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Effects of Temperature, Light, and Relative Humidity on Powdery Mildew of Begonia. J. A. Quinn, Graduate research associate, Department of Plant Pathology, The Ohio State University, Columbus 43210, Present address of senior author: Rohm and Haas Research Laboratories, Spring House, PA 19477; C. C. Powell, Jr., associate professor, Ohio Agricultural Research and Development Center, Wooster 44691. Phytopathology 72:480-484. Accepted for publication 22 June 1981. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1982. DOI: 10.1094/Phyto-72-480.

Under controlled temperature, relative humidity, and light conditions, conidia of Oidium begoniae (the causal agent of powdery mildew of begonia) germinated on glass slides and excised leaves at temperatures ranging 432 C, with the most rapid germination at 2325 C. Haustoria did not form above 30 C. Temperatures of 28 C or above caused reduction or cessation of hyphal growth, inhibition of sporulation, and eventual eradication of the pathogen. Temperatures at 2021 C were optimal for colony development as measured after 7 days of incubation. Decreasing relative humidity caused only slight decreases in conidial germination and the development of mildew colonies. Water killed most submerged conidia within 1030 min. Floating conidia germinated well with little appressorial formation. Sporulation occurred in response to diurnal cycles of light and dark. Changing the onset of the light period changed the time of release of conidia.