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Abstract
The optimum temperature for powdery mildews (Erysiphaceae) as a group is about 21° C, or 4 degrees lower than the average optimum for plant pathogens. Among the powdery mildews the optima for different species are about as follows: Erysiphe graminis, 17° C; E. polygoni, 23°; E. cichoracearum, 22°; Sphaerotheca pannosa, 24°; Uncinula necator, 26°; and Podosphaera leucotricha, 16°. The five criteria of fungus activity used to assay the relation of temperature to powdery-mildew development—number of conidia per conidiophore, rate of maturation of conidia, percentage germination of conidia, length of germ tubes in vitro, disease development, and length of hyphae in vivo—are considered to be of increasing merit in the order given.