None of the current oomycota fungicides are effective towards all species of Phytophthora, Phytopythium, Globisporangium, and Pythium that affect soybean seed and seedlings in Ohio. Picarbutrazox is a new oomyceticide with a novel mode of action towards Oomycete pathogens. Our objectives were to evaluate picarbutrazox to determine i) baseline sensitivity (EC50) to 189 isolates of 29 species, ii) the efficacy with a base seed treatment with three cultivars with different levels of resistance in 14 field environments; and iii) if the rhizosphere microbiome was affected by the addition of the seed treatment on a moderately susceptible cultivar. The mycelial growth of all isolates was inhibited beginning at 0.001µg and the EC50 ranged from 0.0013 to 0.0483 µg a.i. ml-1. The effect of seed treatment was significantly different for plant population and yield in 8 of 14 and 6 of 12 environments, respectively. The addition of picarbutrazox at 1 and 2.5 g a.i. 100 kg seed-1 to the base seed treatment compared to the base alone was associated with higher plant populations and yield in 3 and 1 environment, respectively. There was limited impact of the seed treatment mefenoxam 7.5 g a.i. plus picarbutrazox 1 g a.i. per 100 kg seed-1 on the oomycetes detected in the rhizosphere of soybean seedlings collected at the V1 growth stage. Picarbutrazox has efficacy towards a wider range of oomycetes that cause disease on soybean and this will be another oomyceticide tool to combat early season damping-off in areas where environmental conditions highly favor disease development.
Keywords: Causal Agent; Crop Type; Disease management; Field crops; Oomycetes; Subject Areas; chemical; oilseeds and legumes.