Effect of crop rotation and tillage system on sclerotinia stem rot on soybean

Sclerotinia stem rot (SSR) of soybean is a major disease in the North Central region of the United States. A 3-year study was done to determine if crop rotation and tillage, moldboard plowing then mulch tillage (MP + MT), mulch tillage (MT), and no-till (NT), affected the population density of sclerotia and apothecia, incidence of SSR, and soybean yield. Crop rotation did not significantly affect the number or distribution of sclerotia or SSR incidence, but did affect the number of apothecia and the yield. The highest number of apothecia per square metre was observed in the plots in the continuous soybean rotation. Tillage affected both the number and distribution of sclerotia in the soil profile. Moldboard plowing lowered the number of sclerotia per litre of soil, compared to MT and NT plots, and buried the sclerotia deeper than 10 cm into the soil. Tillage did not affect the total number of apothecia observed, but moldboard plowing did delay emergence of apothecia compared to no-till. The MP + MT plots had the lowest disease incidence and the highest yield, while the NT plots had the highest disease incidence and the lowest yields. One year of moldboard plowing will bury sclerotia at least 10 cm in soil and delay the production of apothecia. How this affects SSR development depends on the other factors involved with disease development.