Management and control of apple powdery mildew

Abstract: After scab and canker, powdery mildew (Podosphaera leucotricha) is the most important disease on apple in the UK. Most cultivars are susceptible to the disease, and some such as Cox are very sensitive with mildew levels as low as 8% mildewed leaves being sufficient to reduce yield and fruit quality. Susceptible cultivars require season long spray programmes amounting to 10-15 sprays to protect shoots and buds and prevent high levels of overwintering mildew. In many orchards control of powdery mildew is difficult. There are several possible reasons for this, but the limited range of fungicides available for control, resulting in heavy reliance on triazole fungicides (myclobutanil and penconazole) and as a result the possibility of reduced sensitivity of mildew to these fungicides could be a contributory factor. The results of two projects are described. Firstly, investigating sensitivity of apple mildew populations to triazole, QoI and SDHI fungicides and secondly, identifying new fungicides, alternative chemicals and biofungicides for apple mildew control. Management of powdery mildew is also discussed.