Gray Leaf Spot (GLS) of corn is being reported in several areas across the state. The fungus Cercospora zeae-maydis causes this common leaf blight. Gray leaf spot has easily identifiable symptoms in a susceptible variety. The lesions are rectangular, which is unusual for fungal leaf blight, and not very large ranging from about 3 mm by 1-6 cm and a light tan color. Lesions can turn a slight grayish color as they mature and the tissue becomes fully necrotic but this is really fairly vague difference in color. The disease probably would have been a lot more aptly named as tan rectangle blight.

The most severe gray leaf spot epidemics usually occur in continuous corn production fields where there is also a substantial amount of corn debris that was infected in the previous season and remains on the soil surface. Not to suggest the spores of the fungus are not blown by the wind into fields that don't have this type of production, but earlier more severe infections are much more likely under the situation of high infested residue and continuous corn. Something to think about with the recent penchant for continuous corn.

Spore production from previously infested residue starts usually in mid to late June depending on environmental conditions. Spores landing on leaves require relative humidity of 95% for successful infection. This disease does not have a rapid turn around from spore to lesion to new spore production like common rust that has been our biggest foliar problem to date this year. For gray leaf spot you will be looking at about an 18 day turn around. This is good in the sense that during the most critical time for leaf bights to cause yield loss, (2 weeks before tasseling to 2 weeks after tasseling) what you see is what you get. But the long turn around time can be very problematic if the lesions all appear right about the time that it's too late to do anything to reduce yield loss.

Progress has certainly been made in the past several years in developing hybrids with some level of resistance to the disease. As with all resistant hybrids keep in mind, that resistance does not mean the hybrid will not be infected by the disease. Resistance in a resistant hybrid can be expressed in a number of ways. Commonly the affect of resistance will be expressed something along the lines of: smaller lesions produced, fewer spores subsequently produced from those lesions, longer time for lesions to develop or fewer overall lesions produced. The resistant hybrids will have lesions develop but the lesions will not 'look' the same as a characteristic lesion on a susceptible hybrid.

Fungicides are useful when conditions favor diseases, especially if you are producing hybrid seed. Apply during the 14-day period before and after tasseling if disease is present at about 15% whole plant infection in field corn. Multiple applications may be necessary when disease pressures are high for seed production fields. However, during periods of high temperatures or dry conditions, disease pressure will be low and additional applications may not be necessary.