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Issue No. 13, Article 7/July 2, 2010

Corn Disease Update

Symptoms of a few foliar diseases are beginning to appear in cornfields across the state.

Anthracnose leaf blight. This disease was discussed in the June 11 issue of the Bulletin. In some fields, symptoms can still be observed on lower leaves. With the rapid growth of corn, spread to upper leaves has not been seen. Depending on conditions in July and August, anthracnose leaf blight can spread to upper leaves later in the season.

Common rust. Common rust has been observed for about the past two weeks in some fields. Cooler temperatures (<75°F) and high relative humidity favor this disease. High temperatures the past few weeks likely will slow or stop the progression of common rust. In general, most dent corn hybrids have good levels of resistance. Considering the recent warm temperatures and the resistance of most hybrids, applications of foliar fungicides specifically for common rust may not be warranted. Foliar fungicides are effective in controlling common rust, and research at the University of Illinois conducted by Dr. Jerald "Snook" Pataky indicated that the strobilurin fungicides evaluated in his trials (Quadris and Headline) had better "curative" control of rust than Tilt fungicide. Symptoms of common rust and southern rust on corn are pictured below.

Symptoms on corn of common rust (left) and southern rust.

Southern rust. Southern rust can cause major yield losses to corn, as nearly all hybrids are susceptible. Southern rust has not yet been reported in Illinois in 2010, but it is progressing northward. The progress can be tracked on the southern corn rust PIPE website. The current southern corn rust map (see below) shows that the disease has been reported as far north as Arkansas.

Southern corn rust observations on June 29. The current map is available on the southern corn rust PIPE site (www.ipmpipe.org).

Gray leaf spot. Conditions in most of Illinois have been very favorable for gray leaf spot so far this season (warm with high relative humidity). Disease symptoms have been observed in some fields on the lower leaves.

Symptoms of gray leaf spot on a corn leaf.

In my field research plots, gray leaf spot symptoms are more obvious in corn-on-corn fields than where corn is following soybean. In University of Illinois foliar fungicide trials where gray leaf spot pressure was significant, products with a strobilurin fungicide component have been the most effective. In addition, results from foliar fungicide trials indicate that bigger yield responses with fungicides are more likely to occur on hybrids with low disease-resistance ratings and high yield potential.--Carl A. Bradley

Carl A. Bradley

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