Comparison of aggressiveness and fungicide sensitivity of four Pythium spp. that cause damping-off of soybean in the United States

Pythium spp. are the most common pathogens associated with soybean damping-off in the United States. The diversity of Pythium spp. recovered is extensive and the relative pathogenicity or aggressiveness of species common across states has varied among studies. We compared the aggressiveness and fungicide sensitivity of 118 isolates of four Pythium spp. (P. lutarium, P. oopapillum, P. sylvaticum, and P. torulosum) from 11 states (AR, IA, IL, IN, KS, MI, MN, ND, NE, SD, and WI). All isolates were screened using seed and seedling assays at two temperatures (13°C and 23°C). Seed rot and root rot severity varied by state of origin, with isolates from AR always the most aggressive. In general, isolates of P. sylvaticum and P. lutarium were more aggressive at 23°C compared with 13°C, while isolates of P. oopapillum and P. torulosum were more aggressive at 13°C than at 23°C. Fungicide sensitivity to five fungicides (metalaxyl, azoxystrobin, ethaboxam, captan, and thiram) was assessed using a plate assay at 13°C and 23°C. The EC50 values for each fungicide-isolate combination varied by state of origin, and were greatest for the isolates from AR. Temperature affected EC50 values for metalaxyl, azoxystrobin, and ethaboxam for P. oopapillum, P. sylvaticum, and P torulosum. The EC50 values for thiram were not affected by temperature for any species. Results from this study indicate that aggressiveness and fungicide sensitivity of Pythium spp. vary geographically, which suggests that management of damping-off of soybean should target local rather than regional Pythium populations.