SELECTION OF RESISTANCE TO VENTURIA-INAEQUALIS USING DETACHED LEAVES FROM IN VITRO-GROWN APPLE SHOOTS

A simple method was developed to screen apple cultivars for resistance to Venturia inaegualis using detached leaves from in vitro-propagated shoots. Leaves of 6 cultivars either susceptible or resistant to V. i naequalis were placed on Petri plates containing water agar or wet Wha tman filter paper, and were inoculated with non-axenic conidial suspen sions. Light microscopy studies indicated that the reaction of resista nt and susceptible cultivars in vitro mimicked reactions in the greenh ouse and field. Susceptible cultivars showed chlorosis 3 weeks after i noculation, and became necrotic after 5 weeks. Leaves from resistant c ultivars showed no symptoms. On susceptible cultivars grown in vitro, multiple appressoria formed per spore, the mycelia proliferated above the cuticle on the surface of the leaf, and sporulation occurred 3-4 w eeks after inoculation. The simplicity of this detached leaf assay all ows not only selection from much larger plant populations than is curr ently possible with greenhouse and field techniques, but also makes po ssible the preservation of the mother plant during identification of r esistant apple clones. This assay could be useful for studies of host- pathogen interactions.