In Vitro Screening of Apple Germplasm for Resistance Against Black Spot Caused by Venturia inaequalis

ABSTRACT Black spot of apple caused by Venturia inaequalis is a major economic problem. Attempts to breed resistant cultivars are complicated by a number of factors, one of which is the lack of an efficient procedure to screen for resistance. This paper describes work aimed at validating an in vitro screening protocol. Shoot apices (2-3 cm), fully expanded leaves, half leaf discs from in vitro raised shoot cultures and callus cultures were inoculated with axenic spore suspensions of V. inaequalis(106 spores/ml) and cultured on water-agar medium. Leaf discs derived from mature trees of apple cv. Granny Smith, McIntosh, Prima, CO-OP 12, CO-OP 26, and SPI-218 were also inoculated. Tissue of the susceptible cultivars exhibited chlorosis, necrosis and spo-rulation of V. inaequalis 3 to 5 weeks after inoculation whereas tissue of CO-OP 12, CO-OP 26, and SPI-218, the resistant cultivars displayed only a hypersensitive response. However, Prima exhibited only limited sporulation of V. inaequalis. Death and brownin...