Effects of Seed Treatments and Yield Potential on Infected Soybean Seed

Diaporthe seed decay is an endemic disease throughout most soybean-growing regions. A major outbreak of Diaporthe seed decay occurred in 2018 in Iowa and other parts of the United States. The most prevalent fungi associated with this outbreak in Iowa were Diaporthe longicolla and D. caulivora. We established field experiments in 2019 and 2020 in two central Iowa locations to evaluate the effects of seed treatments and yield potential of Diaporthe-infected seed lots. Seed of the same variety but originating from different sources were categorized as "good" or "bad" seed depending on the percentage of Diaporthe-infected seed. Four seed treatments were tested, and results indicated that there were differences in plant stand, plant health (normalized difference vegetation index and canopy coverage) (both years), and yield (2019) between both seed lots. The good seed lot had about 9% greater yield than the bad seed lot (P < 0.001), whereas the seed lots had similar yield in 2020 (P = 0.822). Seed treatments improved stand, canopy greenness, and canopy coverage but did not affect yield in either year. For the yield potential experiment, there were minimal differences in plant health and no differences in yield between good and bad seed lots thinned to the same plant population in both years. These results suggest that Diaporthe-infected seed lots with lower germination and plant stand resulted in lower yield, and seed treatments can improve soybean stand, canopy greenness, and coverage. Additionally, Diaporthe-infected seed did not affect soybean growth and development or yield once plant stand was established.