Sudden death syndrome (SDS) of soybean was first detected in the US in 1971 and has remained one of the most damaging soybean diseases in the US. It is caused by a soil-borne fungus, Fusarium virguliforme (Fv), that persists in the soil for multiple years. Currently, conventional management of SDS relies on partially-resistant cultivars and fluopyram seed treatment, but fluopyram seed treatment is expensive and partial resistance does not protect soybeans during severe epidemics. Extended crop rotation is an alternative strategy that has effectively suppressed SDS, but there are barriers to adoption of this strategy in the current agricultural system of soybean producing regions of the US.

Using a long-term cropping system experiment in Iowa (est. 2002), a 2-year corn-soybean rotation system was compared for disease indices, yield, and whole-farm profitability to a 3-year corn-soybean-oat/red clover rotation, and a 4-year corn-soybean-oat/alfalfa-alfalfa rotation. A three-year economic comparison of cropping systems and fluopyram seed treatment was conducted from 2017-2019 to assess the efficacy of SDS suppression, yield, and net economic return of the different management strategies. Results showed that relative to the 2-year rotation, the 3-year and 4-year systems suppressed SDS, and that the 4-year system increased yield an average of 18% and soybean economic return an average of 43%. In contrast, fluopyram reduced yield 3% and economic return 18%. Average whole-rotation profitability of the 2-year system was no different, 40% lower, and 26% higher compared to the 4-year system in 2017, 2018, and 2019, respectively.

The mechanisms of SDS suppression in the longer rotation systems were also explored through greenhouse experiments involving green manure amendments of oat, rye, and alfalfa. Tests of the effects of the soil biota in the 4-year rotation system on SDS development were also conducted. Results indicated that oat and rye green manure substantially reduced root rot severity of soybean by 85% and 67%, respectively, relative to the non-amended control, while alfalfa green manure did not reduce root rot severity. The soil biota also likely drove SDS suppression as live (non-autoclaved soil) demonstrated 91% reduction in root rot, 94% reduction in Fv population, 61% decrease in SDS foliar severity, and 52% increase in soybean plant weight relative to similar autoclaved soil. This is thought to be due to an increase in competition for niche space and nutrients against Fv, as the population of Fv was dramatically suppressed by the presence of robust soil biota.

Overall, these findings show that diversified cropping systems can maintain soybean profitability, and that fluopyram seed treatment would reduce profit in a diversified cropping systems context or when SDS pressure was low. Decomposition of certain green manures such as oat and rye, and soil microbial communities are potent components of SDS suppression through possibly interactive mechanisms of allelopathy and antagonistic microbial populations dominance over Fv. In conclusion, further considerations of crop diversity and crop rotation should be given to build sustainable SDS-suppressive agroecosystems in the long term.