What’s the real deal with soybean cyst nematodes and cover crops?

The North Central IPM Center and the North Central Region’s IPM Coordinators are collaborating to assist our region’s IPM-focused students with professional networking opportunities including travel support to the 9th International IPM Symposium. As part of our travel support program students will present a poster at the IPM Symposium OR submit an article to the NCIPMC Connection newsletter.

This article is submitted by Chelsea J. Harbach who is a student at Iowa State University in Dr. Greg Tylka’s Laboratory. 

The number of acres planted to cover crops annually has been steadily increasing in recent years throughout the United States. Meanwhile, the soybean cyst nematode (SCN) continues to sit atop of the U.S. list of yield-suppressing pathogens. It’s no coincidence, then, that there is an increasing interest regarding the potential for these two factors to interact in the field, particularly with the possibility that cover crops could decrease SCN population densities (numbers).

When considering the potential effects of cover crops on SCN, there are several possibilities. First, it is possible that cover crops could serve as hosts for SCN reproduction, thereby inadvertently increasing SCN population densities in fields where these plants are grown. Many cover crop species have been found to be non-hosts for SCN; however, we cannot say that all cover crops are non-hosts as some species can support SCN reproduction, including crimson clover, field pennycress, and more. Second, it is possible that there is no effect of cover crops on SCN population densities. Lastly, and most interestingly, there is the potential that cover crops may decrease SCN population densities.

There are several mechanisms through which cover crops could decrease SCN population densities:

Mechanism of decreasing SCN numbers or reproductive potential How the mechanism works
Producing nematicidal compounds Some plants, notably members of the Brassicaceae family (which includes radish, mustard, and canola), produce methyl-isothiocyanates as they decompose. These compounds have nematicidal properties.
Serving as a trap crop If SCN juveniles hatch and enter roots of cover crops, it is unlikely they will be able to serve as a host. Being unable to establish a feeding cell, the juveniles would subsequently be “trapped” and die in the plant roots. Ideally, to serve as an effective trap crop, many nematodes would enter the roots to decrease SCN numbers.
Inducing egg hatch As plants grow, the roots in the soil give off compounds called root exudates. It is possible that root exudates could stimulate hatch of SCN juveniles in the fall or spring. If the juveniles hatch at these time points, there is likely no food source for them so they would end up dying of starvation in the soil.
Producing inhibitory allelochemicals Some plant roots may produce inhibitory allelochemicals, either while living or decomposing, that affect other organisms. Such allelochemicals may inhibit the hatch of SCN juveniles, which would overall make the pathogen less productive.

There have been few published reports of how different cover crop species may have a negative effect on SCN numbers. Additionally, there are no data on whether different cultivars within a species of cover crop have differential impacts on SCN numbers.

Some seed companies market their cover crops with benefits like “reduces SCN population densities” or generalize as “decreases nematode populations” but provide no data to substantiate the claims. There are a few reports that suggest that some cover crop species can reduce SCN population densities. The few sets of field data relating to effects of cover crops on SCN are lacking necessary details, were not reproduced over a sufficient range of locations and growing seasons, and/or the results are inconsistent among locations and years.

This all leads to the premise of the cover crop and SCN experiments that I am working on at Iowa State University in Dr. Greg Tylka’s lab. While we have an inkling of an idea about how cover crops could affect SCN population densities, my work is taking a deep dive into this interaction to provide a well-thought-out and comprehensive look at this interaction.  The experiments I am conducting range from very large scale, on-farm strip trials coordinated though a collaborative effort with the Iowa Soybean Association, to laboratory assessments of how root exudates from specific cover crops affect the hatch of SCN juveniles.

The potential benefit of cover crops in decreasing SCN population densities and the scarcity of robust data assessing the implied phenomenon are why I am working on hashing out the effects of cover crops on SCN at Iowa State University. There are researchers at other universities in the North Central region working on this puzzle as well, including Michigan State University, University of Missouri, North Dakota State University, Ohio State University, and perhaps even other institutions. With these combined efforts, there should be a much clearer understanding of the potential benefits and limitations of cover crops for management of SCN in the upcoming years.

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