Gray leaf spot of corn is being reported across the state. The fungus Cercospora zeaemaydis causes this common leaf blight. Gray leaf spot in a susceptible variety has easily identifiable symptoms. The lesions are rectangular, which is unusual for a fungal leaf blight; and not very large, ranging from about 3 millimeters by 1 to 6 centimeters; and are a light tan color. Lesions can turn a slight grayish color as they mature, and the tissue becomes fully necrotic; but this is really a fairly vague difference in color. The disease probably would have been more appropiately named 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 remaining on the soil surface. This is not to suggest that 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 because of highly infested residue and continuous corn.
Spore production from previously infested residue usually starts 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 turnaround from spore to lesion to new spore production like common rust, which has been our biggest foliar problem to date this year. Gray leaf spot has about an 18-day turnaround. This is good in the sense that during the most critical time for leaf blights to cause yield loss (2 weeks before tasseling to 2 weeks after tasseling) what you see is what you get. But the long turnaround time can be very problematic if the lesions all appear during 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. Public inbred lines B68, NC250, NC258, Pa875, Va14, Va17, Va85, T222, and Mo18W have been reported as having resistance to the disease. Use hybrids derived from these lines for resistance.
As with all resistant hybrids, remember 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: smaller lesions produced, fewer spores subsequently produced from those lesions, a longer time for lesions to develop, or fewer lesions produced overall. The resistant hybrids will develop lesions, 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 hybrid seed is being produced. Apply fungicides during the 14-day period before and after tasseling. Multiple applications may be necessary when disease pressures are high. However, during periods of high temperatures or dry conditions, disease pressures will be low, and additional applications may not be necessary.--Suzanne Bissonnette