COLUMBUS, Ohio - Soybean growers should be testing soil samples to determine the presence of soybean cyst nematode, a small round worm that can cause severe yield losses.
Ohio State University plant pathologist Mac Riedel said testing soil samples for nematodes is the first step in a comprehensive soybean cyst nematode management and control program. "Testing soil samples for nematode eggs is a start in helping to keep yields up by managing soybean cyst nematode populations and their feeding habits," said Riedel.
Soybean cyst nematode damages soybeans by feeding on plant roots, robbing the plant of nutrients, and by providing wound sites for fungi to enter. The severity of crop damage and yield loss is dependent on crop rotation and the soybean variety planted.
"The goal is to make sure that populations don't get out of hand," said Riedel. "By calculating the number of eggs in the soil, a farmer can decide if he needs non-host crops (corn or small grains) or resistance soybeans in the rotation scheme.
Yield loss threshold begins at 200 eggs per cup of soil. "One mature female can contain between 200 and 500 eggs," said Riedel, "so it doesn't take much for a farmer to begin losing yields." At 2,000 eggs per cup of soil (equivalent to about 10 females), most soybean varieties susceptible to soybean cyst nematode will be damaged to the point where it's uneconomical to grow them. "We recommend that farmers switch to a resistant soybean variety if they have those kinds of egg numbers in their soils." Farmers seeing egg numbers above 5,000 per cup of soil should avoid growing soybeans altogether. "We've had some farmers with egg numbers so high that we've told them they'd have to grow corn or small grains for the next five or six years to break the nematode cycle. They just stare at us," said Riedel. "It's a hard thing to tell a farmer, especially when it's more economical right now to grow beans rather than grain crops." In addition to calculating nematode egg numbers, Riedel said it's important to practice crop rotation between different crops and among resistant and susceptible nematode soybean varieties due to the nematodes' feeding habits.
"If a farmer only uses one source of resistance year after year, the nematode will eventually adapt and the variety will become useless," said Riedel. A good rule of thumb to follow when planting crops is that the soybean cyst nematode will increase 10-30 times on susceptible varieties in a growing season, but will only decrease by half under a non-host crop.
"For example, if you have a low population of 200 eggs going into a growing season on susceptible beans, your population will go up by 2000 eggs at the end of the season. The next year, if you grow corn that population will decrease by half and if you plant a resistant soybean variety in the third year, that population may drop, say, to 500 eggs. Then the next year you can plant a susceptible soybean variety and not run the risk of losing a lot of yield," said Riedel. "It's a good way to help control the populations while still practicing the needed crop rotation." It may sound easy, but getting farmers to actually follow a strict management plan is difficult. "We keep telling farmers that they need to be more diligent about controlling the soybean cyst nematode," said Riedel. "Farmers can see a 5-10 percent drop in yield without even seeing any above-ground symptoms. And it's not uncommon for farmers who grow susceptible varieties with at least 2,000 eggs per cup of soil present to lose 15 bushels per acre." In addition, farmers are reluctant to plant resistant soybean varieties as they feel that these varieties don't have as high yield potential as susceptible varieties. "Low yields may hold back some growers, but we continually show them that they are throwing away 10-12-14 bushels per acre to the worms by not using resistant varieties," said Riedel.
Ohio State researchers have just completed their third year of study using GPS technology to map and track soybean cyst nematode populations against susceptible and resistant soybean varieties. They've used Wood and Clermont counties as model counties for growing beans and have calculated a 17 bushel per acre increase using resistant soybean varieties over susceptible varieties in soils with nematode egg populations of over 10,000.
"The beauty of it all is that farmers with GPS technology can map and track the nematode populations in their own fields," said Riedel. "If farmers don't control the nematode, it'll finally get to the point where they won't be able to grow beans anymore." For more information on the soybean cyst nematode and soil sampling methods, contact your local OSU Extension county office or log onto http://ppdc.osu.edu/ or http://ohioline.osu.edu/ac-fact/0039.html.