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Influence of Environment and Plant Maturity on Gray Leaf Spot of Corn Caused by Cercospora zeae-maydis. J. C. Rupe, Graduate research assistant, Department of Plant Pathology, University of Kentucky, Lexington 40546; M. R. Siegel(2), and J. R. Hartman(3). (2)(3)Professors, respectively, Department of Plant Pathology, University of Kentucky, Lexington 40546. Phytopathology 72:1587-1591. Accepted for publication 14 June 1982. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-1587.

This study was undertaken to determine why gray leaf spot commonly affects corn grown in eastern, but not in central Kentucky. Weather conditions in locations where gray leaf spot was present were compared with conditions in locations where the disease was not present. The disease was restricted to locations (in eastern Kentucky) having long daily periods of high relative humidity (RH) and leaf wetness (LW). During the months of disease development (August and September) temperatures were the same at sites with and without the disease, but there were more days with 1213 hr of RH>90% and 1113 hr of LW at sites with the disease. Disease progress was monitored in five plantings of corn made at 3-wk intervals. Disease onset and progress were influenced by the physiologic age of the plant. Initial symptoms appeared at or near anthesis on lower leaves and on progressively higher leaves later in the season. Daily aerial spore counts were highest in early afternoon and occurred when there was a rise in temperature, a drop in RH, and drying of leaves. Germination of spores following drying and rewetting was reduced significantly on glass slides and on live tissue in the laboratory. The data suggest that gray leaf spot disease is a problem in eastern Kentucky because there is a high frequency of days having long, uninterrupted periods of LW and high RH. These moist periods may influence spore germination and hence infection.