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RESEARCH HIGHLIGHTS

Susceptibility of cover crop plants to Fusarium virguliforme, causal agent of soybean sudden death syndrome, and Heterodera glycines, the soybean cyst nematode
Wed - December 20, 2017
by Leonor Leandro and Greg Tylka, soybean plant pathologists, Iowa State University

The use of cover crops in corn-soybean rotations is encouraged because of the many benefits -- such as reducing erosion and nutrient loss from fields, improving soil quality, and improving weed control. With the increasing interest in the practice, it is important to know how adding cover crops to the rotation might impact soybean diseases.

Two major pathogens causing yield losses in soybeans in the Midwest are Fusarium virguliforme (Fv), the causal agent of soybean sudden death syndrome (SDS), and Heterodera glycines, the soybean cyst nematode (SCN). To date, it is not known how cover crops may impact these diseases. With checkoff funds provided by the Iowa Soybean Association, the United Soybean Board, and the USDA, we conducted greenhouse studies to evaluate the susceptibility of common cover crop species to infection by these two major pathogens.

The cover crops tested were five legumes species: alfalfa, crimson clover, hairy vetch, pea and red clover, and 11 kinds of grasses and Brassica species:  false-flax, mustard, turnip, corn, millet, oat, ryegrass, rye, sorghum, triticale, and wheat. Knowing the impact that these plants may have on SDS and SCN is important to help farmers make better decisions when choosing cover crops for fields with a history of SDS and/or SCN.

Results for Soybean Cyst Nematode
In the SCN experiments, the leguminous and non-leguminous cover crop plants were grown in soil naturally infested with SCN, and the number of females formed on the roots was determined after 30 days.

We found that very few (zero to five) SCN females formed on the roots of the leguminous cover crop species studied, and females were rarely recovered from the roots of any of the non-leguminous species studied. In contrast, many SCN females formed on the roots of the susceptible soybean control included in the experiments. The results of three different greenhouse experiments with 2 to 3 runs each confirmed that there was little to no SCN reproduction on any of the leguminous and non-leguminous cover crop plants studied.

This research indicates that the cover crops studied in our experiments are not hosts of SCN and that farmers do not need to be concerned about inadvertently increasing SCN population densities if these plants are grown as cover crops in SCN-infested fields.

However, the experiments do not provide information to support or refute claims that some cover crop species reduce population densities of SCN when grown in fields infested with the nematode. Additional greenhouse and field research is underway to determine if suppressive effects are true and, if so, under what conditions.

Results for Sudden Death Syndrome
All of the legume species (alfalfa, crimson clover, hairy vetch, pea and red clover) inoculated with the SDS pathogen (Fv) developed significantly more root rot than the non-inoculated plants of the same species. In addition, the legume species had DNA levels of Fv in roots similar to that of Fv-inoculated soybean, indicating pathogen infection.

However, none of the grass or Brassica cover crop species (false-flax, mustard, turnip, corn, millet, oat, ryegrass, rye, sorghum, triticale, and wheat) inoculated with the SDS pathogen developed root rot, suggesting that these crops are non-hosts or poor hosts.

The results of our greenhouse experiments on SDS showed that the legume cover crop species tested in this study can be infected by Fusarium virguliforme, and are therefore hosts to the SDS pathogen. This indicates that these plant species may help to build-up Fv populations in the absence of a soybean crop. On the other hand, the Brassica species and grasses tested were either poor or non-hosts of Fv, and may be preferred cover crop options for fields with a history of SDS.

Although this study focuses on the ability of Fv to infect live plants, it is known that Fv is able to survive and reproduce on plant residue. It is possible that the residue derived from non-host cover crops could support growth of Fv inoculum between growing seasons, but more research is needed to determine this.

Knowing the impact that cover crops may have on SDS and SCN is important to help farmers make good decisions when planting cover crops in fields with history of these diseases.

Resources
Susceptibility of cover crop plants to Fusarium virguliforme, causal agent of soybean sudden death syndrome, and Heterodera glycines, the soybean cyst nematode. Journal of Soil and Water, 2017.

What’s the real deal with soybean cyst nematodes and cover crops?  Integrated Crop Management, Iowa State University and Extension, November 2017

Integrating Cover Crops into Soybean Rotations - Midwest Cover Crops Council and NCSRP

Managing Cover Crops Series - Purdue University, NCSRP, and Midwest Cover Crops Council Sudden Death Syndrome - Crop Protection Network
Scouting for Sudden Death Syndrome (trifold version)

Soybean Cyst Nematode - NCSRP, 5th Edition
SCN Field Guide - Iowa State University Extension Store