A research study published in the American Phytopathological Society (APS) Plant Disease journal examined the effects of cropping system diversification on management of soybean sudden death syndrome (SDS), caused by Fusarium virguliforme, that provides some insight into preventing the disease for growers. SDS is a major disease that impacts North and South America. Estimates of annual yield losses in the United States due to SDS have ranged between 0.6 and 1.9 million metric tons during the years from 2006 to 2014, accounting for $200 to 750 million in monetary losses over the same time.
The study included a 2-year, 3-year, and 4-year crop rotation as well as a control. The rotations were as follows:
· 2-year: corn-soybean
· 3-year: corn-soybean-oat (with a red clover cover crop)
· 4-year: corn-soybean-oat-alfalfa
The study also examined differing crop practices, with manure application on the 3 and 4-year rotations and reduced rates of fertilizers, as well as weed management regimens. The study took place between 2010 and 2015.
Analysis shows cropping system diversification can be an effective strategy to manage SDS, especially when incorporating oat, clover and/or alfalfa into an annual crop rotation. Even in years when SDS disease pressure was low, the strategies’ effect was consistent over six years. SDS severity was 17 times higher in two-year cropping system plots, compared with extended and diversified systems, specifically four-year systems. F. virguliforme was isolated at higher frequency from roots from the 2-year system compared with the more diversified systems in 2012 and 2013.
Low levels of SDS in 2011 and 2012 may be explained by the amount of precipitation in those years. Development of SDS is highly dependent on soil moisture, and higher-than-average precipitation, especially during soybean reproductive stages, has been associated with SDS epidemic years. All 4 years of the study when severe SDS symptoms were observed had one or more months during the growing season where rainfall greatly exceeded the monthly average.
When it came to yield results, three and four-year systems showed 40 percent greater yields than in two-year systems. Between 50-87 percent of yield variance was explained by SDS incidence; SDS severity caused 30-70 percent variation in yields.
Extended and diversified cropping systems were correlated with lower F. virguliforme population densities. In both soybean and corn plots sampled in 2012 and 2013, the pathogen’s density in the soil was approximately fivefold greater in the two-year system, as compared to the four-year system. F. virguliforme can survive in soil for years as thick-walled chlamydospores and rotation with corn has been shown to be ineffective for reducing F. virguliforme soil populations or SDS symptoms.
The explanation for higher SDS severity and pathogen presence in the two-year system may be the result of the greater frequency of soybean planting, which creates inoculum build-up in the soil. Corn residue can support survival of the SDS pathogen, which could help explain increased SDS in two year rotations. It is possible alternating corn and soybean is more conducive to SDS than is interruption of that cycle with other crops.
Alfalfa and clover are susceptible to F. virguliforme infection, thus pathogen reduction in the extended rotation may be more closely attributed to the inclusion of oats. In addition, oat used in rotation or as a cover crop has been shown to suppress root rot diseases on certain crops because it produces avenacins, which are active saponin compounds that have antimicrobial activity against several fungal pathogens.
Diversified cropping systems, especially those integrated with livestock production, offer numerous potential environmental and agronomic benefits, including improved soil quality, greater nutrient cycling and retention, greater water-holding capacity, lower rates of soil erosion, and improved control of weeds, diseases, and insect pests. The loss of crop diversity in the Midwestern United States, due especially to reductions in small grain, hay, and pasture production, is linked with significant reductions in livestock production in the region, thus the reduction of manure application to fields. The study asserts that adoption of diversified systems will require growers to learn new practices and adapt to new farming equipment, which could serve to benefit growers financially in the long-run, by limiting yield loss.