James E. Kurle

Efficacy of Fungicides on Sclerotinia sclerotiorum and Their Potential for Control of Sclerotinia Stem Rot on Soybean

Sclerotinia stem rot of soybean, caused by Sclerotinia sclerotiorum, is a major disease in the north central region of the United States. One approach to managing Sclerotinia stem rot on soybean is the use of fungicides. S. sclerotiorum was assayed for sensitivity to benomyl, tebuconazole, thiophanate methyl, and vinclozolin in pure cultures on agar medium, inoculated soybean seedlings, detached inoculated leaves, and in experimental field plots. To evaluate the inhibitory effect of four fungicides on growth of S. sclerotiorum in vitro, potato dextrose agar (PDA) was amended with the fungicides at six concentrations. Based on measurements of fungal radial growth, vinclozolin was the most effective in inhibiting S. sclerotiorum mycelial growth at 1.0 μg a.i./ml of PDA. Ranges of reduction of radial growth of 91 isolates of S. sclerotiorum on PDA amended with thiophanate methyl and vinclozolin were 18 to 93% and 93 to 99%, respectively, when compared with the nonamended agar control. Benomyl, thiophanate methyl, and vinclozolin applied to greenhouse-grown seedlings prevented S. sclerotiorum from expressing symptoms or signs on leaf tissue. Detached leaves sprayed with thiophanate methyl and then inoculated with mycelial plugs of S. sclerotiorum did not express symptoms or signs. Of 13 different environments in Illinois, Indiana, Ohio, and Wisconsin from 1995 through 2000, six had low Sclerotinia stem rot incidence (<1%), three environments had low to moderate Sclerotinia stem rot incidence (5 to 25%), and four environments had high Sclerotinia stem rot incidence (>25%). When disease incidence was high, no consistent control of Sclerotinia stem rot was observed with benomyl or thiophanate methyl using different application systems. However, under low disease incidence, spray systems that were able to penetrate the canopy reduced the incidence of Sclerotinia stem rot an average of 50%.

Introduction of Asian Soybean Rust Urediniospores into the Midwestern United States—A Case Study

In 2005, weekly rain samples collected at 124 National Atmospheric Deposition Program/National Trends Network (NADP/NTN) sites in the eastern and central United States were screened for Asian soybean rust (ASR; Phakopsora pachyrhizi) urediniospores. Application of a quantitative polymerase chain reaction method detected P. pachyrhizi DNA in the filter residue of rain samples collected during the week of 19 to 26 July 2005 in Minnesota, Missouri, and South Dakota. To determine the geographic origin of ASR urediniospores in those weekly composite samples, back air trajectories of the lifted condensation and mixed boundary layers were calculated for each rain event within the week, by sampling site. The calculations, based on the hybrid single-particle lagrangian integrated trajectory model, pointed to source areas in eastern and southern Texas. In a separate case, DNA of P. pachyrhizi was detected in a 28 June to 5 July 2005 rain sample from an eastern Texas site. Back trajectories pointed to southern Texas and the Yucatan Peninsula in Mexico as potential source areas of ASR urediniospores. Vertical motions of those back trajectories indicated a ventilation of the boundary layer in the upwind areas, suggesting the possible injection of urediniospores into the free troposphere where they can be transported for long distances before wet deposition.

Specific detection of Phialophora gregata and Plectosporium tabacinum in infected soybean plants using polymerase chain reaction

The fungal deuteromycetes Phialophora gregata and Plectosporium tabacinum are associated with soybean plants. P. gregata causes brown stem rot (BSR) of soybean, whereas P. tabacinum is a frequent cohabitant of soybean stems. The role of P. tabacinum in soybean growth and in the development of BSR is not known. Traditional methods of isolating and differentiating these two fungi require up to 3 weeks to complete. In order to effectively study the interactions among P. gregata, P. tabacinum, and the soybean plant, we developed specific primers for P. tabacinum based on its rDNA internal transcribed spacer sequences. In combination with specific primers developed previously for P. gregata, the specific primer pairs were used successfully in polymerase chain reactions to detect the targeted fungi in both artificially inoculated and naturally infected soybean plants. Using this technique, we examined 130 soybean plants collected from natural field environments for the presence or absence of P. gregata and P. tabacinum. Statistical analyses of the results showed that the frequency of co‐occurrence of P. gregata and P. tabacinum in soybean plants was significantly less than expected if the two fungi would occur independently, suggesting that one of the fungi may be inhibitory to the other fungus.

Association mapping and genomic prediction for resistance to sudden death syndrome in early maturing soybean germplasm

Sudden death syndrome (SDS), caused by Fusarium virguliforme, has spread to northern soybean growing regions in the US causing significant yield losses. The objectives of this study were to identify loci underlying variation in plant responses to SDS through association mapping (AM) and to assess prediction accuracy of genomic selection (GS) in a panel of early maturing soybean germplasm. A set of 282 soybean breeding lines was selected from the University of Minnesota soybean breeding program and then genotyped using a genome-wide panel of 1536 single-nucleotide polymorphism markers. Four resistance traits, root lesion severity (RLS), foliar symptom severity (FSS), root retention (RR), and dry matter reduction (DMR), were evaluated using soil inoculation in the greenhouse. AM identified significant peaks in genomic regions of known SDS resistance quantitative trait loci cqSDS001, cqRfs4, and SDS11-2. Additionally, two novel loci, one on chromosome 3 and another on chromosome 18, were tentatively identified. A ninefold cross-validation scheme was used to assess the prediction accuracy of GS for SDS resistance. The prediction accuracy of single-trait GS (ST-GS) was 0.64 for RLS, but less than 0.30 for RR, DMR, and FSS. Compared to ST-GS, none of multi-trait GS (MT-GS) models significantly improved the prediction accuracy due to weak correlations between the four traits. This study suggests both AM and GS hold promise for implementation in genetic improvement of SDS resistance in existing soybean breeding programs.

Symptomatic and Asymptomatic Host Range of Fusarium virguliforme, the Causal Agent of Soybean Sudden Death Syndrome

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 F. 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 F. 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.

Cultivar Preference and Genotype Distribution of the Brown Stem Rot Pathogen Phialophora gregata in the Midwestern United States

Brown stem rot (BSR), caused by Phialophora gregata f. sp. sojae, is an important yield-limiting disease of soybean (Glycine max) in the midwestern United States. Midwestern populations of P. gregata are separated into genotypes A and B based on intergenic spacer sequences of nuclear ribosomal DNA. Genotype A causes both leaf and stem symptoms, and genotype B typically causes internal stem symptoms only. Data are limited on the geographic distribution of genotypes A and B. It is not well understood whether cultivars may be infected preferentially by a genotype. Field plots were established at five sites in Illinois, three sites in Wisconsin, and two sites in Minnesota in two different years. Soybean cvs. Bell, BSR101, Dwight, Sturdy, Williams 82, LN92-12033, and LN92-12054 were sown with two to four replications at each field site. From each plot, 5 to 10 stems were harvested arbitrarily at the R8 growth stage and assayed by polymerase chain reaction to detect the A and B genotypes. Both pathogen genotypes were detected at all locations except Urbana, where only genotype A was detected, and St. Paul, where only B was detected. Genotype A was the predominant genotype detected in susceptible cvs. Williams 82 and LN92-12054, with 70 and 78% of infected stems, respectively, positive for A. The other susceptible cultivar, Sturdy, yielded predominantly genotype A at four of the seven Illinois and Wisconsin locations where both pathogen genotypes were present, but yielded predominantly B at the Minnesota location where both genotypes were detected. Genotype B was the predominant type detected in partially resistant cvs. Dwight, LN92-12033, and Bell, with 56, 85, and 99% of the infected stems, respectively, testing positive for B.

Interactions between iron-deficiency chlorosis and soybean cyst nematode in Minnesota soybean fields

Experiments were conducted in four commercial fields differing in severity of iron-deficiency chlorosis (IDC), and soybean cyst nematode (SCN) in Waseca and Lamberton, Minnesota to determine the interaction between the IDC and SCN. Each experiment was a randomized complete block with a factorial treatment design including 23 cultivars with or without traits of resistance to SCN, and IDC. The study illustrated the interactive effects of the two defensive traits on the diseases and soybean yields. IDC rating was higher in SCN-susceptible than SCN-resistant soybean, suggesting SCN infection increased IDC. Resistance to IDC apparently increased SCN reproduction due to better soybean plant growth. Yield response to the defensive traits depended on the disease pressures in a field. When both IDC and SCN were present in a field, deploying SCN-resistance was the best solution to the problems. However, SCN-resistance suppressed soybean yields when used in fields without the disease problems. IDC-resistance increased yield of SCN-susceptible cultivars, but it did not result in detectable yield benefit of SCN-resistant cultivars in SCN-infested sites. Effective use of the defensive traits for management of IDC and SCN requires specific knowledge of the disease problems present in a field.

Pathotype Diversity of Phytophthora sojae in Eleven States in the United States

Pathotype diversity of Phytophthora sojae was assessed in 11 states in the United States during 2012 and 2013. Isolates of P. sojae were recovered from 202 fields, either from soil samples using a soybean seedling bioassay or by isolation from symptomatic plants. Each isolate was inoculated directly onto 12 soybean differentials; no Rps gene or Rps 1a, 1b, 1c, 1k, 3a, 3b, 3c, 4, 6, 7, or 8. There were 213 unique virulence pathotypes identified among the 873 isolates collected. None of the Rps genes were effective against all the isolates collected but Rps6 and Rps8 were effective against the majority of isolates collected in the northern regions of the sampled area. Virulence toward Rps1a, 1b, 1c, and 1k ranged from 36 to 100% of isolates collected in each state, while virulence to Rps6 and Rps8 was less than 36 and 10%, respectively. Depending on the state, the effectiveness of Rps3a ranged from totally effective to susceptible to more than 40% of the isolates. Pathotype complexity has increased in populations of P. sojae in the United States, emphasizing the increasing importance of stacked Rps genes in combination with high partial resistance as a means of limiting losses to P. sojae.

Resistance and partial resistance to Phytophthora sojae in early maturity group soybean plant introductions

Phytophthora sojae, an important yield limiting pathogen of soybean, causes seed, seedling, root, and stem rots. Losses caused by P. sojae can be controlled by both major gene and partial resistance. Early maturity group (MG) soybeans are an increasingly important crop in northwestern Minnesota and eastern North Dakota. Early MG plant introductions (PIs) from the USDA Soybean Germplasm Collection and early MG public and private cultivars were evaluated for resistance and partial resistance to P. sojae. Of the 113 PIs, PI438445, and PI438454 exhibited resistance to P. sojae races 4, 7, 17, and 28 indicating they may possess either Rps1c, Rps1k, previously unidentified or multiple resistance gene to Phytophthora sojae (Rps) genes. Because they exhibited partial resistance equal to or greater than the standard check cultivar Conrad, three early MG soybean cultivars (MN0902, MN0302, and 91B53) were selected as standard checks to evaluate early MG PIs for partial resistance. Sixty-nine PIs were evaluated for partial resistance to P. sojae races 7 and 25 using the inoculum layer method. Of this group of PIs, 22 had the same level of partial resistance as Conrad to P. sojae race 7 while 19 had the same degree of partial resistance to race 25. Twelve PIs had same level of partial resistance as Conrad to both P. sojae races 7 and 25. The PIs and cultivars identified in this study will be of great value in developing early MG soybean cultivars suitable for planting in Canada and the northern United States.

Pythium, Phytophthora, and Phytopythium spp. Associated with Soybean in Minnesota, Their Relative Aggressiveness on Soybean and Corn, and Their Sensitivity to Seed Treatment Fungicides

Pythium spp. cause seed decay, damping-off, and root rot in soybean and corn; however, their diversity and importance as pathogens in Minnesota are unknown. Our objectives were to identify the Pythium spp. present in Minnesota soybean fields, determine their aggressiveness on corn and soybean, and investigate their sensitivity to seed treatment fungicides. For identification, sequences obtained using internal transcribed space ITS4 and ITS1 primers were compared with reference sequences in the National Center for Biotechnology Information database. Seedling and soil samples yielded over 30 oomycete species. Aggressiveness was determined using two methods; a seed assay, which also examined temperature effects on aggressiveness, and a seedling assay. Of 21 species evaluated, seven Pythium spp. were pathogenic on both soybean and corn, reducing root growth by 20% or more while two Pythium and one Phytopythium spp. were pathogenic only on soybean. Aggressiveness of many isolates increased as temperature increased from 15°C to 25°C. The sensitivity of 10 pathogenic species to azoxystrobin, ethaboxam, mefenoxam, pyraclostrobin, or trifloxystrobin was tested. EC50 values for mefenoxam and ethaboxam were 10-2 of those to strobilurin fungicides. Pythium spp. in Minnesota are diverse and a significant cause of seedling disease on soybean and corn. Most Pythium spp. isolated in this study were more sensitive to mefenoxam and ethaboxam than to strobilurin fungicides.