"Some fields made 50 bushels of soybeans, but SDS-infested fields - planted to the same variety - yielded only 30 bushels."
Jerry Cox, Chaffee, MO, is referring to sudden-death syndrome, the "stealth" killer of otherwise-healthy beans. In the decade since his first SDS outbreak, the disease has periodically returned, sometimes severe, sometimes barely noticeable.
"My best defense against SDS is variety selection," says Cox. "Over the past 10 years, I've learned which soybean varieties are least susceptible to the disease."
To date, none are truly immune to the SDS-causing fungus, Fusarium solani. But that should change in the next few years, believes David Lightfoot, biogeneticist at Southern Illinois University. Lightfoot has developed a DNA marker system that lets him identify and track resistance genes.
"We've found a gene that prevents the fungus from moving into soybean roots," says Lightfoot. "It will be relatively easy to move this gene into soybean varieties.
"We're looking for more genes that preclude movement of the fungus into plant roots," Lightfoot adds.
Southern Illinois University is the hub of a four-state research effort to combat SDS. Team members in Arkansas, Illinois, Iowa and Missouri are studying different aspects of the puzzling syndrome. For one thing, the fungus itself invades only roots of soybean plants, but produces toxins that poison the entire plant.
"In infested fields, the fungus can be found in soybean roots shortly after planting," says Al Wrather, a University of Missouri plant pathologist and SDS research team member. As part of his research, Wrather screens germ plasm for root infection with the F. solani fungus. He notes that, while there is no proven cause-and-effect relationship between soybean cyst nematode and the SDS organism, the two pests are linked.
"Both favor light-textured, well-drained soils," he says. "You are more likely to have problems with both SDS and cyst nematodes on these soils. That makes it important to choose varieties on the basis of what problems you anticipate."
John Rupe, University of Arkansas plant pathologist, agrees.
"There's a strong connection with cyst nematode and SDS," he says. "Many cultivars that show susceptibility to cyst nematode are also more susceptible to SDS."
Rupe also is studying moisture in relation to SDS outbreaks.
"Where the fungus exists, SDS may show up in spots in a field any year," he points out. "But when you plant a susceptible variety and get wet weather at blooming or just before, up to 80% yield loss can occur."
Until the SDS-resistant genes Lightfoot tracks are bred into commercial soybean varieties, SDS researchers recommend that growers with the fungus in their fields choose varieties carefully.
"Pick a soybean that is at least somewhat resistant to SDS," Wrather advises. "Susceptibility varies widely among varieties."