When Iowa State University SCN guru Greg Tylka wanted to plant test plots in a field with no cysts, he chose a university plot that hadn't had seen soybeans for more than a decade. In March 2000, he divided an 80 × 160' section of the plot into quarters and collected 10 core samples from each quadrant. All four quadrants tested negative for soybean cyst nematode (see top chart).
In May of 2000, Tylka divided the area into 64 smaller, 10 × 20' plots for his checkoff-sponsored trials and tested again for SCN, taking 10 core samples/plot. Samples showed that low SCN populations did exist in six out of the 64 plots, with one plot testing at 500 eggs/100 cubic centimeter of soil (see middle chart).
A third sampling of the plots in October 2000 revealed even more startling results. Only eight plots tested negative for SCN. The plot that tested at 500 eggs/100 cubic centimeters in May jumped to 19,700 eggs/100 cubic centimeters in October. The highest density recorded was 37,000 eggs/100 cubic centimeters in a plot that had tested negative just six months earlier (see bottom chart).
“It's mostly the sampling, not the cysts moving that cause the inaccuracy,” Tylka says. “Cysts are extremely aggregated and can't move around much in the soil. We just missed them when we sampled the plots in March and May.”
While the testing results surprised Tylka somewhat, he doesn't believe the situation is unusual. “I've been telling people for years that just because you don't find SCN doesn't mean it isn't there,” he says. “The sampling procedure we used was probably 1,000 times more intensive than what we suggest to farmers, and we still missed it.”
That's why Tylka recommends that when you find any SCN in a field, it pays to plant the entire field with an SCN-resistant soybean variety. “Our recommendation, once you find SCN in a field, remains a six-year rotation of corn, SCN-resistant soybeans, corn, SCN-resistant variety, corn, SCN-susceptible variety.”
Tylka stresses that it's important to include an SCN-susceptible variety once in the six-year rotation to maintain non-resistant strains of SCN.
“Resistance is the only thing we have going for us in the battle against SCN,” he says. “If we lose that, we're in trouble.”
For more management information on SCN, check Tylka's Web site at www.soybeancyst.info.