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Two soybean diseases to watch for in 2011

Gil Gullickson 01/17/2011 @ 1:03pm Crops Technology Editor for Successful Farming magazine/Agriculture.com

Frogeye leaf spot is a disease that’s increasingly becoming a problem in soybean fields in states like Illinois. Your first step in fighting this fungal disease is to plant frogeye leaf spot resistant varieties, says Carl Bradley, University of Illinois  (U of I) Extension plant pathologist.

Fungicides also can control this disease, but they are most effective on susceptible varieties. A 2010 U of I trial at Belleville, Illinois, examined fungicide response one variety susceptible to frogeye leaf spot and three others resistant to the disease.

“We saw a 10 to 11 bushel per acre response on the susceptible variety,” says Bradley. However, no statistically significant differences resulted on the resistant varieties.

In 2010, university plant pathologists confirmed frogeye leaf spot resistance to a strobiluirn fungicide in Tennessee. To prevent resistance from building up to strobilurin fungicides, Bradley recommends treating frogeye leafspot with a triazole fungicide when use is warranted. Topsin M could also be used in place of triazole fungicides. It belongs to a different chemistry class than a triazole or a strobilurin.

 If other diseases are present, use a combination of a triazole and strobilurin,” says Bradley.

What about white mold?

Many soybean fields infested with white mold in 2009 went back to corn last year. Since 2011 marks soybean’s turn in a corn-soybean rotation, will white mold still be a threat?

Two components of the disease triangle—host and pathogen—will be present this year Soybeans, of course, are the host. Ditto for the pathogen. White mold inoculum is contained in hardened structures called sclerotia. Mushroom structures known as apothecia sprout from the sclerotia. Spores produced from the apothecia can infect soybean flower petals, thus causing disease. The disease sloughed off soybean plants in 2009, and more sclerotia formed. Sclerotia can reside in soil 8 to 10 years or longer.

The third component—weather—is unknown. Similar weather to 2009—cooler-than normal temperatures and rainfall prior to and throughout flowering—will prompt white mold infestations. Absent these conditions, though, you can rest easy.

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