Suppression of Soybean Diseases Through the Use of Cover Crops

Suppression of Soybean Diseases Through the Use of Cover Crops

Summary

Evaluations of the impact of fall cover crops (rye, rape, canola, mustard, and fallow) on diseases in spring planted soybean crops continued at 6 locations in Illinois in 2013. Reductions in severity levels of Rhizoctonia root rot and foliar diseases were seen in rye plots in some instances. Cover crop induced soil suppressiveness to sudden death syndrome was observed in greenhouse bioassays, but only in soils from one location, as were reductions in populations of soybean cyst nematode. No differences in populations of other pathogens or microbial community structures were detected among soils from the various cover crop treated plots. 

Objectives/Performance Targets

·   Establish fall cover crop plantings at six locations in Illinois

·   Establish soybean plantings in the spring after incorporation of the cover crops

·   Collect soil samples to evaluate disease suppressiveness and monitor pathogen populations and non-pathogen microbial communities.

·   Monitor root and foliar disease severity levels throughout the season.

·   Collect soybean yield data.

·   Prepare information articles on the project for outreach.

Accomplishments/Milestones

Cover crop establishment

Cover crop plots were established in the fall of 2012 in on-farm and on-station trials in three regions of Illinois (six locations in all). On-farm trial treatments included cereal rye, rape, and fallow. On-station trials included cereal rye, rape, canola, mustard, and fallow. The mustard crop winterkilled in most locations. The rye crop established well, and the rape and canola crops established well in some locations, but poorly in others, based on fall rainfall amounts. Cover crop biomass was incorporated into the soil in the on-station trials, but left standing in the on-farm trials, as this fit with the standard practices used by the farmer-cooperators.

Biomass and soil samples

Biomass levels were assessed for each cover crop plot at all six locations were quantified prior to burn down (and incorporation) in the spring of 2013. Rye produced the largest levels of biomass at each location. Biomass levels produced by rape and canola were statistically equal at all locations. No mustard biomass was present in the spring, as a result of winterkill.

Soil samples were then collected shortly after planting of the soybean crops, several weeks after incorporation of the cover crops. Subsamples were collected from each soil sample (from each treatment plot) and immediately frozen at -80 °C for later use in DNA analysis. The remainder of the soil samples were stored at 4°C until used in greenhouse bioassays for disease suppressiveness. DNA was extracted from these samples and used to quantify populations of selected soybean pathogens including Fusarium virguliforme, Phytophthora sojae, Heterodera glycines, Phialophora gregata, Macrophomina phaseolina, and Colletotrichum truncatum. No differences in pathogen populations, determined through DNA analysis, were detected among the cover crop treatment soils. However, evaluations of soybean cyst populations using direct egg counts showed a reduction in egg counts in the rye treated soils in some locations. ARISA analysis of microbial community structures was conducted to determine if there were general changes in community structure resulting from cover crop treatments. While the community structures in the soils collected from the various trial locations were fairly distinct, there were no apparent changes in community structure resulting from the cover crop treatments.

Soybean establishment and disease severity

In the 2011 trials in which soybean plots were infested with the fungal pathogen R. solani at the UIUC on-station location, significant stand reductions were seen in the fallow and mustard cover crop plots, better establishment in the rape and canola treated plots, and almost no reduction in soybean stand establishment in the rye treated plots. Reductions in soybean stand establishment occurred in the both the R. solani and F. virguliforme infested plots in 2012, but the levels of reduction were not as large as those observed in 2011. In 2013 we again saw higher stand counts in the rye plots that were inoculated with R. solani, as compared to the fallow/inoculated plots at the UIUC location. There also were differences in the levels of Rhizoctonia root rot severity in 2012 and 2013 in the R. solani infested plots. Infestation with the sudden death syndrome pathogen, F. virguliforme in 2012 and 2013 did not result in increased levels of SDS severity as compared to the non-infested plots, and there were no significant differences in SDS disease levels among the cover crop treated plots that were infested with the SDS pathogen. At all of the other trial locations, where plots were not intentionally infested with pathogens, there were no significant differences in soybean stand establishment associated with cover crop treatments in 2013, with the exception of the Western Illinois Allison Farm, where the highest stand counts were observed in in the rye and rape plots.

Plants were evaluated for foliar and root diseases in the late vegetative stages and early reproductive soybean growth stages and for vascular diseases (sudden death syndrome [SDS] and brown stem rot) in the late reproductive stages (R6).  Levels of foliar diseases were low at all locations as a result of hot and dry conditions over most of Illinois during the growing season. Such conditions are not conducive for most foliar diseases. In most locations there were no observed effects of cover crop treatments on foliar disease levels.

Soil suppressiveness bioassays

Greenhouse bioassays of soils collected from the cover crop plots were conducted to quantify levels of disease suppressiveness to two pathogens, R. solani and F. virguliforme on soybean seedlings. Differences in disease suppressiveness associated with cover crop treatments were observed at some locations in some years to both pathogens. Increased suppressiveness to R. solani was observed in soils collected from rye plots at both the Ayres Farm and the WIU farm in 2012, and from the canola plots at the UIUC location in 2013. Increased suppressiveness to R. solani was also seen in soils collected from the rape and mustard plots at the WIU farm. Increased levels of suppressiveness to F. virguliforme were seen in soils collected from the rape plots at both the WIU and Hunt Farm locations in 2012 and from the rye plots in 2013. Increased suppressiveness to F. virguliforme was also seen in soils collected from the rye plots at the Hunt farm in 2012.

Impacts and Contributions/Outcomes

The results from the third year of this study showed some similarities and some differences over the three years. We are seeing mixed results between years and between trial locations. There was some impact of cover cropping, especially the rye treatments, on lowering disease levels, increasing disease suppressiveness of soils, and increasing yields, but not at all locations, and results have varied from year to year at specific locations.  The information obtained so far is promising, in that it does indicate that cover crops can reduce both root and foliar diseases of a following soybean crop and result in increased yield levels, especially in situations where Rhizoctonia root rot is a problem. This information will encourage soybean farmers in the Midwest to consider adding cover crops to their rotation schedules, resulting in less soil erosion and increased soil health. The results of this study were presented at various events in 2013, including summer field days, extension conferences, and scientific meetings.

Collaborators: