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What's up for corn and soybean diseases

Gil Gullickson 04/04/2012 @ 9:29am Crops Technology Editor for Successful Farming magazine/Agriculture.com

The telltale small yellow lesions are early symptoms of frogeye leaf spot. This disease, once mainly limited to Southern soybeans, is increasing in the Midwest.

Diseases are stealthy yield robbers that can plague your corn and soybeans. Fortunately, steps like scouting and fungicide use can curb their impact. Here are some answers to questions about diseases and ways to control them.

1. Disease thresholds: do they exist?

Brown spot, frogeye leaf spot, and Cercospora leaf spot are all soybean diseases that can be curbed by fungicides.

But there are no thresholds developed for these diseases, says Mark Carlton, Iowa State University (ISU) Extension field agronomist. Start scouting at R1 (beginning flowering) and watch for disease movement and rainy conditions with high humidity. These factors can increase the odds of a positive payback from fungicides of up to 8 to 10 bushels per acre. Fields with continuous no-till beans, particularly with frogeye leafspot, also favor a fungicide payoff.

In corn, the threshold for gray leaf spot (GLS), developed in the mid-1990s, consists of lesions on the third leaf below the ear leaf or higher occurring on 50% of the plants at tasseling. This threshold is being reevaluated because of the new hybrids and fungicide products that have been developed since then. For now, however, the threshold stands.

It's also important to consider high-risk cases, such as fields in continuous corn with high residue in river bottom areas, says Carlton. Hybrid ratings for GLS resistance should also be considered.

“Timing is important,” says Carlton. He advises applying fungicides following tasseling and continuing into the R2 (blister) stage.

2. Will fungicides aid hail-damaged corn?

BASF officials say Headline fungicide products can do it. BASF obtained a supplemental label for Headline from federal regulators in 2009 that includes benefits other than fungal disease control, including better hail tolerance.

Yield responses on hail-damaged corn from Headline can coincide with 12- to 16-bushel-per-acre responses that BASF says it consistently sees from Headline applications on corn, says Nick Fassler, BASF technical market manager for fungicides.

Responses hinge upon yield potential and the severity of plant damage. Headline won't help corn with heavily stripped leaves. It is beneficial, though, to less damaged plants. It can protect those new leaves forming below the tassel that contribute to yield during grain fill, says Fassler.

University of Illinois (U of I) tests in 2007 and 2008 showed no interaction between simulated hail damage and foliar fungicide for yield or foliar diseases severity. Scientists simulated hail-damaged corn with a grass trimmer. Researchers applied two strobilurin fungicides – Headline and Quadris – at tasseling and compared it against simulated damaged corn with no strobilurin fungicide application.

“No interaction between simulated hail damage and foliar fungicide was observed for yield or foliar disease severity,” says Carl Bradley, U of I Extension plant pathologist.

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