If you're tempted to plant soybeans before May in the Midwest, pathologists have some advice: Consider the risk of sudden death syndrome (SDS).
Early planting, combined with above-average moisture and moderate temperatures through the vegetative stage of soybean growth, is conducive to SDS. The early planting becomes a risk factor, although the conditions that follow it determine whether the disease will actually take hold.
“A rash of extremely early planting — in mid- to late April — was probably a major reason there was such a problem with SDS in 2000,” says Don Hershman, University of Kentucky plant pathologist. But post planting environmental conditions were less conducive to SDS in most areas for 2001 and 2002.
“Field observations, along with research from Kentucky and elsewhere, have confirmed that SDS is more likely to be associated with early planting than with later planting,” he adds. “That's why we rarely see SDS in doublecrop beans.”
Greg Shaner, Purdue University plant pathologist, says the majority of SDS samples submitted to Purdue come from early planted fields.
“What's more,” Shaner notes, “Purdue research shows no yield advantage to early planting. It stresses the plants and increases the risk for disease.”
Why the link between early planting and SDS?
“The disease is initiated when the organism Fusarium solani f. sp. glycines infects soybean roots during seedling and early growth stages,” explains Hershman. “Later in the season, the toxins produced by the fungus lead to the above-ground symptoms of SDS.
“Because root infection by the SDS pathogen is needed for SDS to occur, there is a connection with the planting date,” he notes. “Cool, wet soils at planting favor infection by the SDS pathogen. The warmer soils that come with later planting dates are not conducive to f. solani infection.”
Hershman says a study by X.B. Yang, Iowa State plant pathologist, showed a greater risk of SDS for soybeans planted at soil temperatures of 59° or less. Once the soil temperature reached 68°, the risk of SDS dropped considerably.
Steve Butzen, agronomic information specialist for Pioneer Hi-Bred, notes that April planting should be avoided in Midwestern fields with a history of SDS. He points out that Pioneer and university planting date studies have shown little or no yield advantage for planting soybeans in the Midwest before May 10.
“Delaying planting is difficult for farmers,” says Yang. “So I advise them to plant their SDS-infected fields last.”
Soybean specialists recommend planting soybean varieties with a degree of SDS resistance in fields with a history of SDS.
“Although there are no soybean varieties with complete resistance to SDS, some have partial resistance,” says Brian Diers, University of Illinois soybean breeder.
Check the University of Illinois Varietal Information Program at web.aces.uiuc.edu/vips  for the level of SDS resistance in many Mid-western commercial varieties. The site also includes info on yield, susceptibility levels to other diseases and protein and oil content.