The identity of fungi responsible for causing seedling blights of soybean is underway in nine states and the collection of fungal parasites from a wide range of environments, soils and cropping histories is 85% complete. Hundreds of pure cultures of Pythium spp., Phytophthora spp., Fusarium spp., and Rhizoctonia spp. are currently being identified and processed.

Progress has been made developing molecular diagnostic assays for identification of fungal seedling pathogen. As more isolates are identified they will be used to help the completion of this objective.

At North Dakota State University, molecular work was used to correctly identify Fusarium acuminatum and F. tricinctum in the isolate collection of Fusarium. Based on the analysis of three loci, all 21 isolates that were previously believed to be F. acuminatum were F. tricinctum. They believe that other researchers are having the same problems when using NCBI to identify some of these Fusarium species. It is extremely important that these Fusarium species be correctly identified when reporting results.

Michigan State University researchers have developed and refined pathogenicity and virulence assays for Pythium spp. isolated as part of the USDA-NIFA sponsored survey of oomycete pathogens of soybean seedlings. To date 13 oomycete species have been screened for aggressiveness in seed and seedling based pathogenicity assays. Additional assays are still being developed including the influence of temperature on disease expression.

Experiments were conducted at the University of Minnesota to determine the effect of environmental conditions, temperature and soil moisture, on damping off, seedling rots, seedling growth, and root rot symptom expression. Results obtained during past months indicate that: 1) amount of infection increases with increasing temperature, 2) symptom expression (root rot, seedling rot, and damping off) caused by Fusarium solani is not correlated with the amount of infection, 3) instead symptoms of infection such as the percentage of root loss and root rot symptoms are more severe in saturated soils at either low or high temperatures (30 C). Stand is most severely affected at low temperatures in saturated soils.

Researchers in two states are investigating baseline fungicide sensitivity by establishing fungicide concentrations. Initial work has been completed on Fusarium and Rhizoctonia isolates from Iowa and Illinois, respectively.

Researchers in Iowa and Kansas are developing inoculation assays for the various seedling disease fungi. These assays will be used to compare various inoculation strategies, pH, temperature, soil texture, and water potential influences upon seedling disease development. In Arkansas, studies determining the effects of high soil temperatures on seedling disease were conducted. Soybean seed were treated with either fludioxonil plus mefenoxam, metalaxyl, and PCNB+Vitavax. Initial results indicate that fludioxonil plus mefenoxam and metalaxyl both improved stands at 28 and 32 C.

A study in Arkansas was initiated comparing the effects of seed quality and a broad spectrum seed treatment (fludioxonil, mefenoxam, and azoxystrobin) on emergence at 25, 28, 30, 32, and 35 C. Seed treatment significantly improved stands of both high and low quality seed at 25 C, especially with low quality seed. As temperatures increased the effect of seed treatment lessened and overall stands were lower at 35oC than at lower temperatures. The experiment will be repeated with the addition of 38oC or higher to find the upper limit in soil temperature that allows seed emergence.

Soil from a long-term rotation study was collected earlier this spring and planted to the soybean cultivar Hutcheson. Isolations of Pythium spp. and Fusarium spp. were made and sent to Michigan State University and Southern Illinois University, respectively, for identification to species. Three hundred twenty isolates of Pythium spp. were identified. The most prevalent species were P. spinosum, P. irregular, P. paroecandrum, and P. sylvaticum. There were eight other species recovered at very low incidences. Isolate distribution was affected by rotation. Further analysis of the data is underway. Isolates of Pythium from the long-term rotation study that had been identified to species are being taken out of storage and will be tested for pathogenicity. The initial tests will use the seed plate assay.