Influence of Soybean Tissue and Oomicide Seed Treatments on Oomycete Isolation

Soybean seedlings are vulnerable to different oomycete pathogens. Seed treatments containing the two antioomycete (oomicide) chemicals, metalaxyl-M (mefenoxam) and ethaboxam, are used for protection against oomycete pathogens. This study aimed to evaluate the influence of these two oomicides on 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%), P. heterothallicum (14.3%), P. ultimum var. ultimum (11.8%), P. attrantheridium (7.9%), and P. irregulare (6.6%), which accounted for 61.7% of the isolates collected. Oomicide sensitivity to ethaboxam and mefenoxam was assessed for >300 isolates. There were large differences in ethaboxam sensitivity among oomycete species, with effective concentrations to reduce optical density at 600 nm by 50% compared with the nonamended control (EC₅₀ values) ranging from <0.01 to >100 μg/ml and a median of 0.65 μg/ml. Isolates with insensitivity to ethaboxam (>12 μg/ml) belonged to the species P. torulosum and P. rostratifingens but were sensitive to mefenoxam. Oomicide sensitivity to mefenoxam ranged from <0.01 to 0.62 μg/ml with a median of 0.03 μg/ml. The mean EC₅₀ value of the five most abundant species to ethaboxam ranged from 0.35 to 0.97 μg/ml of ethaboxam and from 0.02 to 0.04 μg/ml of mefenoxam. No shift in sensitivity to mefenoxam or ethaboxam was observed as a result of soybean seed treatment or year relative to the nontreated 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, seed treatments containing mefenoxam or metalaxyl plus ethaboxam can be effective in reducing the probability of oomycete isolation from soybean roots.