Studies on strawberry mildew, caused by Sphaerotheca macularis (Wallr. ex Fries) Jaczewski*

SUMMARY Cross‐inoculation trials have shown that Sphaerotheca macularis on strawberries is a specialized form of this fungus. Conidia germinated consistently well on dry healthy leaf surfaces. Cardinal temperatures for germination of conidia were 2°, 20° and 35° C., while infection and sporulation were not recorded below 5° and 13° C, respectively. Conidia retained their viability for periods up to 5 weeks at 0° C. in a saturated atmosphere, and for short periods under more exacting environmental conditions. Low humidities, although injurious to conidia, especially at high temperatures, had no effect on growth and sporulation of the fungus. Light too had no effect on conidial germination and development of the fungus. Free water had a lethal effect on conidia and no germination was recorded on wet leaf surfaces. A diurnal periodicity of spore liberation, with a maximum in the afternoon, has been observed. Rain has a drastic effect on spore dispersal, reducing the numbers trapped to very low levels. There was a steep falling gradient in the number of spores trapped both with increasing distance and height from the source. Heavy shade, high humidities and temperatures between 60 and 80° F. were conducive to the production of perithecia. There was no evidence to show that strawberry mildew is heterothallic. Inoculation experiments have shown that the perithecia do not play any essential part in the life history of the fungus.