November 30, 1999

If soybean yields are disappointing, test soil for nematode

If you were disappointed in your soybean yields this year, you may need to test your fields for soybean cyst nematode (SCN). That means sending a soil sample to a diagnostic lab, says John Bilotta, educator in Meeker County with the University of Minnesota Extension Service.

"SCN has been positively identified in many of the Minnesota counties south of an east-west line through St. Cloud," says Bilotta. "Seventy percent of the soil samples sent to the Southern Research and Outreach Center's Nematology Lab at Waseca have tested positive for SCN, according to soybean educator Dave Schwartz."

Bilotta cites results of a recent survey of Minnesota soybean growers who know their fields are infested with SCN. Most reported a yield reduction of at least 1-5 bushels per acre, with many having losses of 6-10 bushels per acre. These losses often occur even when above-ground symptoms are minor. Most producers experience yield losses and early symptom development without discovering the presence of SCN. "This delay in proper diagnosis results in development of very large nematode populations that are difficult to manage or reduce," says Bilotta.

The first step in managing SCN is to identify infested fields through detection sampling, Bilotta points out. Fields with a history of stable or declining soybean yields should be soil sampled to determine the presence and population level of nematodes. A diagnostic lab can check the sample to determine the number of SCN eggs in 100 cubic centimeters of soil.

A second management step is to use level-of-infestation soil sampling the year before soybeans are to be planted in fields known to contain SCN. Sampling results show the severity of infestation and can serve as the basis for management decisions, such as variety selection.

"Yield from susceptible varieties is directly correlated to the SCN population level in the soil," says Bilotta. "The higher the population, the lower the soybean yield. If test results some back from 0-200, the potential yield impact on susceptible varieties will be less. If levels are from 200-2,000, resistant varieties are recommended. Levels from 2,000-5,000 indicate that rotation out of soybeans may be a good idea.

"Keep in mind, however, that any levels over 200 may hurt yields on less productive soils. Your yield patterns can serve as indicators of the tolerance or severity of SCN problems."

Bilotta says rotation of soybeans is the best method of decreasing SCN levels in the soil. The levels should decrease about 10 percent for each year a non-host crop is planted, a lower rate of decrease than scientists previously thought. For fields with very high SCN levels, rotating away from soybeans for three or more years may be the best strategy.

For further information on soil testing for SCN, selecting resistant varieties, or other SCN management strategies, contact your county office of the University of Minnesota Extension Service. You can also pick up a SCN-testing packet there that describes the testing procedure.

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Source: John Bilotta, (320) 693-5275
Editor: Joseph Kurtz, (612) 625-3168, pkurtz@extension.umn.edu

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URL: http:// www.extension.umn.edu/extensionnews/1999/soybeanyeilds.html  This page was updated May 21, 2002 .
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