Effect of crop rotation and soil amendments on Sclerotinia stem rot on soybean in two soils

The effect of 4-yr soybean rotations with corn (CCCS vs. CCSS vs. SSSS) and soil amendments (urban compost vs. mineral fertilizer) on Sclerotinia stem rot of soybean (disease severity index, apothecia number, sclerotia survival), caused by Sclerotinia sclerotiorum, was studied on two soils (clay loam and sandy loam) at Saint-Hyacinthe, in southwestern Québec, from 1999 to 2002. In 2002, the 3-yr-corn rotation had a significant effect on the disease severity index (DSI) at the clay loam site; DSI was greatly reduced by rotation (5.2%) compared with monoculture (44.3%). At the sandy loam site, an interaction between rotation and fertility amendments reduced DSI from 71.7% in the monoculture to 37.8% in the 3-yr-corn rotation amended with compost. Canonical redundancy analysis (RDA) of the DSI-apothecia matrix in 2002 confirmed the suppressiveness of the 3-yr-corn rotation and revealed a significant conducive effect on DSI of compost at the clay loam site. At the sandy loam site, RDA of the DSI-apothecia matrix confirmed suppressive interaction between corn-soybean rotations and compost amendment. Key words: Sclerotinia stem rot of soybean, crop rotation, urban compost, multiple regression, canonical redundancy analysis, permutations test