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CHAMPAIGN, Ill. — Cover crops are known for their agronomic benefits, but lesser known for their impact on pests such as soybean cyst nematodes.
Chelsea Harbach, University of Illinois plant pathologist, conducted trials on the cover crops/SCN relationship and spoke of her findings during Crop Management Conference.
“The use of cover crops has increased over the past decade, and that largely has to do with the known agronomic benefits that we see from using cover crops that include building soil health, nutrient retention, erosion control and weed reduction,” Harbach said.
“However, when we’re introducing cover crops into our agricultural system we’re introducing the potential for this new interaction between the plant pathogen and the cover crop.”
She conducted the three-year study that began in 2016 at Iowa State University under the guidance of her then-adviser Gregory Tylka, nematologist and Iowa Soybean Research Center director.
The trials include lab-based assays, greenhouse experiments and small plot field studies. Her trials included two cultivars each of cereal rye, mustard, oilseed radish, annual ryegrass, daikon radish and mixes. The control treatments included non-cover crop, a non-host control using tomatoes and SCN susceptible soybean, Williams 82.
Experiments were conducted in the laboratory and greenhouse to determine if cover crops have the potential to serve as trap crops, in the greenhouse to find if the crops affect SCN population densities under controlled conditions and in the field to assess how cover corps affect SCN population densities in multi-year, multi-location and rotations.
Three possible outcomes were anticipated — cover crops would be a favorable host; there would be no impact on SCN; and SCN population densities would decrease.
A favorable host trap crop could stimulate SCN hatching. The nematodes would then enter the cover crop roots that aren’t a suitable host and the nematodes would die.
There also was the notion that the trap crop would stimulate SCN hatch. The roots would give off root exudates compounds. Those compounds interact with the eggs in the soil to stimulate the hatch and with no favorable host the nematodes die.
Another concept investigated is the possibility of root exudates residing in the soil after the cover crops are gone and the cash crop is planted and if those exudates produce allelochemicals that either inhibits hatch or kill the nematodes.
Findings
“There was no hatch stimulation from any of the cereal cover crops. It looks like crimson clover might be a hatch stimulant and maybe other legumes, but we definitely know there isn’t any hatch stimulation by any of the other cover crops,” Harbach said.
“It will be interesting to take this a step further to see if any of these cover crops can serve as a trap crop because if you have a cover crop that serves as a trap crop you would think that it may also stimulate a hatch. So, we’ll keep an eye on that crimson clover.
“We do know that overall there was a decrease in SCN population for all of these cover crops over the 60 days in the greenhouse.”
Small plot studies in Iowa over three years began in 2016 and monitored SCN density. The cover crop was seeded into the standing cash crop in late August, early September. Soil samples to determine SCN population were collected on seeding day, in mid-November and after cover crop termination.
The small plot results showed no significant reduction in SCN populations across years, locations, treatments or sample dates under three experimental conditions.
Other Trials
Harbach noted other similar trials and their outcomes.
“Field studies for soybean cyst nematode are pretty difficult because this pathogen has patchy distribution in the field. So, the field studies that have been conducted to date have not resulted in any significant effects of cover crops in reducing SCN, and there hasn’t been any difference between the cover crops and the controls,” she said.
“There have also been greenhouse studies conducted and this is kind of the same story where the results were not significant between the cover crops and control.
“We also have this conundrum of cover crop seed companies advertising cover crop mixes or specific cultivars as reducing soybean cyst nematode without the data to back that up. It suggested that some cultivars suggest they can reduce nematode populations, not specifically SCN, but we’re seeing it reduces nematodes, but we don’t have data to back that up.
“Would I rely on cover crops to control SCN? No, but while I don’t recommend cover crops for controlling SCN populations that doesn’t mean we shouldn’t continue to use cover crops for the other agronomic benefits.”