novel technique for evaluation of apple (Malus x domestica Borkh.) cultivars for susceptibility to powdery mildew

A technique for the evaluation of apple cultivars for susceptibility to powdery mildew caused by Podosphaera leucotricha (Ell. & Ev.) E. S. Salmon (anamorph Oidium farinosum Cooke) was tested at the Pacific Agri-Food research Centre (PARC), Summerland, BC. Shoots of test cultivars [Braeburn, Creston, Elstar, Empire, Fuji strain BC2, Gala strain Royal, Jonagold, McIntosh, Delicious red strain Bisbee, Shamrock, Silken, Sunrise, and one breeding selection (11w-17-85)], approximately 6 cm in length, were grafted on to branches of mature Jonagold trees in the spring and evaluated in the summer of the same year and in subsequent years after growth on the host tree. Incidence and severity of powdery mildew were evaluated in 1994, 1995, 1997, and 1998. They were not evaluated in 1996 because approximately half the grafts from the previous year did not grow and had to be replaced. The highest incidence of powdery mildew averaged 84% for Gala, compared with only 15% for Bisbee Delicious with the lowest incidence. Gala had the highest number of P. leucotricha conidia per cm 2 of leaf area (4.9 × 10 4 ) compared to Bisbee Delicious (1.2 × 10 4 ). Some of the grafted cultivars were also evaluated for primary powdery mildew and fruit russetting in 1998 after the growth for three seasons. Jonagold had the highest numbers of primary infections and russetted fruit. Key words: Podosphaera leucotricha, breeding, cultivar resistance, pest management