No. 17 Article 2/July 30, 2010

Be on the Lookout for Southern Corn Rust

Reports of southern corn rust observations in Illinois are being received. Southern rust, caused by the fungus Puccinia polysora, is different from common rust, which is caused by Puccinia sorghi. (See the photograph below to compare symptoms of the two.)

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

Southern rust symptoms are small orange pustules in masses primarily on the upper leaf surface. Chlorosis around the pustules and on the bottom surface of the leaves also may be present. (See photo below.) Common rust is observed as larger cinnamon-colored pustules that may be scattered on either the upper or lower leaf surface. Conditions that are conducive for southern rust development are high temperatures (80-90°F) and high relative humidity.

Chlorosis (yellowing) around southern corn rust pustules on the upper leaf surface (left) and lower leaf surface of corn.

Nearly every corn hybrid planted in Illinois is susceptible to southern corn rust. Late-planted corn will be at most risk for yield being affected. Foliar fungicides are effective in managing the disease. Applications after the R3 (milk) stage on corn that was planted in April will be less likely to provide a yield benefit.

Southern rust was observed at a University of Illinois corn foliar fungicide trial at Dixon Springs in 2009. The yield response to foliar fungicides applied at R1 ranged from 5 to 37 bu/A when compared to the nontreated control (see photo below).

Severity of southern corn rust on a nontreated leaf (left) compared to a fungicide-treated leaf in a University of Illinois foliar fungicide trial at Dixon Springs in 2009.

It is important to scout late-planted corn for southern corn rust. If you suspect the disease but need help with identification, send samples to the University of Illinois Plant Clinic.--Carl Bradley

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