Beneficial effects of fungicide seed treatments for soybean cultivars with partial resistance to Phytophthora sojae

Phytophthora sojae is a yield-limiting soybean pathogen in areas where soils remain saturated for long periods of time. P. sojae has been successfully managed with single dominant resistance genes (Rps genes). The proportion of fields with populations of P. sojae capable of causing susceptible interactions with many of the Rps genes has increased in number. The fungicides metalaxyl and mefenoxam have been used both as in-furrow and seed treatments to provide protection against damping-off caused by P. sojae. To determine the plant age when partial resistance and Rps genes are effective against P. sojae, we evaluated a greenhouse assay in which soybean seeds were planted and inoculated with a zoospore suspension to compare the disease reaction of soybean seeds and seedlings. Efficacy of different fungicide rates also was evaluated using the cultivar with partial resistance with this inoculation technique. Seeds and seedlings of a cultivar with high levels of partial resistance were susceptible to infection by P. sojae while those of a cultivar with an Rps gene were resistant. For the cultivar with partial resistance, reductions in percent emergence and the number of damped-off seedlings were significantly higher for plants inoculated at the day of planting compared to inoculations of plants with unifoliates present (5 days after planting). Results also indicate that fungicide seed treatment on cultivars with partial resistance may be beneficial when the environmental conditions that favor P. sojae infections occur prior to soybean emergence. This greenhouse assay appears to be useful in examining overall fungicide efficacy; however, it did not detect consistent and quantifiable differences in rates of seed treatment fungicides.