Sudden Death Syndrome Symptoms Research Articles

Exploring soybean cultivar susceptibility to sudden death syndrome: Insights into isoflavone responses and biocontrol potential

Sudden Death Syndrome (SDS) caused by Fusarium tucumaniae is a significant threat to soybean production in Argentina. This study assessed the susceptibility of SY 3 × 7 and SPS 4 × 4 soybeans cultivars to F. tucumaniae and studied changes in root isoflavone levels after infection. Additionally, the biocontrol potential of plant-growth promoting rhizobacteria (PGPR) against SDS was also examined. Our results demonstrated that the SY 3 × 7 cultivar exhibited higher disease severity and total fresh weight loss than SPS 4 × 4. Both cultivars showed induction of daidzein, glycitein, and genistein in response to infection, with the partially resistant cultivar displaying significantly higher daidzein levels than the susceptible cultivar at 14 days post infection (dpi) (2.74 vs 2.17-fold), declining to a lesser extent at 23 dpi (0.94 vs 0.35-fold, respectively). However, daidzein was not able to inhibit F. tucumaniae growth in in vitro assays probably due to its conversion to an isoflavonoid phytoalexin which would ultimately be an effective fungal inhibitor. Furthermore, the PGPR bacterium Bacillus amyloliquefaciens BNM340 displayed antagonistic activity against F. tucumaniae and reduced SDS symptoms in infected plants. This study sheds light on the varying susceptibility of soybean cultivars to SDS, offers insights into isoflavone responses during infection, and demonstrates the potential of PGPR as a biocontrol strategy for SDS management, providing ways for disease control in soybean production.

Integration of Host Resistance, Seed Treatment, and Seeding Rate for Management of Sudden Death Syndrome, a Disease of Soybean Caused by Fusarium virguliforme

Field experiments were conducted in Illinois, Indiana, Iowa, Michigan, and Wisconsin, United States, and Ontario, Canada, in 2019 and 2020 to evaluate the integrated effects of host resistance, seed treatment, and seeding rates on root rot (RR) and foliar symptoms of sudden death syndrome (foliar disease index [FDX]) and soybean yield. Seed treatments included a nontreated control and fluopyram in 2019. In 2020, commercial base treatment, base + fluopyram, and base + pydiflumetofen were tested. The base treatment included metalaxyl + pyraclostrobin + fluxapyroxad + clothianidin. The 2019 nontreated control and the 2020 base treatment were considered controls in the analysis because previous studies showed that base treatments do not provide control for sudden death syndrome. The seed treatments were tested on susceptible and moderately resistant (MR) cultivars, which were planted at three seeding rates: 272,277, 346,535, and 420,792 seeds/ha. To mitigate concern that disease pressure may impact treatment, three high disease pressure (>20% FDX) site-years out of the 15 total site-years were grouped and analyzed separately. Seed treatment with fluopyram or pydiflumetofen both reduced FDX and protected yield. Fluopyram reduced RR by about 10%, but RR was not different between pydiflumetofen and the base treatment in 2020. Both seed treatments reduced FDX, but reduction was greater for fluopyram (43.2%) than for pydiflumetofen (24.3%) based on 2020 results. Seeding rate had no effect on foliar symptoms, but the highest seeding rate showed increased RR in 2019 and greater yield both years. Performance of MR cultivars was inconsistent across both years. In 2019, MR cultivars reduced RR by 8.9%; however, in 2020, the MR cultivar had more RR than the susceptible cultivar. Additionally, FDX was only reduced in the MR cultivar in 2020. Although host resistance and seeding rate did not individually impact disease development and yield in every site-year, we showed that integrating seed treatment, host resistance, and adequate seeding rates helped maximize yield in fields with sudden death syndrome.

Deep learning-based phenotyping for genome wide association studies of sudden death syndrome in soybean.

Using a reliable and accurate method to phenotype disease incidence and severity is essential to unravel the complex genetic architecture of disease resistance in plants, and to develop disease resistant cultivars. Genome-wide association studies (GWAS) involve phenotyping large numbers of accessions, and have been used for a myriad of traits. In field studies, genetic accessions are phenotyped across multiple environments and replications, which takes a significant amount of labor and resources. Deep Learning (DL) techniques can be effective for analyzing image-based tasks; thus DL methods are becoming more routine for phenotyping traits to save time and effort. This research aims to conduct GWAS on sudden death syndrome (SDS) of soybean [Glycine max L. (Merr.)] using disease severity from both visual field ratings and DL-based (using images) severity ratings collected from 473 accessions. Images were processed through a DL framework that identified soybean leaflets with SDS symptoms, and then quantified the disease severity on those leaflets into a few classes with mean Average Precision of 0.34 on unseen test data. Both visual field ratings and image-based ratings identified significant single nucleotide polymorphism (SNP) markers associated with disease resistance. These significant SNP markers are either in the proximity of previously reported candidate genes for SDS or near potentially novel candidate genes. Four previously reported SDS QTL were identified that contained a significant SNPs, from this study, from both a visual field rating and an image-based rating. The results of this study provide an exciting avenue of using DL to capture complex phenotypic traits from images to get comparable or more insightful results compared to subjective visual field phenotyping of traits for disease symptoms.

Effect of Seed Treatment and Foliar Crop Protection Products on Sudden Death Syndrome and Yield of Soybean.

Sudden death syndrome (SDS), caused by Fusarium virguliforme, is an important soilborne disease of soybean. Risk of SDS increases when cool and wet conditions occur soon after planting. Recently, multiple seed treatment and foliar products have been registered and advertised for management of SDS but not all have been tested side by side in the same field experiment at multiple field locations. In 2015 and 2016, seed treatment fungicides fluopyram and thiabendazole; seed treatment biochemical pesticides citric acid and saponins extract of Chenopodium quinoa; foliar fungicides fluoxastrobin + flutriafol; and an herbicide, lactofen, were evaluated in Illinois, Indiana, Iowa, Michigan, South Dakota, Wisconsin, and Ontario for SDS management. Treatments were tested on SDS-resistant and -susceptible cultivars at each location. Overall, fluopyram provided the highest level of control of root rot and foliar symptoms of SDS among all the treatments. Foliar application of lactofen reduced foliar symptoms in some cases but produced the lowest yield. In 2015, fluopyram reduced the foliar disease index (FDX) by over 50% in both resistant and susceptible cultivars and provided 8.9% yield benefit in susceptible cultivars and 3.5% yield benefit in resistant cultivars compared with the base seed treatment (control). In 2016, fluopyram reduced FDX in both cultivars by over 40% compared with the base seed treatment. For yield in 2016, treatment effect was not significant in the susceptible cultivar while, in the resistant cultivar, fluopyram provided 3.5% greater yield than the base seed treatment. In this study, planting resistant cultivars and using fluopyram seed treatment were the most effective tools for SDS management. However, plant resistance provided an overall better yield-advantage than using fluopyram seed treatment alone. Effective seed treatments can be an economically viable consideration to complement resistant cultivars for managing SDS.

Soybean Sudden Death Syndrome Causal Agent Fusarium brasiliense Present in Michigan.

Sudden death syndrome (SDS), caused by members of Fusarium solani species complex (FSSC) clade 2, is a major and economically important disease in soybean worldwide. The primary causal agent of SDS isolated to date in North America has been F. virguliforme. In 2014 and 2016, SDS symptoms were found in two soybean fields located on the same farm in Michigan. Seventy Fusarium strains were isolated from roots of the SDS-symptomatic soybeans in two fields. Phylogenetic analysis of partial sequences of elongation factor-1α, the nuclear ribosomal DNA intergenic spacer region, and the RNA polymerase II beta subunit revealed that the primary FSSC species isolated was F. brasiliense (58 and 36% in each field) and the remaining Fusarium strains were identified as F. cuneirostrum, F. phaseoli, an undescribed Fusarium sp. from FSSC clade 2, and strains in FSSC clade 5 and FSSC clade 11. Molecular identification was supported with morphological analysis and a pathogenicity assay. The soybean seedling pathogenicity assay indicated that F. brasiliense was capable of causing typical foliar SDS symptoms. Both root rot and foliar disease severity were variable by strain, just as they are in F. virguliforme. Both FSSC 5 and FSSC 11 strains were also capable of causing root rot, but SDS foliar symptoms were not detected. To our knowledge, this is the first report of F. brasiliense causing SDS in soybean in the United States and the first report of F. cuneirostrum, F. phaseoli, an as-yet-unnamed Fusarium sp., and strains in FSSC clade 5 and FSSC clade 11 associated with or causing root rot of soybean in Michigan.

Interaction Between Seed Treatment and Variety on Sudden Death Syndrome Symptoms and Soybean Yield

Interaction Between Seed Treatment and Variety on Sudden Death Syndrome Symptoms and Soybean Yield

Quantitative trait loci underlying host responses of soybean to Fusarium virguliforme toxins that cause foliar sudden death syndrome.

Soybean deploys multiple genetic mechanisms to confer tolerance to Fusarium virguliforme toxins. This study revealed that F. virguliforme culture filtrates could be used in mapping QTL underlying foliar SDS resistance. Sudden death syndrome (SDS) is a major soybean disease throughout most of the soybean growing regions in the world including the United States. The disease is caused by the fungal pathogen, Fusarium virguliforme (Fv). The fungus produces several toxins that are responsible for development of interveinal leaf chlorosis and necrosis, which are typical foliar SDS symptoms. Growing of resistant cultivars has been the most effective method in controlling the disease. The objective of the present study was to identify quantitative trait loci (QTL) underlying host responses of soybean to Fv toxins present in culture filtrates. To accomplish this objective, two recombinant inbred line (RIL) populations, AX19286 (A95-684043 × LS94-3207) and AX19287 (A95-684043 × LS98-0582), segregating for SDS resistance were evaluated for foliar symptom development by applying two screening protocols, the stem cutting and the root feeding assays. The AX19286 population revealed two major and seven minor QTL for SDS resistance. In the AX19287 population, we identified five major QTL and three minor QTL. The two QTL mapped to Chromosome 7 [molecular linkage group (MLG) M] and Chromosome 20 (MLG I) are most likely novel, and were detected through screening of the AX19287 population with stem cutting and root feeding assays, respectively. This study established that Fv culture filtrates could be employed in mapping QTL underlying foliar SDS resistance. The outcomes of the research also suggest that multiple genetic mechanisms might be used by soybean to overcome the toxic effects of the toxins secreted by the pathogen into culture filtrates.

Effect of fungicide seed treatments on Fusarium virguliforme infection of soybean and development of sudden death syndrome

Sudden death syndrome (SDS), caused by Fusarium virguliforme (Fv), is a major yield-limiting disease of soybean in North America. Infection of soybean seedling roots by Fv results in severe root damage; therefore, fungicide seed treatments could potentially reduce these early-season infections and reduce severity of foliar symptoms that typically occur later in the season. Multiple fungicide seed treatment combinations were evaluated for their effects on Fv infection, DNA concentrations in roots, soybean root development, and SDS development in the field, greenhouse and laboratory trials. Several seed treatments decreased root disease symptoms compared with the non-treated inoculated control in the laboratory assay, and the biological seed treatment, Bacillus pumilus, significantly decreased seedling development and increased SDS root disease compared with the non-treated inoculated control. In the greenhouse, Fv DNA concentrations in roots were reduced by a treatment combining mefenoxam + thiophanate-methyl + azoxystrobin + Bacillus pumilus + prothioconazole + fludioxonil compared with the non-treated control; however, the reduction in Fv DNA did not improve root growth or decrease SDS symptoms compared with the non-treated control. Field trials were conducted in Valmeyer, IL in 2008 and in Urbana, IL in 2008 and 2009. Seed treatments had no effect on the concentration of Fv DNA in soybean roots and had very little effect on root morphology. At the Valmeyer location, most seed treatments significantly decreased SDS symptoms compared with the control. In summary, no consistent, significant effects of the seed treatments evaluated in this study on SDS or Fv root infection were observed. Therefore, soybean growers should continue to utilize other practices for SDS management until new seed treatments with consistent efficacy in controlling SDS are available.

Effects of an Experimental Seed Treatment from DuPont on Sudden Death Syndrome Symptoms and Soybean Yield

Sudden death syndrome (SDS) is a soybean disease that perennially limits yields in the Kansas River Valley. Soybean cyst nematode (SCN) and saturated soils contribute to the severity of the disease. Selecting varieties with some degree of tolerance to SDS is the only cultural practice that can reduce the severity of SDS and improve yields. Variety selection alone, however, doesn’t necessarily make soybean production profitable; an added complication is managing irrigation scheduling to avoid saturated soils. A study with seed treatments applied to soybean was conducted at the Kansas River Valley Experiment Field in 2014, with treatments applied to two soybean varieties susceptible to SDS. The study was irrigated earlier and more often than normal for soybean to promote the disease. In the most severely infested plots, more than 50% of the leaf area expressed symptoms of SDS by the R6 growth stage. Treatments with an experimental seed treatment from DuPont (Wilmington, DE) reduced the amount of foliar disease in all varieties and increased yields up to 10 bu/a, or more than 25%.

Effects of Seed Treatment on Sudden Death Syndrome Symptoms and Soybean Yield

Sudden death syndrome (SDS) is a soybean disease that perennially limits yields in the Kansas River Valley. The presence of soybean cyst nematode (SCN) and saturated soils have been implicated in contributing to the severity of the disease. Selecting varieties with some degree of tolerance to SDS is the only cultural practice that can potentially reduce the severity of SDS and improve yields. Variety selection alone, however, cannot improve the production of soybeans to make them profitable. The challenge of trying to manage irrigation scheduling to avoid saturated soils further complicates efforts to increase productivity with irrigation while still avoiding SDS. A study with seed treatments applied to soybean was conducted at the Kansas River Valley Experiment Field in 2014, with treatments applied to a soybean variety with a high level of tolerance to SDS. The study was irrigated earlier and more often than normal to promote the disease. The most severely infested plots had more than 50% of the leaf area expressing symptoms of SDS by the R6 growth stage. Treatments with ILeVO from Bayer CropScience (Research Triangle Park, NC) reduced foliar symptoms and increased yields up to 12 bu/a, or more than 25%. These results are similar to those in a 2013 study of varieties with SDS tolerance ranging from very susceptible to more tolerant; the yield increase was up to 16 bu/a, or 40% with the ILeVO seed treatment.

Contributions of Fusarium virguliforme and Heterodera glycines to the disease complex of sudden death syndrome of soybean.

BackgroundSudden death syndrome (SDS) of soybean caused by Fusarium virguliforme spreads and reduces soybean yields through the North Central region of the U.S. The fungal pathogen and Heterodera glycines are difficult to manage.Methodology/Principal FindingsThe objective was to determine the contributions of H. glycines and F. virguliforme to SDS severity and effects on soybean yield. To quantify DNA of F. virguliforme in soybean roots and soil, a specific real time qPCR assay was developed. The assay was used on materials from soybean field microplots that contained in a four-factor factorial-design: (i) untreated or methyl bromide-fumigated; (ii) non-infested or infested with F. virguliforme; (iii) non-infested or infested with H. glycines; (iv) natural precipitation or additional weekly watering. In years 2 and 3 of the trial, soil and watering treatments were maintained. Roots of soybean ‘Williams 82’ were collected for necrosis ratings at the full seed growth stage R6. Foliar symptoms of SDS (area under the disease progress curve, AUDPC), root necrosis, and seed yield parameters were related to population densities of H. glycines and the relative DNA concentrations of F. virguliforme in the roots and soil. The specific and sensitive real time qPCR was used. Data from microplots were introduced into models of AUDPC, root necrosis, and seed yield parameters with the frequency of H. glycines and F. virguliforme, and among each other. The models confirmed the close interrelationship of H. glycines with the development of SDS, and allowed for predictions of disease risk based on populations of these two pathogens in soil.Conclusions/SignificanceThe results modeled the synergistic interaction between H. glycines and F. virguliforme quantitatively in previously infested field plots and explained previous findings of their interaction. Under these conditions, F. virguliforme was mildly aggressive and depended on infection of H. glycines to cause highly severe SDS.

First Report of Soybean Sudden Death Syndrome Caused by Fusarium virguliforme in South Africa

Soybean (Glycine max (L.) Merr.) is an important crop in many countries and production is currently increasing (from 311,450 ha in 2010 to 516,500 ha in 2013) in South Africa. On 27 February 2013 in the Lydenburg/Badfontein area, Mpumalanga Province, on a no-till commercial farm planted to soybean cultivar PAN 737 (Roundup Ready, maturity group 7) under irrigation for a second consecutive season, leaf symptoms typical of soybean sudden death syndrome were observed and reported by a farmer (3). The symptoms developed at the R6 growth stage (near physiological maturity) of the soybean plants. Leaf symptoms were interveinal chlorotic blotches that became necrotic while the veins remained green. These symptoms appeared throughout the plant but were most severe on the top leaves. Some of the severely affected leaflets dropped off with the petioles remaining attached to the plant. The vascular tissue in the upper taproot and lower stem turned gray-brown, but the pith remained white. Roots of the affected plants had decayed lateral roots. Surface disinfested root pieces with rot symptoms and spores directly from blue sporodochia on the rotten root were plated on potato dextrose agar amended with novostreptomycin 0.04 g/L (PDA+). Slow growing Fusarium isolates with blue to purple masses of sporodochia were consistently obtained from diseased plants. Cultures were single-spored and plated on PDA+. Growth rate of cultures on PDA+ was on average 6 to 9 mm after 5 days at 20°C. The morphology of the isolates fit the description of Fusarium virguliforme in Aoki et al. (1). Sequence analyses of the nuclear ribosomal internal transcribed spacer (ITS) and partial translation elongation factor (EF-1a) gene of the recovered eight isolates revealed that these isolates matched 99.6% with F. virguliforme O'Donnell & T. Aoki (Accession Nos. KF648835 to KF648850), one of the soybean sudden death syndrome causing species found in North and South America (1). All isolates are identical in each loci except that three isolates had one nucleotide deletion and two insertions at the EF-1a loci. The isolates are deposited at the national culture collection in Pretoria (PPRI13434 to PPRI13441). A glasshouse bioassay was conducted to test the pathogenicity of eight single-spored isolates by inoculating pasteurized planting medium (1:1:1 ratio of sand, perlite, and soil) with a layer of infested sand-bran medium (2) to each pot (13 cm in diameter) and covered with 2 cm of planting medium (4) after planting 20 seeds of soybean cultivar PAN 737. There were three pots per isolate randomized in a complete block design trial. All the South African F. virguliforme isolates tested induced leaf and root rot symptoms of sudden death syndrome on the soybean seedlings under glasshouse conditions after 4 weeks of inoculation. The fungus was re-isolated on PDA+ from diseased roots of the soybean seedlings to fulfill Koch's postulates. This is the first record of F. virguliforme in South Africa, and as an important component of soilborne diseases of soybean it may pose a major threat to the South African soybean industry.

Detecção de Fusarium spp. e Trichoderma spp. e antagonismo de Trichoderma sp. em soja sob plantio direto

This study aimed i) to quantify the occurrence of Fusarium spp. and Trichoderma spp. in rhizospheric soil, with and without symptoms of Sudden Death Syndrome (SDS) in eight soybean genotypes; ii) morphologically identify isolates of Fusarium spp. from roots with SDS; iii) evaluate the antagonism between Trichoderma spp. and Fusarium spp. isolates from rhizospheric soil and roots from with and without SDS, respectively; and iv) characterize through the ITS1-5.8S-ITS2 region of rDNA the isolates of Trichoderma spp. with better performance in the direct confrontation. The sampling of soil and roots was performed in an experimental area located in Cruz Alta, RS, Brazil. In the laboratory, serial dilutions of soil samples, counting of the number of Colony Forming Units (UFCs/g-1 of rhizospheric soil) were performed as well as isolation for identification of isolates of Fusarium spp. and Trichoderma spp. and testing of direct confrontation. There were significant differences between the population of Trichoderma spp. in the rhizosphere of plants with and without symptoms of SDS. For the population of Fusarium spp., significant difference was observed only in the rhizosphere of plants without symptoms of SDS. In diseased roots the following species were identified: F. solani, F. avenaceum, F. graminearum, F. oxysporum and F. verticillioides. In the test of direct confrontation, eight isolates of Trichoderma spp. achieved the best performance in the antagonism to Fusarium spp. and Trichoderma spp. from areas with symptoms of SDS had a higher control efficiency in vitro. These isolates showed high similarity to the species of T. koningii agregate.

Detecção de Fusarium spp. e Trichoderma spp. e antagonismo de Trichoderma sp. em soja sob plantio direto

This study aimed i) to quantify the occurrence of Fusarium spp. and Trichoderma spp. in rhizospheric soil, with and without symptoms of Sudden Death Syndrome (SDS) in eight soybean genotypes; ii) morphologically identify isolates of Fusarium spp. from roots with SDS; iii) evaluate the antagonism between Trichoderma spp. and Fusarium spp. isolates from rhizospheric soil and roots from with and without SDS, respectively; and iv) characterize through the ITS1-5.8S-ITS2 region of rDNA the isolates of Trichoderma spp. with better performance in the direct confrontation. The sampling of soil and roots was performed in an experimental area located in Cruz Alta, RS, Brazil. In the laboratory, serial dilutions of soil samples, counting of the number of Colony Forming Units (UFCs/g-1 of rhizospheric soil) were performed as well as isolation for identification of isolates of Fusarium spp. and Trichoderma spp. and testing of direct confrontation. There were significant differences between the population of Trichoderma spp. in the rhizosphere of plants with and without symptoms of SDS. For the population of Fusarium spp., significant difference was observed only in the rhizosphere of plants without symptoms of SDS. In diseased roots the following species were identified: F. solani, F. avenaceum, F. graminearum, F. oxysporum and F. verticillioides. In the test of direct confrontation, eight isolates of Trichoderma spp. achieved the best performance in the antagonism to Fusarium spp. and Trichoderma spp. from areas with symptoms of SDS had a higher control efficiency in vitro. These isolates showed high similarity to the species of T. koningii agregate.

Synergism in the interaction of Fusarium virguliforme with Heterodera glycines in sudden death syndrome of soybean

Sudden death syndrome (SDS) of soybean caused by Fusarium virguliforme has increased in geographical distribution in the North Central region of the U.S. The disease often co-occurs with the widely distributed Heterodera glycines. The extensiveness of the interaction of SDS and H. glycines at different soil moisture levels is not fully defined. Further, it is not clear if Meloidogyne spp., that recently is more frequently observed in soybean areas, forms a disease complex with F. virguliforme. In a multi-factor factorial-designed mi-croplot experiment, SDS symptoms and yield parameters were determined following infestation with F. virguliforme and either H. glycines or M. incognita at different soil moisture regimes in different microbial backgrounds rendered by soil fumigation. On soybean ‘Williams 82’, susceptible to the three pathogens, H. glycines increased severity of foliar symptoms of SDS irrespective of being naturally present or added to previously fumigated soil, while yield parameters declined in the plots co-infested with F. virguliforme. This increase was synergistic in some contexts as determined by Gowing’s formula. In pre-season-fumigated soil, only plants in plots concomitantly inoculated with H. glycines and F. virguliforme had more severe SDS than plots with no nematode or M. incognita amendment when the plots were heavily watered (P < 0.01). No synergism between F. virguliforme and M. incognita was detected. The results provided the conceptual framework of the synergistic interaction of H. glycines and F. virguliforme in a disease complex under different soil moisture conditions.

Investigation of the Fusarium virguliforme fvtox1 mutants revealed that the FvTox1 toxin is involved in foliar sudden death syndrome development in soybean

The soil borne fungus, Fusarium virguliforme, causes sudden death syndrome (SDS) in soybean, which is a serious foliar and root rot disease. The pathogen has never been isolated from the diseased foliar tissues; phytotoxins produced by the pathogen are believed to cause foliar SDS symptoms. One of these toxins, a 13.5-kDa acidic protein named FvTox1, has been hypothesized to interfere with photosynthesis in infected soybean plants and cause foliar SDS. The objective of this study is to determine if FvTox1 is involved in foliar SDS development. We created and studied five independent knockout fvtox1 mutants to study the function of FvTox1. We conducted Agrobacterium tumefaciens-mediated transformation to accomplish homologous recombination of FvTox1 with a hygromycin B resistance gene, hph, to generate the fvtox1 mutants. Approximately 40 hygromycin-resistant transformants were obtained from 10(6) conidial spores of the F. virguliforme Mont-1 isolate when the spores were co-cultivated with the A. tumefaciens EHA105 but not with LBA4044 strain carrying a recombinant binary plasmid, in which the hph gene encoding hygromycin resistance was flanked by 5'- and 3'-end FvTox1 sequences. We observed homologous recombination-mediated integration of hph into the FvTox1 locus among five independent fvtox1 mutants. In stem-cutting assays using cut soybean seedlings fed with cell-free F. virguliforme culture filtrates, the knockout fvtox1 mutants caused chlorophyll losses and foliar SDS symptoms, which were over twofold less than those caused by the virulent F. virguliforme Mont-1 isolate. Similarly, in root inoculation assays, more than a twofold reduction in foliar SDS development and chlorophyll losses was observed among the seedlings infected with the fvtox1 mutants as compared to the seedlings infected with the wild-type Mont-1 isolate. These results suggest that FvTox1 is a major virulence factor involved in foliar SDS development in soybean. It is expected that interference of the function of this toxin in transgenic soybean plants will lead to generation of SDS-resistant soybean cultivars.

The receptor like kinase at Rhg1-a/Rfs2 caused pleiotropic resistance to sudden death syndrome and soybean cyst nematode as a transgene by altering signaling responses.

BackgroundSoybean (Glycine max (L. Merr.)) resistance to any population of Heterodera glycines (I.), or Fusarium virguliforme (Akoi, O’Donnell, Homma & Lattanzi) required a functional allele at Rhg1/Rfs2. H. glycines, the soybean cyst nematode (SCN) was an ancient, endemic, pest of soybean whereas F. virguliforme causal agent of sudden death syndrome (SDS), was a recent, regional, pest. This study examined the role of a receptor like kinase (RLK) GmRLK18-1 (gene model Glyma_18_02680 at 1,071 kbp on chromosome 18 of the genome sequence) within the Rhg1/Rfs2 locus in causing resistance to SCN and SDS.ResultsA BAC (B73p06) encompassing the Rhg1/Rfs2 locus was sequenced from a resistant cultivar and compared to the sequences of two susceptible cultivars from which 800 SNPs were found. Sequence alignments inferred that the resistance allele was an introgressed region of about 59 kbp at the center of which the GmRLK18-1 was the most polymorphic gene and encoded protein. Analyses were made of plants that were either heterozygous at, or transgenic (and so hemizygous at a new location) with, the resistance allele of GmRLK18-1. Those plants infested with either H. glycines or F. virguliforme showed that the allele for resistance was dominant. In the absence of Rhg4 the GmRLK18-1 was sufficient to confer nearly complete resistance to both root and leaf symptoms of SDS caused by F. virguliforme and provided partial resistance to three different populations of nematodes (mature female cysts were reduced by 30–50%). In the presence of Rhg4 the plants with the transgene were nearly classed as fully resistant to SCN (females reduced to 11% of the susceptible control) as well as SDS. A reduction in the rate of early seedling root development was also shown to be caused by the resistance allele of the GmRLK18-1. Field trials of transgenic plants showed an increase in foliar susceptibility to insect herbivory.ConclusionsThe inference that soybean has adapted part of an existing pathogen recognition and defense cascade (H.glycines; SCN and insect herbivory) to a new pathogen (F. virguliforme; SDS) has broad implications for crop improvement. Stable resistance to many pathogens might be achieved by manipulation the genes encoding a small number of pathogen recognition proteins.

The Fusarium virguliforme Toxin FvTox1 Causes Foliar Sudden Death Syndrome-Like Symptoms in Soybean

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.

Effects of crop rotation of soybean with corn on severity of sudden death syndrome and population densities of Heterodera glycines in naturally infested soil

Sudden death syndrome, caused by Fusarium virguliforme, and the soybean cyst nematode, Heterodera glycines, combined cause the highest yield losses in soybean. The objective of this study was to determine the effectiveness of corn rotated annually with soybean on reducing severity of sudden death syndrome (SDS) and if such crop rotation is beneficial to soybean root health and thus improves disease management strategies. Experiments were conducted from 2003 to 2006 through two cycles of a corn–soybean rotation on two commercial fields in Indiana. With one exception, the rotation of soybean with corn did not provide yield benefits compared to monoculture of soybean. Severity of foliar and root symptoms of SDS in rotation plots were never less than in soybean monoculture plots. At one location, soybean monoculture resulted in suppression of SDS compared to the corn–soybean rotation, while H. glycines reproduced freely. At the other location, monoculture of soybean resulted in suppressiveness against H. glycines, while SDS was limited in all treatments . The data suggest that soil suppressiveness can independently impact the pathogens that are important in SDS development. Because H. glycines can increase SDS symptoms, its suppression may also reduce severity of SDS. Current production systems consisting of yearly rotation of soybean with corn are highly vulnerable to the development of severe soil-borne disease complexes. The simple year-to-year rotation of corn and soybean is not considered sustainable. While monoculture of soybean resulted in some disease suppression in these trials, reliance on monoculture may be detrimental due to other environmental considerations beyond the scope of these trials. Including other crops may be beneficial in improving the sustainability of soybean and corn production systems.

Comparative analysis and characterization of the soybean sudden death syndrome pathogen Fusarium virguliforme in the northern United States

Sudden death syndrome (SDS), caused by Fusarium virguliforme, has occurred in several US states and Ontario since 1985 but is a new problem in the northern states, including Minnesota. The characteristics and distribution of F. virguliforme and SDS in Minnesota and other northern soybean production areas were unknown. In 2006 and 2007, SDS was confirmed in 21 counties from the eastern to western borders of Minnesota and to 45°N based on symptoms and a polymerase chain reaction assay. It was also confirmed that F. virguliforme and Phialophora gregata, the cause of brown stem rot, can simultaneously infect a single plant. Isolates of F. virguliforme from soybeans in Minnesota were morphologically identical to those from other states and induced typical SDS symptoms on soybean seedlings. Isolates (n = 15) obtained from 15 fields in 10 Minnesota counties were compared genotypically and phenotypically to a group of 13 isolates of F. virguliforme from other states. Genetic similarity was very high among isolates from Minnesota, Iowa, Missouri, and Illinois based on DNA sequences from the intergenic spacer region, the translation elongation factor 1α gene, and the internal transcribed spacer region. Growth rates and production of macroconidia at several temperatures differed among isolates. Virulence varied among isolates from different fields and states based on disease reactions on susceptible and moderately resistant soybean cultivars in greenhouse tests. Results support the hypothesis that SDS in Minnesota may stem from the establishment of a clonal US population of F. virguliforme that is spreading north and west, thus increasing production risk for soybean grown in northern states. Key words: Fusarium virguliforme, Fusarium solani, root rot, population biology, DNA sequences, soybean. Le syndrome de la mort subite (SMS), causé par Fusarium virguliforme, a été documenté dans plusieurs États américains et en Ontario depuis 1985, mais dans les États du nord, incluant le Minnesota, c’est un problème récent. Les caractéristiques et les aires de distribution de F. virguliforme et du SMS au Minnesota et dans les autres régions productrices de soja du nord étaient inconnues. En 2006 et 2007, la présence du SMS a été confirmée, par reconnaissance des symptômes et PCR, dans 21 comtés du Minnesota s’étendant de l’est à l’ouest de l’État, et ce, jusqu’au 45° de latitude Nord. On a également confirmé que F. virguliforme et Phialophora gregata, responsable de la pourriture brune des tiges, peuvent infecter simultanément un même plant. Des isolats de F. virguliforme provenant de plants de soja du Minnesota étaient morphologiquement identiques à ceux provenant d’autres États et induisaient l’apparition des symptômes typiques du SMS chez les plantules de soja. Des isolats (n = 15) obtenus de 15 champs situés dans 10 comtés du Minnesota ont été comparés, sur les plans génotypiques et phénotypiques, à 13 isolats de F. virguliforme provenant d’autres États. En ce qui a trait aux séquences d’ADN extraites de la région de l’espaceur intergénique, du gène du facteur d’élongation 1α de la traduction et de la région de l’espaceur transcrit interne, elles présentaient une similarité génétique frappante malgré que les isolats provenaient du Minnesota, de l’Iowa, du Missouri et de l’Illinois. Par ailleurs, le taux de croissance ainsi que la production de macroconidies variaient chez les isolats exposés à des conditions différentes de température. En outre, selon des tests visant à évaluer les réactions à la maladie, tests effectués en serre sur des cultivars de soja réceptifs et moyennement réfractaires, le taux de virulence variait aussi chez les isolats provenant de différents champs et de divers États. Les résultats supportent l’hypothèse voulant qu’au Minnesota le SMS provienne de l’établissement d’une population clonale états-unienne de F. virguliforme qui s’étend vers le nord et l’ouest, ce qui accroît les risques relatifs à la production de soja dans les États du nord. Mots-clés : Fusarium virguliforme, Fusarium solani, pourridié, biologie des populations, séquences d’ADN, soja.