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
“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
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
“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,”
“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
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
Members of the mustard family are used in the three other
“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
“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
“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.”