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Stay vigilant for soybean cyst nematode

Agriculture.com Staff 02/26/2007 @ 2:45pm

In many areas, timely August rains helped pump 2006 U.S. soybean yields to a record production level of 3.19 billion bushels.

However, burgeoning production doesn't mean it was a quiet year for soybean pests last year. "There was well over $300 million of damage in Illinois from soybean cyst nematode (SCN)," says Terry Niblack, University of Illinois Extension nematologist. "It wasn't a quiet year for soybean cyst nematode."

Part of the reason SCN doesn't pop up on your radar is its stealthy manner. "The only time you see symptoms is on the yield monitor when you're harvesting your fields," says Niblack. "We don't see symptoms when it's just soybean cyst nematode alone."

Most often, symptoms pop up when other stressors surface, such as seedling diseases or iron chlorosis.

Further disconcerting news is that SCN-resistant varieties are no longer bulletproof. Niblack notes she's fielding more and more reports about yield losses on SCN-resistant varieties.

"Most of our resistant varieties come from the same source of resistance," she says. "The nematodes have adapted to that source of resistance."

In Illinois, the SCN types that infest most fields are Type 2 or Type 1.2. These nematodes are the ones that attack resistant varieties with the most popular source of resistance, PI88788.

Another unsettling point: No industry resistance standards exist for SCN-resistant varieties. "Just because it's labeled for resistance doesn't mean it's necessarily resistant," Niblack says. "One percent resistance can still be labeled as resistant."

If you farm in areas of excellent root growth, it's a good bet you'll have large numbers of SCN this year.

"In 2006, there wasn't a lot of root restriction," Niblack says. "We had nice fat juicy root systems, which means we had lots of nematodes. My prediction is we will have much higher nematode populations this spring."

Most soybean ground is already infested in Illinois. A 2005-2006 survey in Illinois showed 84% of Illinois soybean fields had SCN. Planting corn on soybean ground -- as most farmers do in the Corn Belt -- is your first line of defense against SCN. This can drop SCN eggs counts. Soil sampling can detect if numbers are still sufficient to cause damage.

Resistant varieties remain a good way to deal with SCN. However, choose them carefully. Of fields with SCN in Illinois, Niblack notes that 75% of those SCN populations already have adapted at some level to available resistant varieties.

In Illinois, a good way to compare varieties is the Variety Information Program for Soybeans. You can access the Web site at http://web.aces.uiuc.edu/VIPS. Look under the SCN resistance column for the resistance source and the levels of resistance in each variety. Farmers then may match the resistance source against the nematode type in their field that the soil test has detected.

Other states also have SCN-resistance information. For example, Iowa farmers can check out Iowa State University SCN-resistant soybean variety trial results from 1996 through 2006 at www.isuscnvarietytrials.info.

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