As soybeans reached pod fill and later productive stages this year, many growers saw sudden death syndrome (SDS), a yield-robbing soybean disease, take a toll on their fields. "We saw SDS showing up extensively this year, especially in northern Illinois and parts of Iowa," says Jim Trybom, Pioneer agronomy research scientist. "In general, August is the time when the disease starts to show on the leaves. And it was widespread this year."
The past several years, SDS has become an issue in most soybean-growing areas and often is ranked second only to soybean cyst nematode (SCN) in causing decreased yields and economic losses. The root-rotting organism infects bean plants very early in the growing cycle; however, aboveground symptoms do not appear until late summer when the fungus produces a toxin that damages the leaves. In recent years, SDS has had a more profound impact in the U.S., moving into high-producing soybean regions. Early planting and cool, moist conditions early in the growing season often result in higher incidence of SDS.
Beans were planted in cool, moist soil in late-April to mid-May
"Because SDS is more weather-related, its impact and reach can vary year to year and area to area," says Jean Liu, a Pioneer plant pathologist. "This year in April, many areas had warm conditions, then two or three weeks of cooler weather in May. Growers who planted soybeans shortly before or during the period of cool, moist conditions (from late April to mid-May) need to pay attention, because SDS fungus can infect roots as early as seedling emergence. Early infection would aggregate the problem and cause greater yield reduction compared to late infection."
Once detected, there is little growers can do because the disease manifests itself from the bottom up. Although foliar symptoms and defoliation are trademarks of SDS, the fungus itself does not spread to the leaves but rather produces toxins that are transported to the leaves while the fungus only colonizes the roots and base of the stem. For this reason, foliar fungicides are not effective in reducing damage to soybeans from SDS.
Foliar fungicides are not effective in reducing damage from SDS
Instead, growers need to pay attention to signs of SDS now so decisions can be made to reduce potential exposure in next year's crop, says Iowa State University plant pathologist X.B. Yang. He recommends growers their seed sales representative or area ISU Extension crop specialist to confirm the presence of SDS, then monitor how severely it is impacting their fields. The disease usually is more severe if SCN (soybean cyst nematode) is also a problem in the field.
"Growers must clearly understand the extent of infection in each of their fields to effectively manage SDS," Trybom says. "If SDS is identified, growers need to maintain the field history and select varieties with higher tolerance to SDS in the future. Variety selection and good field drainage are some of the best tools available to counter most disease threats."
Tips to help manage and hopefully avoid SDS problems in future
In addition, growers should focus on planting the most problematic fields last, managing SCN, improving field drainage, reducing compaction, evaluating tillage systems and reducing other stresses on the crop, says ISU's Yang.
To assist growers in choosing resistant varieties, Pioneer researchers rate products in multiple test sites with historical SDS occurrence, says Trybom. These sites are irrigated and/or planted early to encourage SDS development. Tolerance data are collected and analyzed across years to determine the appropriate SDS tolerance score. Due to continued improvements in breeding for this trait, Pioneer now has varieties that score as high as 7 for SDS tolerance on a 1 to 9 scale, with 9 being most tolerant.
Trybom says the company also is aggressively advancing seed products and traits to provide new levels of tolerance to SDS, which is a complex trait requiring researchers to stack multiple genes for stronger and more effective resistance. Molecular markers are being developed to aid with breeding in making these complex genetic stacks. In the future, growers will need soybean varieties that possess new sources of resistance to stay ahead of SDS and other diseases.