Gray leaf spot (GLS) of corn (Zea mays L.), caused by the fungus Cercospora zeaemaydis (CZM) (Tehon and Daniels) has increased in incidence and severity with increasing use of no-tillage and continuous corn practices. This disease can be yield limiting. Corn hybrids were evaluated under natural disease pressure for three years (1989, 90, and 91) at two locations (Montgomery and Wythe Co., VA). Yield losses ranged from 2127.4 kg/ha (Wythe Co., 1991) to 4242.2 kg/ha (Wythe Co., 1990). It was estimated that 77% of the variability in yield was due to GLS. Fungicides were evaluated for the control of GLS over three years on a susceptible hybrid, Pioneer Brand 3320. All fungicides, with the exception of mancozeb, provided significant control over nontreated check in all years. Benomyl, propiconazole and terbutrazole were the most effective fungicides. As much as 93% of the variablilty in yield was attributed to blighting. Reduction in blighting also increased the kernel weight. The toxin, cercosporin, produced by CZM was evaluated for its ability to elicit differential responses in corn germplasm by three methods, ie., vein inoculation, root, and shoot uptake. No consistant differential reponses were found with vein inoculation, but 31-day old plants were significantly more sensitive to the toxin than 21-day old plants, as measured by lesion width. Root and shoot uptake of the toxin by inbred germplasm produced lesions that resembled those produced by CZM in the field. Microscopic, yellow fluorescing crystals were found associated with necrotic tissue from toxin-treated inbreds. Significantly more injury occurred to toxin-treated inbreds exposed to light than to darkness. By chromatographic analysis, 407.1-1076.7 ng of toxin/g of tissue was recovered from leaf lesion extracts of plants exposed to light. Five inbreds (B73, H99, Va59, NC250a, and NC264) showed consistent and differential responses to the toxin. H99 and NC250a showed differential responses to the same concentration of toxin, thus suggesting that some germplasm are more sensitive to the toxin than others. Tests using the toxin as a means to identify resistant germplasm did not provide reliable predictions of germplasm response to CZM in the field.