Phytophthora, Phytopythium, and Pythium species that cause early-season seed decay and pre-emergence and post-emergence damping off of soybean are most commonly managed with seed treatments. The phenylamide fungicides metalaxyl and mefenoxam, and ethaboxam are effective toward some but not all species. The primary objective of this study was to evaluate the efficacy of ethaboxam in fungicide mixtures and compare those with other fungicides as seed treatments to protect soybean against Pythium, Phytopythium, and Phytophthora species in both high-disease field environments and laboratory seed plate assays. The second objective was to evaluate these seed treatment mixtures on cultivars that have varying levels and combinations of resistance to these soilborne pathogens. Five of eight environments received adequate precipitation in the 14 days after planting for high levels of seedling disease development and treatment evaluations. Three environments had significantly greater stands, and three had significantly greater yield when ethaboxam was used in the seed treatment mixture compared with treatments containing metalaxyl or mefenoxam alone. Three fungicide formulations significantly reduced disease severity compared with nontreated in the seed plate assay for 17 species. However, the combination of ethaboxam plus metalaxyl in a mixture was more effective than either fungicide alone against some Pythium and Phytopythium species. Overall, our results indicate that the addition of ethaboxam to a fungicide seed treatment is effective in reducing seed rot caused by these pathogens commonly isolated from soybean in Ohio but that these effects can be masked when cultivars with resistance are planted.

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Species of Phytophthora, Phytopythium, and Pythium affect soybean seed and seedlings each year primarily through reduced plant populations and yield. Oxathiapiprolin is effective at managing several foliar diseases caused by some Oomycetes. The objectives of these studies were to evaluate oxathiapiprolin i) in a discriminatory dose assay in vitro; ii) as a soybean seed treatment on i) a moderately susceptible cultivar in 10 environments; iii) compare environments with low and high precipitation on the impact of seed treatment on plant population and yield and iv) a seed treatment mixture on cultivars with different levels of resistance in 4 environments. There was no reduction in growth in vitro among 13 species of Pythium at 0.1µg ml-1. Soybean seed treated with the base fungicide plus oxathiapiprolin (12 and 24 µg a.i. seed-1) alone, oxathiapiprolin (12 µg a.i. seed-1) plus mefenoxam (6 µg a.i. seed-1), or oxathiapiprolin (24 µg a.i. seed-1) plus ethaboxam (12.1 µg a.i. seed-1) contributed to higher yield in those environments that received > 50 mm of precipitation within 14 days after planting (dap) compared to those that received less. Early plant population and yield were significantly higher for seed treated with oxathiapiprolin (24 µg a.i. seed-1) + metalaxyl (13.2 µg a.i. seed-1) compared to nontreated for 6 of 7 cultivars in at least 1 of 4 environments. Oxathiapiprolin combined with another oomycota fungicide applied to seed has the potential to be used to protect soybean plant establishment and yield in regions prone to poor drainage following high levels of precipitation.