Influence of soybean tissue and oomicide seed treatments on oomycete isolation

Soybean seedlings are vulnerable to different oomycete pathogens. Seed treatments containing the two anti-oomycete (oomicide) chemicals, metalaxyl-M (mefenoxam), and ethaboxam are used for the protection against oomycete pathogens. This study aimed to evaluate the influence of these two oomicides on the isolation probability of oomycetes from soybean taproot or lateral root sections. Soybean plants were collected between the first and third trifoliate growth stages from five Midwest field locations in 2016 and four of the same fields in 2017. Oomycetes were isolated from taproot and lateral root. In 2016, 369 isolation attempts were completed resulting in 121 isolates from the taproot and 154 isolates from the lateral root. In 2017, 468 isolation attempts were completed, with 44 isolates from the taproot and 120 isolates from the lateral roots. In three of nine site-years, the probability of isolating an oomycete from a taproot or lateral root section was significantly different. Seed treatments containing a mixture of ethaboxam and metalaxyl significantly reduced the probability of oomycete isolation from lateral roots in Illinois in 2016 and 2017, but not in other locations, which may have been related to the heavy soil type (clay loam). Among the 439 isolates collected from the two years sampled, 24 oomycete species were identified, and community compositions differed depending on location and year. The five most abundant species were Pythium sylvaticum (28.9%), Pythium heterothallicum (14.3%), Pythium ultimum var. ultimum (11.8%), Pythium attrantheridium (7.9%), and Pythium irregulare (6.6%) which accounted for 61.7% of the isolates collected. Oomicide sensitivity to ethaboxam and mefenoxam was assessed for more than 300 isolates. There were large differences in ethaboxam sensitivity among oomycete species with EC50 ranging from < 0.01 to > 100 μg ml-1, and a median of 0.65 μg ml-1. Isolates with insensitivity to ethaboxam (> 12 μg ml-1) belonging to the species Pythium torulosum and Pythium rostratifingens but were sensitive to mefenoxam. Oomicide sensitivity to mefenoxam ranged from < 0.01 to 0.62 μg ml-1 with a median of 0.03 μg ml-1. The mean EC50 of the five most abundant species to ethaboxam ranged from 0.35 to 0.97 μg ml-1 of ethaboxam and from 0.02 to 0.04 μg ml-1 of mefenoxam. No shift in sensitivity to mefenoxam or ethaboxam was observed due to soybean seed treatment or year relative to the non-treated seed controls. In summary, this study contributed to the understanding of the composition of oomycete populations from different soybean root tissues, locations, years, and seed treatments. Finally, the effectiveness of seed treatments containing mefenoxam or metalaxyl plus ethaboxam can be effective in reducing the probability of oomycete isolation from soybean roots.