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Relationship of Temperature and Humidity to Development of Mycosphaerella Lesions on Chrysanthemum. R. E. McCoy, Former Graduate Student, Department of Plant Pathology, Cornell University, Ithaca, New York 14850, Present position of senior author: Assistant Professor (Plant Pathologist), Agricultural Research Center, University of Florida, Ft. Lauderdale 33314; A. W. Dimock, Professor, Department of Plant Pathology, Cornell University, Ithaca, New York 14850. Phytopathology 62:1195-1196. Accepted for publication 28 April 1972. DOI: 10.1094/Phyto-62-1195.

The development of stem lesions on florists' chrysanthemum inoculated with Mycosphaerella ligulicola was observed from 10 to 30 C at two relative humidities. In cultivar Fred Shoesmith, lesions are indeterminate and enlarge when conditions are favorable. The lesion-development: temperature curve closely parallels that for linear extension of the fungus on potato-dextrose agar (minimum > 3 C, optimum 26 C, maximum 30 C) except for a slightly narrower temperature range. The rate of lesion growth is greatest at humidities near saturation; relative humidities of 50% result in approximately half the values obtained at saturation.