No. 18 Article 7/July 25, 2008

Common Rust of Corn Arrives in Illinois a Bit on the Early Side

Common rust of corn, caused by the fungus Puccinia sorghi, is a relatively common disease that is generally observed every summer in Illinois. This year, it seemed to arrive a little earlier than usual. In late June and early July, observations of common rust were beginning to be reported in the state.

Common rust is generally most severe when cool to moderate temperatures are present, and it does not spread well when temperatures exceed 80°F. Conditions that keep leaves wet for an extended period (frequent rains, dew, etc.) are also favorable for common rust development. Similar to other rust diseases in Illinois, spores of the common rust fungus are blown in from the south. Symptoms of common rust appear as raised pustules on the leaves. These pustules may appear in "bands," which is due to infection that occurred while the leaf was in the whorl.

Common rust pustules on a corn leaf.

Some hybrids have good specific and/or general resistance to common rust. Hybrids with specific resistance provide high levels of resistance to specific races of the rust fungus, but not against all races that may be present. Hybrids with general resistance may slow the progression of common rust, but they do not provide complete resistance; however, general resistance is not limited to particular races of the rust fungus.

Foliar fungicides that are registered for use on corn are highly effective against common rust. The window of application is generally considered to be sometime between VT (full tassel emergence) and R2 (blister). The question I've been getting the past two weeks is "Should I spray a fungicide on my corn to control common rust?" I think the answer may depend on a few considerations. With fields that are currently at R1 (silking) and approaching R2 (blister), the benefit with a fungicide may not be as great as what could be observed in late-planted corn. However, one should also consider other risk factors (see "Making Profitable Fungicide Applications in Corn" in issue 15 of the Bulletin, July 3, 2008), and make those part of the fungicide decision-making process. With late-planted fields that may not yet have tasseled or are near tasseling, the risk for yield loss due to rust (and a few other foliar diseases) does increase.

In 2007, a late-planted corn fungicide research trial (planted in early July) was conducted in Urbana at the Crop Sciences Research and Education Center. This late-planted trial had a high level of common rust severity. Applications of Headline fungicide (9 fl oz per acre) were made at V15, VT, and R2. In this trial, the untreated check had a final disease severity rating of 68% and yielded 160 bu/A. The final rust severity in the Headline-applied treatments ranged from 33% to 40%, and yield ranged from 176 to 194 bu/A. Headline was the only fungicide product used in this particular trial, but other products available for use on corn (Quadris, Quilt, Stratego) have good efficacy on common rust as well. The level of yield loss due to common rust in this research trial is not typical, but under high rust severity and late-planted corn, yield losses can occur.

Common rust developing on late-planted corn.

Effect of Headline fungicide (9 fl oz per acre) on control of common rust and yield of corn in a late-planted fungicide trial at Urbana in 2007.

--Carl A. Bradley

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