Figure 1. Gray leaf spot.

Figure 2. Common rust.

Figures 3a (above) and 3b (below) . Lesions (a) likely caused by a genetic mutation, possibly a disease lesion mimic, tend to begin on the lower leaves (Figure 3b, courtesy of Clint Koester, Aurora Coop).

July 13, 2007

Over the past few weeks we've begun to see the development and increase of gray leaf spot (Figure 1) on the lower leaves of corn in southern counties of Nebraska. In the most severe cases, a few lesions have developed on the ear leaf, where it can begin to impact yield. Common rust (Figure 2) also has been observed. This is NOT the same as southern rust, which was a serious problem in much of eastern Nebraska last year. For more information differentiating rust diseases in corn, refer to the NebGuide, Rust Diseases of Corn in Nebraska, G1680. Serious infections of fungal diseases, particularly on the ear leaf and above, may justify fungicide treatment given current higher corn prices. It will be important for fields to be monitored closely where foliar diseases have developed early and to especially monitor those fields that were planted late and with susceptible hybrids.

We've also observed substantial common smut on leaves and stalks in some fields. This is likely due to early injury from insect feeding or late spring hail, high winds and sandblasting. The eyespot that developed on the lower leaves earlier in the season will likely dissipate with the return of hot temperatures.

The most common symptom on corn samples being received at the UNL Plant and Pest Diagnostic Clinic lately is not likely caused by a disease, but instead a genetic disorder that causes lesions under stressful conditions. The symptom has been reported previously in Nebraska, particularly in 1999 and 2005 (Figures 3a and 3b), and across multiple hybrids from several companies. The symptoms behave and appear similar to a residue-borne foliar disease because they tend to begin on the lower leaves and spread up the plant, but no pathogen has been confirmed in samples. A likely cause of this symptom is a genetic mutation called a disease lesion mimic. For more information on this see this month's feature article and abstract by Dr. Gurmukh S. Johal, Purdue University, on the American Phytopathological Society's Web site.

Tamra Jackson
Extension Plant Pathologist, Lincoln