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The widespread drought made 2012 a banner year for SCN in Iowa. “In 25 years I’ve never seen as much reproduction as we did last year,” says Greg Tylka, Iowa State University (ISU) Extension nematologist and plant pathologist. “We don’t understand exactly why SCN seems to thrive when it’s dry, but we also know that nematodes grow best in the greenhouse under dry conditions. “
Conversely, the 2013 growing season began in Iowa with record rains, delaying soybean planting. “But by June 2,” Tylka says, “SCN females were reported on soybean roots in sandy soils just 26 days after planting. In a normal spring, we expect to see female nematodes 40 days after planting.
Very often, the only visible sign of soybean cyst nematode is an egg sac on soybean roots.
One tough customer
“SCN is the ultimate pathogen,” says Greg Tylka, Iowa State University (ISU) Extension nematologist and plant pathologist. “It causes yield loss directly and indirectly by making other things, like soybean sudden death syndrome (SDS) and soybean brown stem rot (BSR), worse. We don’t know completely how it works, but it is a very consistent relationship,” Tylka says. “SCN has a unique biology that makes it very difficult to control. High reproductive capability, genetic diversity and variable responses to different cultural practices and control methods all contribute to it being the leading cause of soybean yield loss.”
SCN populations build rapidly because females’ very large ovaries enable them to lay several hundred eggs at a time, Tylka says. “Its fairly short lifecycle allows four or five generations to occur in one season.
“SCN females mate with many different males, providing tremendous genetic diversity,” Tylka says. This impedes resistance management because SCN can respond differently to various control methods. The genetic diversity is one of the reasons that SCN has been able to reproduce on soybean varieties with the PI 88788 and Peking sources of SCN resistance.”
And SCN has the unique ability to live in dormancy for 10 or more years without food, Tylka says. “Many farmers have infested fields and do not know it’s there. Infestations can increase undetected for five to 10 years. Soybean yields decrease steadily but the plants don’t look sick.”