of environmental law,” results in an inconsistent system for addressing water pollution, with regulation for the majority of urban sources and a voluntary, incentive-based system for much of agriculture (Laitos and Ruckriegle 2013).
Despite more than 40 years of largely voluntary efforts by federal, state, and local government, and tens of billions of US dollars of investment in conservation, nationwide progress on nutrient control has not yet been achieved. Concentrations of nitrogen (N)…
Sudden death syndrome (SDS) of soybean caused by Fusarium virguliforme is one of the most damaging diseases of soybean in the US. Management of SDS relies on selection of resistant varieties, cultural practices to improve soil drainage, and avoiding planting in cool, wet soils. The effectiveness of crop rotation for SDS management is not well understood. In this study, the effect of long-term crop rotations in reducing SDS was studied in 2010 and 2011, in a field trial established in Iowa since 2002. The trial included three rotation treatments: corn-soybean, corn-soybean-oat/red-clover, and corn-soybean-oat/alfalfa-alfalfa. SDS incidence and severity, root rot and root growth, yield, and F. virguliforme and soybean cyst nematode (SCN) populations in soil were assessed. In both years, the 2-year rotation showed greater (P<0.001) SDS incidence and severity, and lower yield, compared to the 3- and 4-year rotations. Roots from the 2-year rotation showed more severe rot and reduced growth (P<0.05) compared to the longer rotations. SCN populations did not differ among rotations. Quantification of F. virguliforme in soil using real-time PCR suggested a greater pathogen population in the 2-year rotation compared to the 3-year rotation, but the mechanisms behind the reduction in SDS need further clarification. These findings suggest that long-term crop rotations may offer an alternative management practice for reducing risk of SDS.
Nitrate lost from agricultural soils is an economic cost to producers, an environmental concern when it enters rivers and lakes, and a health risk when it enters wells and aquifers used for drinking water. Planting a winter wheat cover crop (CC) and/or use of controlled tile drainage-subirrigation (CDS) may reduce losses of nitrate (NO) relative to no cover crop (NCC) and/or traditional unrestricted tile drainage (UTD). A 6-yr (1999-2005) corn-soybean study was conducted to determine the effectiveness of CC+CDS, CC+UTD, NCC+CDS, and NCC+UTD treatments for reducing NO loss. Flow volume and NO concentration in surface runoff and tile drainage were measured continuously, and CC reduced the 5-yr flow-weighted mean (FWM) NO concentration in tile drainage water by 21 to 38% and cumulative NO loss by 14 to 16% relative to NCC. Controlled tile drainage-subirrigation reduced FWM NO concentration by 15 to 33% and cumulative NO loss by 38 to 39% relative to UTD. When CC and CDS were combined, 5-yr cumulative FWM NO concentrations and loss in tile drainage were decreased by 47% (from 9.45 to 4.99 mg N L and from 102 to 53.6 kg N ha) relative to NCC+UTD. The reductions in runoff and concomitant increases in tile drainage under CC occurred primarily because of increases in near-surface soil hydraulic conductivity. Cover crops increased corn grain yields by 4 to 7% in 2004 increased 3-yr average soybean yields by 8 to 15%, whereas CDS did not affect corn or soybean yields over the 6 yr. The combined use of a cover crop and water-table management system was highly effective for reducing NO loss from cool, humid agricultural soils.
Copyright © by the American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America, Inc.
Fusarium virguliforme (syn. F. solani f. sp. glycines) is the primary causal pathogen responsible for soybean sudden death syndrome (SDS) in North America. Diagnosis of SDS is difficult because symptoms can be inconsistent or similar to several soybean diseases and disorders. Additionally, quantification and identification of F. virguliforme by traditional dilution plating of soil or ground plant tissue is problematic due to the slow growth rate and plastic morphology of F. virguliforme. Although several real-time quantitative PCR (qPCR) based assays have been developed for F. virguliforme, the performance of those assays does not allow for accurate quantification of F. virguliforme due to the reclassification of the F. solani species complex. In this study, we developed a TaqMan qPCR assay based on the ribosomal DNA (rDNA) intergenic spacer (IGS) region of F. virguliforme. Specificity of the assay was demonstrated by challenging it with genomic DNA of closely related Fusarium species and commonly encountered soilborne fungal pathogens. The detection limit of this assay was determined to be 100 fg of pure F. virguliforme genomic DNA or 100 macroconidia in 0.5 g of soil. An exogenous control was multiplexed with the assay to evaluate for PCR inhibition. Target locus copy number variation had minimal impact, with a range of rDNA copy number from 138 to 233 copies per haploid genome, resulting in a minor variation of up to 0.76 Ct values between strains. The qPCR assay is transferable across platforms, as validated on the primary real-time PCR platform used in the North Central region of the National Plant Diagnostic Network. A conventional PCR assay for F. virguliforme detection was also developed and validated for use in situations where qPCR is not possible.
Temporal dynamics of soybean sudden death syndrome (SDS) root and foliar disease severity were studied in growth chamber experiments on susceptible plants exposed to different inoculum densities (0, 100, 101, 102, and 103 conidia g−1 soil) of Fusarium virguliforme. The monomolecular model provided the best fit to describe the progress of root and foliar disease severity over time. Disease severity and area under disease progress curve (AUDPC) both increased in response to increasing inoculum density (P < 0.01), particularly for foliar symptoms. Rate of disease progress increased as inoculum densities increased for both root and foliar disease severities. The incubation period for root and foliar disease severity ranged from 9 to 18 and 15 to 25 days, respectively. Significant differences in root rot severity were most easily detected during the early stages of infection, whereas root rot and foliar severities were only weakly correlated when both were assessed simultaneously at later stages of disease development. Root rot severity assessments performed 15 to 20 days after inoculation (DAI) were most highly correlated (r > 0.9, P < 0.01) with foliar disease severity assessments performed 30 to 50 DAI. Root biomass was reduced by up to 67% at the three highest inoculum densities, indicating the aggressiveness that F. virguliforme possesses as a root rot pathogen on soybeans.
Soybeans were grown in field microplots in sterile, noninfested soil versus soil infested with either Fusarium solani (form A), Heterodera glycines (the soybean cyst nematode), or both. Symptoms of sudden death syndrome occurred on plants in soil containing F. solani or F. solani plus H. glycines. Signs (macroconidia) sometimes occurred on symptomatic roots. Fusarium solani was reisolated from symptomatic plants but not from asymptomatic ones. Histological data further confirmed F. solani as the causal agent and documented the presence of F. solani chlamydospores in infected roots and cysts. Fusarium solani was not isolated from surface-disinfested seeds of infected plants. Irrigation increased disease incidence and severity. Heterodera glycines was not necessary for infection of plants by F. solani; however, when combined with F. solani, leaf symptoms occurred earlier and were more severe. Inoculation with F. solani plus H. glycines increased the incidence of tip dieback of pods, a disorder of uncertain cause. Fusarium solani was isolated in high frequency from roots of symptomatic plants sampled in the South and Midwest. However, F. solani form B was the most common isolate from roots. A significant positive correlation occurred between incidence of the two F. solani forms in roots of symptomatic plants. Key words: Glycine max, Fusarium solani, Heterodera glycines, etiology.
Soybean sudden death syndrome (SDS) pathogens and dry bean root-rot pathogens were studied taxonomically, phylogenetically, and pathologically. Detailed phenotypic comparisons of macro- and microscopic features and phylogenetic analyses of multilocus DNA sequence data, including those on the nuclear ribosomal intergenic spacer region and the single copy nuclear gene translation elongation factor 1-a, indicated that they comprised five distinct species of Fusarium. Two new species causing soybean SDS in Brazil, F. brasiliense and F. cuneirostrum, are formally described. Fusarium cuneirostrum is responsible for soybean SDS in Brazil and dry bean or mung bean root-rot in the United States, Canada, and Japan. Strains of each species, including F. cuneirostrum isolates from dry bean and mung bean and F. phaseoli isolates from dry bean, were inoculated on soybean cultivar Pioneer 9492RR to determine their pathogenicity. Although intraspecific variation in pathogenicity was observed, all the species were able to induce typical SDS symptoms on soybean plants in the artificial inoculation tests. Comparisons of the key diagnostic morphological features reveal that all five species can be diagnosed using conidial morphology.
Seven different 2-year rotations, consisting of barley/clover, canola, green bean, millet/rapeseed, soybean, sweet corn, and potato, all followed by potato, were assessed over 10 years (1997–2006) in a long-term cropping system trial for their effects on the development of soilborne potato diseases, tuber yield, and soil microbial communities. These same rotations were also assessed with and without the addition of a fall cover crop of no-tilled winter rye (except for barley/clover, for which underseeded ryegrass was substituted for clover) over a 4-year period. Canola and rapeseed rotations consistently reduced the severity of Rhizoctonia canker, black scurf, and common scab (18 to 38% reduction), and canola rotations resulted in higher tuber yields than continuous potato or barley/clover (6.8 to 8.2% higher). Addition of the winter rye cover crop further reduced black scurf and common scab (average 12.5 and 7.2% reduction, respectively) across all rotations. The combined effect of a canola or rapeseed rotation and winter rye cover crop reduced disease severity by 35 to 41% for black scurf and 20 to 33% for common scab relative to continuous potato with no cover crop. Verticillium wilt became a prominent disease problem only after four full rotation cycles, with high disease levels in all plots; however, incidence was lowest in barley rotations. Barley/clover and rapeseed rotations resulted in the highest soil bacterial populations and microbial activity, and all rotations had distinct effects on soil microbial community characteristics. Addition of a cover crop also resulted in increases in bacterial populations and microbial activity and had significant effects on soil microbial characteristics, in addition to slightly improving tuber yield (4% increase). Thus, in addition to positive effects in reducing erosion and improving soil quality, effective crop rotations in conjunction with planting cover crops can provide improved control of soilborne diseases. However, this study also demonstrated limitations with 2-year rotations in general, because all rotations resulted in increasing levels of common scab and Verticillium wilt over time.
This article reviews literature about the impacts of cover crops in cropping systems that affect soil and water quality and presents limited new information to help fill knowledge gaps. Cover crops grow during periods when the soil might otherwise be fallow. While actively growing, cover crops increase solar energy harvest and carbon flux into the soil, providing food for soil macro and microorganisms, while simultaneously increasing evapotranspiration from the soil. Cover crops reduce sediment production from cropland by intercepting the kinetic energy of rainfall and by reducing the amount and velocity of runoff. Cover crops increase soil quality by improving biological, chemical and physical properties including: organic carbon content, cation exchange capacity, aggregate stability, and water infiltrability. Legume cover crops contribute a nitrogen (N) to subsequent crops. Other cover crops, especially grasses and brassicas, are better at scavenging residual N before it can leach. Because growth of these scavenging cover crops is usually N limited, growing grass/legume mixtures often increases total carbon inputs without sacrificing N scavenging efficiency. Cover crops are best adapted to warm areas with abundant precipitation. Water use by cover crops can adversely impact yields of subsequent dryland crops in semiarid areas. Similarly, cooler soil temperatures under cover crop residues can retard early growth of subsequent crops grown near the cold end of their range of adaptation. Development of systems that reduce the costs of cover crop establishment and overcome subsequent crop establishment problems will increase cover crop utilization and improve soil and water quality.
The integration of cover crops into cropping systems brings costs and benefits, both internal and external to the farm. Benefits include promoting pest‐suppression, soil and water quality, nutrient cycling efficiency, and cash crop productivity. Costs of adopting cover crops include increased direct costs, potentially reduced income if cover crops interfere with other attractive crops, slow soil warming, difficulties in predicting N mineralization, and production expenses. Cover crop benefits tend to be higher in irrigated systems. The literature is reviewed here along with Michigan farmer experience to evaluate promising cover crop species for four niches: Northern winter (USDA Hardiness Zones 5–6), Northern summer (Zones 5–6), Southern winter (Zones 7–8), and Southern summer (Zones 7–8). Warm season C 4 grasses are outstanding performers for summer niches (6–9 Mg ha ⁻¹ ), and rye ( Secale cereale L.) is the most promising for winter niches (0.8–6 Mg ha ⁻¹ ) across all hardiness zones reviewed. Legume–cereal mixtures such as sudangrass ( Sorghum sudanese L.)–cowpea (Vigna unguiculata L ) and wheat ( Triticum aestivum L.)–red clover ( Trifolium pretense L.) are the most effective means to produce substantial amounts (28 Mg ha ⁻¹ ) of mixed quality residues. Legume covers are slow growers and expensive to establish. At the same time, legumes fix N, produce high quality but limited amounts (0.5–4 Mg ha ⁻¹ ) of residues, and enhance beneficial insect habitat. Brassica species produce glucosinolate‐containing residues (2–6 Mg ha ⁻¹ ) and suppress plant‐parasitic nematodes and soil‐borne disease. Legume cover crops are the most reliable means to enhance cash crop yields compared with fallows or other cover crop species. However, farmer goals and circumstances must be considered. If soil pests are a major yield limiting factor in cash crop production, then use of brassica cover crops should be considered. Cereal cover crops produce the largest amount of biomass and should be considered when the goal is to rapidly build soil organic matter. Legume–cereal or brassica–cereal mixtures show promise over a wide range of niches.
Soybean sudden death syndrome (SDS), caused by Fusarium virguliforme, is an economically important soilborne disease and is a major risk to many soybean [Glycine max, (L.) Merr.] production regions worldwide. Two-year studies were conducted in a greenhouse and in fields to examine survival of the fungus in corn-soybean residues. Corn kernels consistently showed significantly (P<0.05) higher F. virguliforme colony-forming units per gram of dry soil (CFU/g) in a greenhouse and in field micro-plots compared to no additional residue added treatment (control). None of the soil samples from commercial fields in Iowa showed significant (P<0.05) difference in F. virguliforme CFU/g within year of sampling, but between years there were numerical differences but not statistically different in samples if the previous crop had been corn, compared with winter wheat or soybean. In Fusarium spp. not causing SDS, CFU/g were significantly (P<0.05) higher in micro-plots amended with six different corn-soybean residue treatments compared with the control in 2008, while in 2009, only corn stock spread on soil showed significant (P<0.05) difference over the control. Our results suggest that a clean corn harvest could reduce SDS risk by reducing colonization of corn kernels that supports survival of F. virguliforme, while a considerable corn loss during harvest could increase SDS risk.
Cover crops can improve the sustainability and resilience of corn (Zea mays L.) and soybean (Glycine max L. Merr.) production systems. At present, the most widely used cover crops in corn-soybean systems in the upper Midwest United States have been winter cereals. However, there have been isolated reports of corn yield reductions following winter rye (Secale cereale L.) cover crops, and the risk of corn yield reductions will reduce the likelihood of farmers adopting cover crops. Although the exact mechanism is unknown and there are many possible causes of corn yield reductions following winter cereal cover crops, we hypothesize that there may be differences among winter cereal species or cultivars in their effect on corn yield. Additionally, there have been no evaluations of shoot growth and nitrogen (N) uptake of winter cereal cultivars used as cover crops in the upper Midwest. Seven winter rye cultivars, 2 winter triticale (× Triticosecale Wittmack) cultivars, and 3 winter wheat (Triticum aestivum L.) cultivars were planted following soybean harvest and grown as a winter cover crops preceding corn in four years to determine whether the 12 cultivars differed in (1) biomass production and N uptake, and (2) impact on corn yield, harvest population, and other yield parameters. The 12 cover crop cultivars differed in each of the four years for shoot dry weight, shoot N concentration, and total shoot N content. In general, the winter rye cultivars had greater shoot biomass, lower shoot N concentrations, and higher total shoot N contents than the winter triticale and winter wheat cultivars. The winter cereal cultivars decreased corn yield in two of the four years, and the yield effect varied among cultivars. Some cultivars of all three species caused corn yield decreases, with no indication that winter rye had a greater effect than did winter wheat or winter triticale. Four winter rye cultivars did not significantly reduce corn yield in either of the two years in which yield was reduced. In general, the decreases in corn yield following the winter cereal cover crops were related to decreases in harvest population and increases in the number of barren plants, but were not strongly related to cover crop shoot dry weight within years. Our study shows that there are genotypic differences among winter cereal cultivars for their performance as cover crops and their effect on corn yields.
This book, divided into three parts, provides a wide range of information on plant-parasitic nematodes. The chapters in Part I presents the basic structures of nematodes, followed by a chapter on molecular taxonomy, systematics and phylogeny. The subsequent four chapters focus on the major groups of plant-parasitic nematodes, presenting information on their morphology, taxonomy, basic biology and management. Part II deals with nematode biology and plant nematode interactions. Part III focuses on quantitative nematology and management.
Research must focus on management of diseases that cause extensive losses, especially when funds for research are limited. Knowledge of the losses caused by various soybean diseases is essential when prioritizing research budgets. The objective of this project was to compile estimates of soybean yield potential losses caused by diseases for each soybean producing state in the United States from 2006 to 2009. This data is of special interest since the 4-year period summarized in this report, permits an examination of the impact of soybean rust that was first reported in the United States in 2004. Thus, in addition to the goal of providing this information to aid funding agencies and scientists in prioritizing research objectives and budgets, an examination of the impact of soybean rust on soybean yield losses relative to other diseases is warranted. Yield losses caused by individual diseases varied among states and years. Soybean cyst nematode caused more yield losses than any other disease during 2006 to 2009. Seedling diseases, Phytophthora root and stem rot, sudden death syndrome, Sclerotinia stem rot, and charcoal rot ranked in the top six of diseases that caused yield loss during these years. Soybean yield losses due to soybean rust and Sclerotinia stem rot varied greatly over years, especially when compared to other diseases.
Accepted for publication 21 October 2010. Published 22 November 2010.
Fusarium virguliforme causes soybean sudden death syndrome (SDS) in the United States. The disease was first observed in Arkansas in 1971, and since has been reported in most soybean-producing states, with a general movement from the southern to the northern states. In addition to F. virguliforme, three other species, Fusarium brasiliense, Fusarium crassistipitatum, and Fusarium tucumaniae, have been reported to cause SDS in South America. Yield losses caused by F. virguliforme range from slight to 100%. Severely infected plants often have increased flower and pod abortion, reduced seed size, increased defoliation, and prematurely senescence. Foliar symptoms observed in the field are most noticeable from mid to late reproductive growth stages. To manage SDS, research on crop rotations, soil types, tillage practices, seed treatments, and the development and utilization of host resistance has been investigated. This review focuses on what is known about F. virguliforme, the management of SDS in the United States, and how genetic engineering along with other traditional management options may be needed as integrated approaches to manage SDS.
Current greenhouse screening methods for soybean [Glycine max (L.) Merr.] sudden death syndrome (SDS) are not sensitive enough to evaluate progeny of resistant x resistant crosses, which can possess resistance alleles at up to 12 quantitative trait loci (QTL). The objective of the study was to modify the current greenhouse screening method, the layer method, to increase its sensitivity in distinguishing SDS-resistant lines. Three experiments were conducted using infested sorghum [Sorghum bicolor (L.) Moench] as inoculum. In Exp. 1, five different inoculum densities (1:1, 1:5, 1:10, 1:15, and 1:20 inoculum:soil ratio) were compared, and the optimum density to distinguish genotypes was identified. In Exp. 2, the association between SDS field ratings and greenhouse severity scores was determined. In Exp. 3, the inoculum layer method was compared to the proposed modified mixed method. Inoculum ratios 1:15 and 1:20 showed the largest difference in SDS ratings between soybean genotypes. The best time to assess disease severity was 30 to 36 d after planting (DAP), with the highest correlation with field ratings at 36 DAP (r = 0.82, p < 0.05). The proposed modified method shifted the frequency distribution of SDS scores toward a normal curve. The findings indicate that the modified screening method using a 1:20 inoculum:soil ratio correlated well with field data and provided adequate screening of lines possessing up to 12 SDS resistant QTL without negatively impacting germination.
Kolander, T. M., Bienapfl, J. C., Kurle, J. E., and Malvick, D. K. 2012. Symptomatic and asymptomatic host range of Fusarium virguliforme, the causal agent of soybean sudden death syndrome. Plant Dis. 96:1148-1153. Sudden death syndrome, caused by Fusarium virguliforme, is an important disease of soybean in the United States. Fifteen species of crops, weeds, or prairie plants were evaluated for their potential as hosts of F virguliforme. Root and foliar symptoms and plant biomass were assessed following greenhouse inoculation studies. Root colonization of F virguliforme was determined with isolations and with polymerase chain reaction assays. Soybean, alfalfa, pinto and navy bean, white and red clover, pea, and Canadian milk vetch developed root necrosis. Soybean, alfalfa, and red clover also developed foliar symptoms following inoculation. Sugar beet and canola did not develop symptoms but had significant reductions in biomass, suggesting that they are also hosts of E virguliforme. Corn, wheat, ryegrass, pigweed, and lambsquarters did not develop symptoms. However, these species appeared to be asymptomatic hosts because quantities of pathogen DNA detected in inoculated roots were similar to quantities detected in inoculated soybean roots. These results suggest that the number and diversity of hosts for E virguliforme are greater than previously reported. The likely broad host range limits the efficacy of crop rotation and indicates that crops other than soybean can be damaged by F virguliforme and maintain or increase inoculum in soil.
Sudden death syndrome, caused by Fusarium solani f. sp. glycines, has caused severe damage to soybean production in recent years. One way to control sudden death syndrome is with resistant cultivars. Over a 3-year period, 2,335 publicly and privately developed soybean entries were inoculated and evaluated for their response to F solani f. sp. glycines under greenhouse conditions. The entries were compared with the susceptible check, Great Lakes 3302 (GL3302), and the moderately resistant checks, plant introductions (Pis) 520733 and 567374. Thirty-eight entries were identified with moderate levels of resistance. Based on foliar ratings, there were no differences (P < 0.05) between the Roundup Ready and conventional cultivars. In all, 90 ancestral lines that represent 99% of the genes in modern U.S. cultivars and 55 lines found in the pedigrees of public cultivars reported to have some resistance were evaluated for their response to F solani f. sp. glycines. Nine ancestral lines (Aoda, Kim, Jackson, Sioux, Mammoth Yellow, T 117, PI 171450, PI 54615-1, and PI 71506) and 12 cultivars or experimental lines (Ina, D83-3349, LN98-4340. LN83-2356, Hartwig, Harosoy, Bedford, Merit, Cutler, Calland, Hill, and Evans) had disease ratings not significantly different (P < 0.05) from PI 520733 or PI 567374. PI 54610. a putative ancestral line, also was found to be moderately resistant.
The soybean cyst nematode (SCN), Heterodera glycines, is a major soybean yield-limiting factor, and the use of resistant cultivars is one of the most effective means to manage the nematode. During the past decade, a number of resistant cultivars in maturity groups I and II have been developed and made available to growers. A total of 47 resistant cultivars and nine susceptible cultivars were evaluated at 15 SON-infested field sites and two noninfested sites during 1996 to 1998 in Minnesota. As expected, more nematodes developed on susceptible cultivars than on resistant cultivars. Egg density on susceptible cultivars increased by 1.9- to 10.6-fold during the growing season at 12 sites and did not change at the other three sites. Average egg density decreased over time for resistant cultivars at all sites, except where the initial egg density was low (less than or equal to 455 eggs per 100 cm(3) soil). Nematode reproduction factors (Rf = egg density at harvest/egg density at planting) for individual resistant and susceptible cultivars were highly consistent across the eight sites where initial SCN density was more than 1,000 eggs per 100 cm(3) soil. Resistance, however. varied among the cultivars, with the average Rf of individual resistant cultivars across the sites ranging from 0.3 to 1.7. Resistant cultivars produced an average yield of 3,082 kg/ha compared with 2,497 kg/ha by susceptible cultivars at eight of 10 sites where egg density at planting was greater than 700 eggs per 100 cm(3) soil. In contrast, no difference in yield was observed between resistant and susceptible cultivars at sites where egg density at planting was lower than 500 eggs per 100 cm3 soil. Yield differences between resistant and susceptible cultivars increased with increasing initial SCN egg density. In six fields infested with initial densities of more than 5,000 eggs per 100 cm(3) soil, resistant cultivars produced 28.4% (676 kg/ha) more yield on average than susceptible cultivars. Soybean yield increased when cultivars with increasing resistance to the SCN (lower Rf or females formed on roots) were grown in fields infested with SCN. Average relative yield (yield of a cultivar/average yield of all resistant cultivars at a site) of individual resistant cultivars across all SON-infested sites ranged from 0.76 to 1.10. Yield consistency of soybean cultivars was low among the different sites, indicating that many other factors affected yield. Our results suggest growing resistant cultivars is an effective method to manage SCN in Minnesota while minimizing yield loss due to SCN.
Research must focus on management of diseases that cause extensive losses, especially when funds for research are limited. Knowledge of yield suppression caused by various soybean diseases is essential when prioritizing research. The objective of this project was to compile estimates of soybean yield suppression due to diseases in the USA from 1996 to 2007. The goal was to provide information to help funding agencies and scientists prioritize research objectives and budgets. Yield suppression due to individual diseases varied among years. Soybean cyst nematode suppressed USA soybean yield more from 1996 to 2007 than any other disease. Phytophthora root and stem rot ranked second among diseases that most suppressed yield seven of 12 years. Seedling diseases and charcoal rot also suppressed soybean yield during these years. Research and extension efforts must be expanded to provide more preventive and therapeutic disease management strategies for producers to reduce disease suppression of soybean yield.
Accepted for publication 25 February 2009. Published 1 April 2009.
Sudden death syndrome (SDS), caused by the fungal pathogen Fusarium virguliforme, causes significant yield reductions in soybean [Glycine max (L.) Merr.] in the United States. Appropriate recommendations to manage SDS for growers in Iowa and the Upper Midwest are limited. The research objective was to determine the response of SDS foliar disease incidence, severity, and yield to row spacing and seeding rate. In 2008 and 2009, at two Iowa locations, in fields with histories of SDS, SDS-susceptible and SDS-resistant cultivars were planted at 38- and 76-cm row spacing at seeding rates of 185,000, 309,000, and 432,000 seeds ha(-1) in plots infested with and without the pathogen. Sudden death syndrome incidence and severity were very low; however, infested plots had greater SDS disease incidence and severity than uninfested plots. A row spacing x infestation interaction indicated 7% greater yield in narrow rows (38 cm) than wide rows (76 cm) in uninfested plots, with no yield advantage to narrow rows in infested plots. Soil infestation reduced soybean seed mass (7%) in narrow rows, explaining the yield reduction for narrow rows with greater SDS. The two highest seeding rates had increased SDS incidence but yielded 9% greater than the lowest seeding rate. The susceptible cultivar had greater SDS incidence and severity and yielded 7% less than the resistant cultivar. This study indicates that in infested plots with greater SDS symptom expression, the yield advantage of narrow rows may be negated; therefore, cultivar selection is crucial when planting in narrow rows to maximize yield.
Fusarium virguliforme causes sudden death syndrome (SDS) in soybean. The pathogen has never been isolated from diseased foliar tissues; therefore, one or more toxins have been considered to cause foliar SDS development. Cell-free F. virguliforme culture filtrates containing a toxin causes foliar SDS in soybean. A low-molecular-weight protein of approximately 13.5 kDa (FvTox1), purified from F. virguliforme culture filtrates, produces foliar SDS-like symptoms in cut soybean seedlings. Anti-FvTox1 monoclonal antibodies raised against the purified FvTox1 were used in isolating the FvTox1 gene. In the presence of light, recombinant FvTox1 protein expressed in an insect cell line resulted in chlorosis and necrosis in soybean leaf disks that are typical foliar SDS symptoms. SDS-susceptible but not the SDS-resistant soybean lines were sensitive to the baculovirus-expressed toxin. The requirement of light for foliar SDS-like symptom development indicates that FvTox1 induces foliar SDS in soybean, most likely through production of free radicals by interrupting photosynthesis.
Sudden death syndrome, caused by Fusarium solani f. sp. glycines, has caused increased losses in soybean production in recent years. This study was done to identify potential sources of resistance to sudden death syndrome. Using a greenhouse screening procedure, 6,037 soybean plant introductions (PIs) were compared with a susceptible check, Great Lakes 3302, and two moderately resistant checks, PI 520.733 and PI 567.374, for resistance to sudden death syndrome. Only 57 PIs had foliar disease ratings that were not significantly different from PI 567.374 (P less than or equal to 0.05) 3 weeks after inoculation. Six PIs had lower ratings than PI 567.374 at 4 weeks after inoculation, while none had lower area under the disease progress curve (AUDPC) values. When comparing the PIs to PI 520.733, 209 PIs had foliar disease ratings not significantly different from PI 520.733 (P less than or equal to 0.05) 3 weeks after inoculation. Eight PIs had significantly lower disease severity ratings 4 weeks after inoculation, and 38 PIs had significantly lower AUDPC values than PI 520.733. Additionally, root lesion lengths were measured 4 weeks after inoculation and ranged from 25.2 to 41.5 mm for all the PIs; none of the entries had smaller lesion lengths than the susceptible check Great Lakes 3302. The correlation between lesion length and disease foliar severity rating was not significant. There also were no plant morphological characteristics (i.e., flower color or seed coat color) associated with higher sudden death syndrome foliar symptoms. Eighteen PIs previously identified as moderately resistant with differing agronomic traits were inoculated with five different isolates of Fusarium solani f. sp. glycines. Results indicated that resistance in these 18 PIs was effective against all five isolates of Fusarium solani f. sp. glycines. Isolate Mont-1 caused the greatest disease severity ratings. These PIs that exhibited low foliar severity ratings may provide new sources of resistance for the development of new sudden death syndrome-resistant lines and cultivars.
The effects of alfalfa, red clover, and perennial ryegrass as cover crops on soybean cyst nematode (SCN) and soybean and corn yields were evaluated in Waseca, Lamberton, and Rosemount, MN. The cover crops were interseeded in soybean at 0 or 2 wks after planting soybean in 2002 and killed with herbicide before planting corn in 2003. As expected, SCN-susceptible soybean supported higher SCN population density than SCN-resistant soybean. Reduction of SCN population density by red clover (up to 40%) and alfalfa (up to 55%) was observed in some sampling occasions at Lamberton and Rosemount, probably due to reduced soybean growth, but the effect was inconsistent. No significant reduction of SCN population by the two crops was detected at Waseca. While perennial ryegrass did not affect SCN population density in most cases, up to 46% higher egg population densities were observed in the perennial ryegrass treatment as compared to the control at Waseca. SCN-resistant soybean produced higher yield than susceptible soybean at all sites. While alfalfa reduced soybean yield at Lamberton (up to 50%) and Rosemount (up to 11%), red clover and perennial ryegrass reduced soybean yield only at Lamberton (up to 38%) and Waseca (up to 34%), respectively. No difference in corn yield was observed at Waseca. At Lamberton, alfalfa and red clover planted at the time of planting soybean reduced corn yield in the following year 17 and 13%, respectively, and perennial ryegrass planted 2 wks after planting soybean reduced corn yield 13%. At Rosemount, significant reduction of corn yield was observed with red clover (15-21%) interseeded in SCN-susceptible soybean and with alfalfa (12%) and red clover (12%) interseeded in SCN-resistant soybean at the time of planting soybean. The results suggest that an even later planting date of cover crops in soybean may reduce yield loss due to competition and make these cover crops more appropriate for use in the soybean-corn rotation in Minnesota.
The soybean cyst nematode (SCN), Heterodera glycines, is a major yield limiting pest of soybean (Glycine max). Current SCN management strategies include resistant soybean varieties and rotation with non-host crops. Increased adoption of the early soybean production system (ESPS) combined with a greater incidence of winter weeds from no-tillage practices may increase the population density of SCN during the noncrop period since several winter weeds and cover crops are hosts for SCN. A field experiment with a split-strip design was conducted over three years to evaluate whether SCN reproduced on weeds and cover crops during the noncrop period. Winter weeds and crimson clover roots were examined for females and cysts with eggs. The soil was sampled to determine if an increase in SCN egg population density occurred. We found no indication of SCN reproduction on winter weeds during the noncrop period or on the cover crop crimson clover.
Accepted for publication 2 November 2006. Published 26 February 2007.
Root diseases caused by soil-borne pathogens are often main constraints in legume crop production. Changes towards organic farming practices have recently contributed to an increase in legume cropping, mainly for nitrogen supply purposes, and these have raised concerns about unacceptable build-up of soil-borne pathogen inocula. This study aimed to evaluate the impact of frequent legume cropping on such inocula build-up, and had emphasis on Aphanomyces euteiches, an important pathogen causing pea root rot in Sweden. Field experiments with legume monocultures were established, and the effect of these monocultures on disease development and yields in subsequent pea, broad bean and snap bean was measured. Isolates of Aphanomyces spp., from several legumes, were tested for host ranges and characterised by means of morphological, biochemical and molecular methods. A survey of legume-specific soil-borne pathogens in fields under frequent legume cropping in northern Spain was also undertaken.
Several legumes were found to be hosts for A. euteiches, and this pathogen was isolated from field-grown alfalfa, snap bean, pea, sweet clover and vetch. The Swedish isolates of A. euteiches were assigned two putative pathotypes, pea- and vetch-specific. Other species of Aphanomyces had a wide host range among legumes, but these did not induce disease symptoms. Sequencing of ITS1 - 5.8S - ITS2 rDNA region and RFLP of AT-rich DNA allowed appropriate delineation of these Aphanomyces spp. Monocultures of the tested legume crops affected the inoculum potential of A. euteiches differently. Pea, broad bean, snap bean, vetch and sweet clover were almost equally efficient in inoculum build-up and markedly affected subsequent pea and broad bean yields. Monocultures of alfalfa, birdsfoot trefoil, red, white, and Persian clover had lower impact on disease development and yields in subsequent crops. Thielaviopsis basicola and Rhizoctonia solani were the most prevalent pathogens in pea and snap bean fields in Spain and significantly affected yield. Climatic factors and soil properties favoured prevalence of these pathogens in Spain, whereas A. euteiches was most prevalent under Swedish conditions. It is concluded that intensive legume cropping will, on many soil types not be sustainable in the long-term due to the build-up of soil-borne pathogen inoculum.
A total of 66 plants in 50 species were inoculated with eggs and juveniles of soybean cyst nematode, Heterodera glycines. Roots were stained and observed for penetration and development of the nematode. Twenty-six plants were not penetrated; twenty-three were penetrated, but there was no development of the nematode; eight were penetrated with some nematode development; two were penetrated and had considerable nematode development, but few nematodes, if any, matured; and seven were penetrated with many nematodes maturing. The penetration of nonhosts may imply some susceptibility and that populations eventually would build up on the penetrated plants. Plants not penetrated may be useful as rotation plants because no reproduction would occur.
Experiments were established in field microplots to examine the association between Heterodera glycines and the blue form of Fusarium solani in sudden death syndrome of soybean (SDS). Foliar disease symptoms occurred on more plants per plot, appeared 3 to 7 days earlier, and were more severe on plants grown in plots infested with F. solani + H. glycines than on those inoculated with F. solani only. Yields were suppressed only in treatments that included the nematode. Numbers of H. glycines cysts and second-stage juveniles were significantly lower in plots infested with F. solani + H. glycines than with the nematode alone. Fusarium solani was able to infect cysts and eggs.
The effects of the blue form of Fusarium solani, the causal agent of sudden death syndrome (SDS), on Heterodera glycines were examined in the greenhouse. Roots of soybean cv. Coker 156 were inoculated with either H. glycines alone or F. solani + H. glycines in combination. Population levels of H. glycines were reduced 47% in the presence of F. solani. Life-stage development of H. glycines increased 3% in 30 days in the presence of F. solani. Fusarium solani colonized epidermal and cortical cells adjacent to developing juveniles of H. glycines and the nematode-induced syncytia within the soybean root tissue. At 40 days after inoculation, F. solani was isolated from 37% of the cysts in soil recovered from the F. solani + H. glycines combination treatment. Fusarium solani significantly affected H. glycines population density, life-stage development, and succeeding populations.
ABSTRACT The soybean cyst nematode, Heterodera glycines, and the fungus that causes sudden death syndrome (SDS) of soybean, Fusarium solani f. sp. glycines, frequently co-infest soybean (Glycine max) fields. The interactions between H. glycines and F. solani f. sp. glycines were investigated in factorial greenhouse experiments with different inoculum levels of both organisms on a soybean cultivar susceptible to both pathogens. Measured responses included root and shoot dry weights, H. glycines reproduction, area under the SDS disease progress curve, and fungal colonization of roots. Both H. glycines and F. solani f. sp. glycines reduced the growth of soybeans. Reproduction of H. glycines was suppressed by high inoculum levels but not by low levels of F. solani f. sp. glycines. The infection of soybean roots by H. glycines did not affect root colonization by the fungus, as determined by real-time polymerase chain reaction. Although both pathogens reduced the growth of soybeans, H. glycines did not increase SDS foliar symptoms, and statistical interactions between the two pathogens were seldom significant.
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