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Development and Survival of Cercospora zeae-maydis Germlings in Different Relative-Humidity Environments. P. R. Thorson,Department of Plant Pathology, Iowa State University, Ames 50011; C. A. Martinson, Department of Plant Pathology, Iowa State University, Ames 50011. Phytopathology 83:153-157. Accepted for publication 30 October 1992. Copyright 1993 The American Phytopathological Society. DOI: 10.1094/Phyto-83-153.

Conidia of Cercospora zeae-maydis were atomized onto polysulfone membrane discs and were incubated for 6 h at 100% relative humidity (RH). Discs were subjected to a range of relative humidities (39100%) by suspending discs above glycerol solutions maintained at 25.0 0.03 C. When germlings were maintained at 95 or 100% RH, germ tube elongation continued, and appressoria formed in 23 days. However, germ tube elongation did not occur when discs were subjected to relative-humidity environments below 95%. Germlings survived at 60% RH for 6 days, at 70% RH for 8 days, at 80% RH for 10 days, and at 90% RH for 15 days. When returned to 95% RH, viable germ tubes elongated and produced appressoria within 72 h. When discs were exposed for 6 days to daily 12-h periods of 95% RH and to 7090% RH for the remaining 12 h, approximately 80% of the germlings produced at least one appressorium. Only 2230% of the germlings produced at least one appressorium when the alternate 12-h period was at 60% RH or lower. When germinated conidia were placed in a 95% RH environment for periods of 6, 8, 10, or 12 h daily, with the balance of each day spent in 60 or 80% RH environments, germ tube development and appressorium formation proceeded at a rate that correlated with the accumulated hours spent at 95% RH.

Additional keywords: corn, gray leaf spot, maize.