COLUMBUS, Ohio - If a soybean grower is experiencing low yields this season, especially throughout northern Ohio, soybean cyst nematode may be to blame.
Mac Riedel, an Ohio State University plant pathologist, said the summer drought provided ideal conditions for soybean cyst nematode development. Soil samples from several counties in northern Ohio recorded egg counts ranging from 10,000-20,000 per cup of soil - populations so high that planting a soybean variety next spring, including SCN-resistant, may prove economically unwise.
"SCN generally does better when it's droughty and many of the soils in the northern Ohio area are sandy loam, which the nematodes just love. There's a lot of open pore space in the soil and the oxygenation is pretty good," said Riedel. "So when we are getting sample after sample of 10,000-20,000 eggs per cup of soil, that indicates not only good reproduction, but it also indicates that probably some growers are not having their populations checked and nematode control is getting away from them." High egg counts were found in Wyandot, Seneca and Sandusky counties, a primary soybean area no stranger to SCN.
"High populations in Wyandot surprised us because for years the county has reported low egg levels," said Riedel. "Sandusky was the first county where soybean cyst nematode was identified and Seneca is also considered an 'old-find' county, so growers have had a long exposure to the pest. So why populations are so high now is not entirely known, but it's a very troublesome find." The discovery of economically devastating SCN levels is a reminder to growers of the importance of sampling their fields in the fall for SCN egg counts to determine how the pest will need to be controlled come spring.
"SCN is a perfectly manageable pathogen if you know it's there. There's no reason a grower should suffer any losses from SCN," stressed Riedel. "We know a lot about its life cycle and its management, but you have to know it's there to take advantage of that. And there's only one way to find out if you've got it and that's to sample for it." The easiest way for growers to test their fields is to collect five to eight soil samples throughout several points in the field using a one-inch agronomic soil core. The samples should then be mixed and sent to a soil-testing laboratory to obtain a representative count of nematode eggs. Ohio State University C. Wayne Ellett Plant and Pest Diagnostic Clinic houses such a laboratory.
Yield loss threshold of SCN begins at 200 eggs per cup of soil. At 2,000 eggs per cup of soil, most susceptible soybean varieties suffer damage to the point where it's uneconomical to grow them. At 5,000 eggs per cup of soil, growers should avoid growing soybean varieties altogether, even resistant varieties.
"With susceptible varieties, when there is stress on the crop or poor nutrition, you can detect economic losses under 1,000 eggs per cup of soil. So at the 10,000 range, susceptible beans are producing very little yield and resistant beans will probably have root damage and won't yield optimally," said Riedel.
Additionally, since SCN populations tend to increase 10-fold on host crops, planting another soybean crop next spring into a field with 20,000 eggs per cup of soil is likely to increase populations to 200,000 eggs per cup of soil - forcing the grower to plant non-host crops for five to six years to bring populations below economic threshold.
"That outlook is obviously not economically viable for a grower. So it's important to get out there and test fields to determine what populations you do have," said Riedel. He noted production numbers in some fields during the late 1980s-early 1990s showed only a six-to-eight-bushel-per-acre harvest in fields with nematode populations of 200,000 eggs per cup of soil or more.
"It's a rude awakening to realize how devastating SCN can actually be when populations get high enough," he said.
Additional information on SCN, including a list of resistant soybean varieties for the 2003 growing season, can be found by logging on to http://ipm.osu.edu.