Soybean Monoculture Is Associated with Suppression of Foliar Sudden Death Syndrome Expression Without Consistent Reductions in Pathogen Levels in Ontario Agroecosystems

Sudden death syndrome (SDS) and soybean cyst nematode (SCN) are major yield-limiting diseases in North American soybean production, with limited effective management options. Long-term soybean monoculture has been reported to suppress SDS and SCN, but the mechanisms, onset, and persistence of such suppression remain poorly understood. To study these mechanisms, a six-year field study (2018–2023) was conducted at two Ontario sites with contrasting disease histories: Chatham (conducive) and Essex (suppressive). We evaluated suppression development and resilience across soybean monoculture (SSSSSS) and corn–soybean rotations (SCSCSC/CSCSCS), using eight cultivars differing in SDS and SCN resistance across two maturity groups. In Chatham, disease index (DX) progressively declined under monoculture; the most susceptible cultivar, HS11RY07, declined from a mean DX of 89 to 43 by year six, with corresponding yield increases, and rotational yield advantages diminished. In Essex, introducing corn rotation increased SDS symptoms during soybean phases; monoculture yields became comparable to rotation in later years. Importantly, suppression developed without corresponding reductions in Fusarium virguliforme and SCN populations, which remained variable across years, suggesting that monoculture may disrupt pathogen effectiveness rather than eliminating it. This decoupling of pathogen abundance and disease severity is consistent with soil-mediated biological suppression; the microbial drivers are addressed in subsequent work.