A Usual Summer Suspect – Gray Leaf Spot

Published on: 11:41AM Jun 13, 2011

Eric Tedford

Will it be a hot and humid summer? Will gray leaf spot be prevalent over much of the Midwest and Eastern corn-growing regions? As temperatures and humidity rise, these conditions pave the way for the development of gray leaf spot. As one of the most damaging and yield-robbing diseases in corn, gray leaf spot is a fungal disease that eats away at leaf tissues, inhibiting a plant’s production of energy and impacting yield. Information published by The Ohio State University Extension says potential yield losses from gray leaf spot generally range from 5-40 bu/A, and losses as high as 90-100 percent have been reported. Additionally, gray leaf spot can reduce grain fill by up to 50 percent. In addition to reducing grain fill, gray leaf spot can greatly compromise the photosynthetic integrity of the leaves which leads to weaker stalks and increased lodging. 

If you regularly scout your field, you can spot the brownish-gray, rectangular lesions early on. Gray leaf spot typically originates on the lower leaves, generally around the time when tasseling occurs. As lesions on the bottom leaves sporulate spores disperse to the middle and upper canopy. Gray leaf spot lesions start developing during the late-vegetative and the early reproductive growth stages of the plant. In warm, humid weather the lesions develop quickly and can kill entire leaves.
According to Kansas State University Research and Extension, conditions for gray leaf spot development are ideal when temperatures are between 70 and 85 degrees Fahrenheit, but the disease can thrive in the hottest summer conditions if adequate moisture is available.

If a highly susceptible hybrid is being grown and lesions are found on the third leaf below the ear leaf, a curative and preventive fungicide application will be beneficial. Strobilurin fungicides can be helpful in preventing and controlling gray leaf spot, and a fungicide with two different modes of action with both preventive and curative activity provides the best defense.

Eric Tedford, Fungicide Technical Brand Manager for Syngenta, provides technical leadership for the development of fungicides. His experience includes fungicide research and development for field crops, development of postharvest fungicides, and global technical development of fungicides. He holds bachelors, masters, and doctorate degrees in plant pathology from the University of Massachusetts, Clemson University, and the University of California (Davis), respectively.