Epidemiology and Control of Powdery Mildew (Podosphaera aphanis) on Strawberry
Strawberry powdery mildew, caused by Podosphaera aphanis, has the potential to cause over 20% yield loss, particularly where strawberries are grown under cover. A holistic approach to the control of strawberry powdery mildew (P. aphanis) is important, since the disease is never absent from the crop. The new disease assessment key was developed to assess strawberry powdery mildew (the old one is for assessing red blotches on leaves, See appendix 8). The results (Chapter 3) showed that the disease is present in the crop when new plants are bought in from a propagator, with 14% of strawberry crowns were infected by P. aphanis in July 2013 and 4% of the strawberry plants had symptoms of powdery mildew in pre-assessment of plants for the 2013 Si nutrient fertigation field experiment. Control measures used in one growing season reduced the disease carry-over, thus reducing the initial inoculum in the following season. The use of a late autumn fungicide spray and a fungicide spray before the plants were covered by fleece in spring reduced the number and maturity of overwintering chasmothecia, thus contributing to a reduction in initial inoculum. The use of silicon (Si) nutrient (foliar spray and root treatment) also suppressed strawberry powdery mildew development (Chapter 4). The results of Area Under Disease Progress Curve (AUDPC) and the rate of epidemic growth curve (r) in 2012 indicated that the high concentration of Si nutrient foliar spray inhibited (r = 0.002, AUDPC = 44) the epidemic build-up of P. aphanis better than the low concentration of Si nutrient (r = 0.012, AUDPC = 51) and untreated (r = 0.018, AUDPC = 70). The Si nutrient root treatment (AUDPC = 12.8) was better in inhibiting strawberry powdery mildew development than the Si nutrient foliar spray treatment. Moreover, the high concentration of Si nutrient foliar spray resulted in fewer chasmothecia compared to the untreated. Si nutrient foliar spray and root treatments increased the concentration of Si in the plants and produced physiological changes in the plants, including wax formation on the adaxial leaf surface, greater leaf thickness and cuticle layer and increased Brix0 value in plants, which all were associated with reduced disease incidence. The integrated use of all these control strategies suppresses disease development so that control is achieved with less use of conventional fungicides.