This year Iowa has had one of the worst soybean sudden death syndrome epidemics since the disease was found in Iowa in 1994. Preliminary research data suggest that corn has much to do with the SDS pathogen and there are things farmers can do about the disease as corn harvest begins. "Management of SDS should start when you harvest cornfields," says X.B. Yang, an Iowa State University Extension plant pathologist.
Yang and his colleagues at ISU have been puzzled by the fact that SDS is now more prevalent in the Corn Belt; the disease was first reported in Arkansas. This summer many people in Iowa reported that SDS occurred in soybean fields where little or no SDS had been seen previously. These fields have been in a corn and soybean rotation for years. And there have been many reports of severe outbreaks of SDS in fields which had been in continuous corn for years.
"We speculated that the rotation with corn may have something to do with it," says Yang. "Specifically, corn may harbor the SDS pathogen in the absence of soybeans."
What has corn got to do with Soybean Sudden Death syndrome?
With funding from the Iowa Soybean Association, Yang and his colleagues conducted studies for two years, both in the greenhouse and in field plots to examine the survival of SDS fungus in corn. "We wanted to see if corn residue harbored SDS pathogen in the absence of soybeans," he explains. "We compared the survival of SDS fungus in two crop residues--corn or soybean--which included different parts of a plant--root, stalk, kernels. We consistently found the highest SDS fungus population in the treatment with corn kernels at a density of average harvest loss."
Yang and his research team repeated the experiments in both greenhouse and in fields, and the results are consistent over the two years. Figure 1 shows the frequency of the SDS fungus isolated from field plots that had a variety of crop residues plus the SDS fungus; Figure 2 shows the frequency of the fungus isolated from similar treatments in the greenhouse.
Soil samples also were collected from one field in each of these counties -- Boone, Black Hawk and Delaware. These fields had SDS in previous growing seasons and were in corn-soybean rotation. Cropping history and yield data were collected from these farms to better understand survival of SDS fungus in soil and on residues. Results from these farms are on par with the results shown in Figures 1 and 2.
ISU findings are consistent with these observations about SDS
"Our findings are consistent with the following observations," says Yang.
1) Severe SDS occurs in corn/soybean rotation fields although little or no previous SDS was observed;
2) Severe outbreaks of SDS occurred after a few years of continued corn production; and
3) Severe SDS was found in seed-corn fields which often have a lot of unharvested kernels due to quality control.
"Although we are yet to experiment to see which types of tillage measures are better at reducing the SDS pathogen in a corn/soybean rotation, our results so far suggest that a nice, clean harvest of corn should help reduce the risk of SDS occurring in soybeans, while a high amount of harvest loss -- corn kernels on the ground -- increases the SDS risk," says Yang.