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RESEARCH HIGHLIGHTS

Cropping System Diversification Reduces Incidence and Severity of Soybean Sudden Death Syndrome (SDS)
Thu - March 8, 2018
by Leonor Leandro and Gwyn Beattie, plant pathologists, and Matt Liebman, agronomist, Iowa State University

In a 6-year study, we assessed the effects of cropping system diversification — encompassing both crop rotations and organic soil amendments — on the incidence of sudden death syndrome (SDS) and soybean yield under field conditions. Sudden death syndrome is caused by the soil fungal pathogen Fusarium virguliforme (Fv). We found that diversification of the soybean-corn rotation with oat, and clover or alfalfa, in conjunction with the use of composted manure amendments, greatly suppressed SDS development and protected soybean yield. The effect was consistent over six years (2010-2015), even when SDS disease pressure was low. The results have been published in the scientific journal Plant Disease.  

The study was conducted on a naturally-infested field trial with a long-term history of cropping system diversification treatments in Iowa. The trial included a 2-year corn/soybean rotation system, a 3-year corn/soybean/oat + red clover system; and a 4-year corn/soybean/oat +alfalfa/alfalfa system. The 2-year system was conventionally managed with synthetic fertilizers, whereas the 3- and 4-year systems received manure and low synthetic fertilizer rates. We also assessed the amount of the pathogen in the soil using a sensitive DNA-based assay.

In every year except 2012 (a drought year with little SDS development), SDS incidence (the number of plants infected) was significantly lower in the 3- and 4- year system  than in the 2-year system (Figure A).

In all six years, SDS severity was highest in the 2-year system plots, and foliar symptom severity was up to 17 times higher in 2-year  plots than the 4-year system plots (Figure B). 
 
Soybean yield was greatly affected by cropping system, and this effect was strongly associated with SDS. Yields in 3- and 4-year system plots were up to 40% greater than yields in 2-year plots. Statistical analysis showed that 50 to 87% of the variation in yield was explained by SDS incidence, and that 30 to 70% of the variation in yield was explained by SDS severity (Figure C).
   
Moreover, we demonstrated that the reduced SDS in the 4-year system was associated with a lower density of the SDS pathogen in soil, compared with the 2-year system. In 2012 and 2013, when the Fv population sizes were estimated after crop harvest, Fv was found to be more than 5 times as abundant in the 2-year than the 4- year system plots. This is significant because SDS is an inoculum-dependent disease — meaning that there is a direct relationship between the amount of the pathogen in the soil and the amount of SDS that can develop.
 
Fusarium virguliforme is a fungal pathogen that can survive in soil for years in the form of thick-walled chlamydospores. Rotation with corn is ineffective for reducing Fv soil populations or SDS symptoms, as SDS outbreaks can occur even after several years of continuous corn. Our study provides further evidence that the corn-soybean production system that is predominant in the Midwest is more conducive to Fv buildup in soil than diversified cropping systems.  

The processes responsible for the effectiveness of cropping system diversification in suppressing SDS and protecting soybean yield remain to be determined. In this study, the cropping sequence was confounded with fertilization regime, as manure and lower rates of chemical fertilizer were used in the 3- and 4-year systems, while only synthetic fertilizers were used in the 2-year system. We can therefore not conclude from this study if the suppression of SDS and Fv density in soil was due to the cropping sequence or fertility regime.

Current management recommendations for SDS emphasize using resistant varieties, improving soil drainage, and reducing soil compaction. More recently, a fungicide treatment has been shown to reduce SDS under some conditions and confer a yield benefit when applied as a seed treatment.

Our study provides the strongest evidence to date that diversified cropping systems offer another approach for SDS management.  Adoption of diversified systems will require growers to learn new practices and adapt to new farming equipment. However, the disease management benefits, combined with the fact that profitability is as high or higher compared with the dominant corn-soybean system, make it a promising alternative to growers.

Resources
Cropping system diversification reduces severity and incidence of soybean sudden death syndrome caused by Fusarium virguliforme - Plant Disease, March 2018
Sudden Death Syndrome - Crop Protection Network, a collaboration sponsored by the North Central Soybean Research Program (NCSRP) and other soybean checkoff programs, 2016
Scouting for Sudden Death Syndrome - Crop Protection Network/NCSRP, 2016
Scouting for Soybean Stem Diseases