A 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 pow dery mildew caused by Podosphaera leucotricha (Ell. & Ev.) E. S. Salmon (an amorph 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, D elicious red strain Bisbee, Shamrock, Silken, Sunrise, and one breeding sel ection (11w-17-85)], approximately 6 cm in length, were grafted on to branc hes of mature Jonagold trees in the spring and evaluated in the summer of t he same year and in subsequent years after growth on the host tree. Inciden ce and severity of powdery mildew were evaluated in 1994, 1995, 1997, and 1 998. 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 inc idence 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 x 10(4)) compared to Bis bee Delicious (1.2 x 10(4)). Some of the grafted cultivars were also evalua ted for primary powdery mildew and fruit russetting in 1998 after the growt h for three seasons. Jonagold had the highest numbers of primary infections and russetted fruit.