URBANA, Ill. — Weed suppression, nitrogen sequestration, improved soil tilth and erosion reductions are among the benefits touted for cover crops, and research has found another advantage to add to the list.

A three-year study by the University of Illinois, Western Illinois University and Southern Illinois University has found evidence that cover crops can suppress soybean diseases.

Results of the study, now in its final year through funding support from the North Central Sustainable Agriculture Research and Education, were released during U of I’s recent Agronomy Day.

The use of cover crops to suppress diseases has been investigated in high-value crop systems such as vegetables and fruit production, but not on a larger scale for corn and soybeans.

Darin Eastburn, U of I Department of Crop Sciences plant pathologist, wanted to take that one step further and see how disease management in fruit and vegetable systems can translate in a large corn-soybean system.

A key question researchers hope to answer is how a cover crop grown off-season affects disease in a soybean or corn crop two to three months later.

“There are a couple of possibilities, and most of these have been evaluated in research trials and found to be valid,” Eastburn said.

“One is you’re increasing the microbial activity in the soil. When you’re adding that crop debris to the soil, you’re basically feeding the microorganisms in the soil.

“You’re increasing the level of activity of the bacteria and fungi in the soil, and that does a number of things. Those microorganisms compete for nutrients, and the idea we’re trying to do is get the nonpathogenic microorganisms to out-compete the organisms that are causing plant diseases.”

The nonpathogenic microorganisms sequester nitrogen, taking the proteins and carbohydrates away from the pathogens making them less likely to cause disease.

Many of these nonpathogenic microorganisms produce antibiotics.

“Microorganisms in the soil maintain their territory. They defend their nutrient sources by chemical warfare. They produce chemicals that keep other things out,” Eastburn said.

“In fact, most of the antibiotics that we use in human medicine come from soil bacteria and soil fungi — microorganisms that live in the soil. We can take advantage of that and have them prevent the development of things that cause plant diseases.

“Some of these are also direct parasites. They’re able to actually eat the pathogens. Whether it’s a soybean cyst nematode eggs or a fungus spore, these parasites can make those less viable, lower the population and reduce the level of diseases.”

Another benefit of this system is it can create reduced systemic host resistance.

“These microorganisms are not pathogens themselves. They don’t cause disease, but they do interact with the plant root system, and their presence stimulates that plant to get its own defense mechanisms in gear,” Eastburn said.

“So they become less-susceptible disease. The presence of nonpathogens makes them less susceptible to infection.”

Cover crops can affect the soil’s chemical and physical characteristics, including temperature.

A layer of cover crop debris on the soil lowers the amount of sunlight reaching the soil, moderating soil temperatures.

The cover crops also affect soil moisture, but not in a way that some believe.

“A lot of growers are concerned that the presence of a cover crop is going to take the moisture out of the soil and reduce the amount of soil moisture, but it’s been shown to do just the opposite,” Eastburn said.

“Moisture in cover crop plots is higher for a number of reasons. One is it has increased the soil organic matter and increases the water holding capacity of the soil.

“Even last year with the drought we saw some of our highest yields in cover crop plots because the moisture level in those plots were higher.”

Some cover crops produce antimicrobial chemicals. As the debris degrades, it releases those chemicals into the soil and suppresses pathogens that cause soybean diseases.

The research plots are located at each of the universities, as well as on-farm trails with cooperating farmers.

Five different treatments are used in the trials, including a fallow plot where nothing was planted in the fall.

“We have cereal rye treatment that fits very well into the corn-soybean rotation planted into the corn or after the corn is harvested,” Eastburn said. “It is established in the fall and then goes dormant and produces a lot of biomass in the spring that we’re able to incorporate into the soil.”

Members of the mustard family are used in the three other treatments.

“We chose these because they do produce a chemical that is supposed to suppress soil-borne fungi. As that crop debris decays, that chemical is released, and the idea was maybe we can suppress some of the soil-borne fungi that cause problems on soybeans,” Eastburn said.

“The mustard is marketed by a company in Idaho, and it says right on the back ‘for control of soil-borne fungi.’ Unfortunately, (mustard) is not winter-hearty here, so it kills out fairly early and so we haven’t seen a lot of effectiveness.

“Both rapeseed and canola produce some of this, as well, rapeseed more so than canola and we are seeing some affects with rapeseed as a cover crop on reducing soil disease.”

Among the areas being looked at in the trials are cover crop biomass, seedling disease severity, both foliar and root diseases, and mid- to late-season disease severity.

Researchers collect soil samples and evaluate pathogen population levels, as well as the population levels of other bacteria and fungi in the soil to evaluate the microbial community structure “and see if we can figure out what kind of microbial community is associated with low disease levels,” Eastburn said. “Ultimately, we are interested in yields, and we also take yield levels.”

Results from the first two years indicate a cover crop’s disease suppression abilities.

“2011 was a great year for rhizoctonia root rot. On-university trials here, we actually add pathogens to the soil to make sure that we get disease,” Eastburn said. “We also added fusarium fungi that causes sudden death syndrome.

“We saw some pretty dramatic results in the first year. In the fallow plots where we added rhizoctonia, there was a 90-percent reduction in seedling emergence.

“In the plots treated with rhizoctonia and had been planted to rye, their level of seedling emergence was almost as good as where we didn’t have rhizoctonia in the first place.”

Rhizoctonia levels weren’t very high the past two years in the plots, and researchers didn’t see dramatic stand establishment differences like what was apparent in 2011.

However, once the plants were dug up and evaluated, there was a difference in the amount of disease between the fallow plots and cover crop fields. The lowest levels were in rye compared to the fallow crop.

“We also saw a difference in the foliar disease – septoria brown spot – on seedlings,” Eastburn said.

“In this case, we saw the highest level of septoria in the fallow plots and the lowest levels in the rye plots. It could be because of that systemic induced resistance phenomena, but in this case it could also be a physical barrier.

“This was a farmer plot where he killed the rye, but did not incorporate it into the soil. He left it standing. The rye was a good three feet tall, which you might think would be terrible for soybeans, but it didn’t affect yield at all.

“The soybeans came up through that standing rye, and I think that rye might have been a physical barrier to infection by the pathogen that causes septoria brown spot.”

The research also focuses on soybean cyst nematodes.

“For the plots here at the university and on-farm in east-central Illinois, we didn’t have large levels of soybean cyst nematodes in the first place. We really didn’t see much of a reduction,” Eastburn said.

“But in western Illinois where they had fairly large populations of soybean cyst nematodes, we did see some pretty dramatic reductions in nematode populations following the cover crop treatment.”

“If you’re looking for advantages to use cover crops, there is the weed suppression issue, the sequestering of nitrogen, preventing soil erosion and now we have evidence that we can get some reduction of diseases as a result of using cover crops.

“Now you’re sole reason may not be to plant a cover crop to manage a disease. But if you’re thinking of various reasons why you want to plant a cover crop, here’s one more piece of the puzzle that says this is a good and feasible idea.”