In vitro screening of apple germplasm for resistance against black spot caused by Venturia inaequalis

摘要

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 all ill 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 (10~6 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 sporulation 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 tit' V. inaequalis. Death and browning of tissues, more commonly found in mature tissues and following wounding did not limit sporulation of V. inaequalis. Scanning electron microscopy (SEM) revealed that V. inaequalis appeared to grow above the cuticle of the leaf surface and to show limited mycelial growth and sporulation in resistant cultivars/selections. The mycelia grew on the cuticle and showed abundant sporulation in all susceptible cultivars within 3 to 5 weeks after inoculation. It is concluded that in vitro screening can be used to select black spot resistant germplasm efficiently.