Temperature relations of powdery mildews

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. Powdery-mildew conidia are shorter-lived than the spores of most fungi: they live only about 2 days in ordinary environments. At lower and higher temperatures they live for longer and shorter times, respectively. Attached spores in situ live longer than detached spores on glass. Fresh conidia on the host surface are more tolerant of high temperature than are the young colonies resulting from the growth of these conidia.