Phytophthora root and stem rot of soybean in Iowa: minimizing losses through an improved understanding of population structure and implementation of novel management strategies
Phytophthora root and stem rot (PRR), caused by the oomycete Phytophthora sojae, is an economically important soybean disease in the North Central Region of the United States, including Iowa. Management of PRR includes the use of specific resistance (Rps genes), partial resistance, and seed treatment fungicides. We had three objectives specific to PRR in Iowa; (i) to determine the within field population diversity of P. sojae in fields with a history of PRR, (ii) to screen 32 PI lines identified with unique resistance to P. sojae with isolates from Iowa and determine their value for future breeding efforts and (iii) to determine if seed treatment fungicides in combination with partial resistant cultivars of soybean would be a useful management tool against PRR.;Previous surveys in Iowa recovered 12 races of the pathogen between 1966 and 2004. These surveys collected a single isolate from each site. We proposed multiple races could be identified from a single site, based on evidence from Ohio, Arkansas and Australia surveys. Three fields in Iowa with a history of PRR were extensively sampled. Soil samples were baited using leaf disc and seedling baiting methods. A total of 36 isolates of P. sojae were collected. Of those, 32 were identified to race or pathotype. We found multiple races and pathotypes in two fields sampled. Eleven races and 13 pathotypes are reported new to Iowa.;Rps genes generally are defeated by P. sojae populations between 8 and 15 years after deployment. Due to the pathogen's ability to defeat Rps genes, new sources of resistance need to be discovered and incorporated into commercial soybean lines. As part of a collaborative study with the North Central Soybean Research program, we screened 32 previously identified plant introduction (PI) lines against isolates of P. sojae collected in Iowa to identify potential resistance beneficial to Iowa soybean growers. We screened the PIs in four fields as well as in the greenhouse using four layer tests with multiple combinations of isolates. We found six PI lines (PI 398693, PI 399079, PI 408029, PI 424487B, PI 567374, and PI 407974B) with possible new sources of resistance to be incorporated into Iowa soybeans.;Partial resistance does not become active until the VC growth stage of the soybean, causing the seed and seedling to be susceptible to disease at planting and early stages of growth. Application of fungicide seed treatments, including the active ingredients metalaxyl and mefenoxam, are suggested in areas of strong disease pressure and favorable environmental conditions for disease development. We evaluated the use of fungicide seed treatments in combination with partial resistance as a potential effective management tool, as part of a collaborative study of the North Central Soybean Research Program. Three fields with a history of PRR were identified and field trials were conducted over two years in a split plot design, with the main plot including cultivar of soybean (2 susceptible; Sloan and Williams, 2 reportedly partial resistant; Stine 2402 and S27-T7, and 2 with Rps genes; Archer with Rps1k and 6 and 2834RR with Rps1k). The subplot included three seed treatments; a control untreated, Apron XLRTM and Apron XLRTM plus Apron MaxxRTM. Evaluations included initial stand counts, disease incidence taken twice in the growing season, final stand counts and yields. Disease pressure was low due to unfavorable environmental conditions for P. sojae development. As results of these conditions, we are unable to make any conclusions to the effectiveness of using fungicide seed treatments in combination with partial resistance as an effective management tool.