Characterization of Strawberry Host Plant Resistance to Powdery Mildew Caused by Podosphaera aphanis

Host plant resistance is an essential tool in plant disease management worldwide. Evaluations of strawberry cultivar resistance to powdery mildew were done previously in California, but many new cultivars have been released since the last evaluation in 1996 and merit evaluation for today's growers. Two studies were conducted over the winter and summer of 2020, evaluating 10 commonly grown cultivars. Powdery mildew-free plants were established in 3-liter pots under high plastic tunnels and after 3 weeks (four- to five-leaf stage) moved into a powdery mildew-infested greenhouse. Disease incidence and severity ratings were taken weekly beginning at the first sign of disease. Ratings taken at 40 (winter) and 41 (summer) days after transfer to the greenhouse were used to make comparisons among cultivars. Significant differences were found in foliar disease severity among cultivars, but none were totally free of disease. Moderately resistant cultivars were 'San Andreas' and 'Sweet Ann'. Highly susceptible cultivars were 'BG 3.324' and 'Royal Royce'. Two field evaluations of the 10 cultivars confirmed the observed relative differences in host resistance under field conditions. This information is valuable to California strawberry growers, who select cultivars based in part on their susceptibility to economically important diseases such as powdery mildew.