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Two soybean diseases to watch for in 2011

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.

“The
risk is up, due to the increased amount of inoculum that’s present,” says Carl
Bradley, University of Illinois Extension plant pathology. “A lot depends on
the weather.”

 

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