The effect of temperature on colony growth by Erysiphe sp. infecting Rhododendron

The growth and reproduction of powdery mildew pathogens is generally encouraged by increasing temperatures, up to 25°C. Germination and germ tube extension of Erysiphe sp. on Rhododendron cv. Elizabeth were optimal at 20°C for conidia originally formed at either 10 or 15°C. During a 50‐day period of colony growth, the viability of conidia formed at 15°C declined but for those formed at 10°C it increased. The expansion of Erysiphe colonies over an initial 12‐day period was favoured by incubation at 15°C compared with 10 or 20°C. In the first 8 to 10 days of growth, secondary and tertiary hyphae formed most rapidly at 20°C. After 12 days, expansion of colonies at 20°C was limited to the area initially infested by primary hyphae, whereas in colonies grown at 10 and 15°C secondary and tertiary hyphae had extended beyond the area first colonized. Small colonies of densely packed hyphae formed at 20°C compared with open spreading colonies observed at 10 and 15°C.