Sudden death syndrome (SDS) and soybean cyst nematode (SCN) bring out the worst in each other. “When the two are together, disease severity tends to be much stronger,” says Anne Dorrance, Ohio State University plant pathologist.
“SCN and SDS go hand in hand,” says Purdue Nematologist Virginia Ferris. “SDS is worse when SCN is present, but the presence of the nematode is not required for SDS to occur.”
In a University of Illinois (UI) study, when SCN was present, there was double the amount of SDS, says UI Nematologist Terry Niblack.
SCN is also a troublemaker with brown stem rot (BSR). The cyst nematode can break soybean varieties’ BSR resistance, according to Iowa State University (ISU) research. “SCN increased BSR disease even in BSR- and SCN-resistant varieties,” says ISU Nematologist Greg Tylka. “And SCN makes BSR progressively worse as SCN numbers increase.
“BSR can be somewhat of a hidden disease that often doesn’t cause obvious leaf symptoms, only internal stem symptoms, so many growers have it without knowing it unless they split open the stems to see the brown rotting inside,” Tylka says. “Checkoff-funded research in the mid-1990s revealed that 72-73% of Iowa and Illinois fields tested had BSR, and 68% of Minnesota fields had it.”
The mechanism for these synergies is not understood. But it appears that additional physiological changes within the soybean root are involved in the synergistic disease development. In ISU studies where soybean roots were split to form two separate root systems, one that was then inoculated with BSR and one with SCN, the disease synergy still occurred. “We still saw increased stem discoloration symptoms even though SCN and the BSR fungus were physically separated on different half-roots,” Tylka says.
A similar split-root study is being conducted by Tylka and ISU’s Leonor Leandro, pathologist, on the SDS and SCN interaction.
How can you slow down these deadly interactions?
• Solve the SCN problem first because SCN “is always present, reducing soybean yields, while fungal diseases such as SDS and BSR develop only when en-vironmental conditions are favorable,” says Niblack.
“Any factor that keeps the soil moist for extended time periods favors SDS, but SCN is the biggest factor tied to SDS,” Dorrance says.
“If you have an SDS problem every year, then the next step after SCN control is to look for SDS resistance,” Niblack says.
• Rotate, rotate rotate. “One year of corn reduces SCN on average about 35%,” says Niblack. “And the important thing is to reduce SCN populations.”
However, Indiana research by Andreas Westphal shows that a single year of corn may be insufficient to reduce SDS pressure in problem fields.
Just as alternating herbicides with different modes of action impedes weed resistance, university experts recommend rotating among different sources of SCN resistance.
“In the past few years, we’ve seen more nematodes reproducing in fields planted with varieties using the traditional source of resistance (PI 88788) which is deployed in many Midwest varieties,” says Tylka. “Although currently, SCN-resistant soybean varieties with PI88788 resistance continue to yield well and keep SCN population densities from increasing.”
When selecting for resistance to SDS, “you should start with SCN-resistant varieties, because SDS severity is worse when SCN is present,” says Daren Mueller, ISU Extension plant pathologist.
• Plant beans later in the spring to try to avoid SDS. But delaying planting will also lower yields, even if SDS is not a problem, notes Mueller. “With early planting, your selection of SDS resistance becomes even more important. If you have a problematic field or are planting a SDS-susceptible variety, planting these fields last may help.”
• Reconsider planting before a rain storm because prolonged wet conditions favor several diseases, Dorrance says.
SDS symptoms are frequently associated with major weather fronts, which bring cooler temperatures and significant rainfall near the flowering stage, says Craig Grau, University of Wisconsin professor of plant pathology emeritus. SDS symptoms commonly first appear 10-14 days after heavy rains and saturated soils.
• Improve drainage in poorly drained fields and avoid compacting soils with farm equipment.