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Pathogenesis of Venturia inaequalis on Shoot-Tip Cultures and on Greenhouse-Grown Apple Cultivars. L. M. Yepes, Department of Plant Pathology, Cornell University, New York State Agricultural Experiment Station, Geneva 14456; H. S. Aldwinckle, Department of Plant Pathology, Cornell University, New York State Agricultural Experiment Station, Geneva 14456. Phytopathology 83:1155-1162. Accepted for publication 7 June 1993. Copyright 1993 The American Phytopathological Society. DOI: 10.1094/Phyto-83-1155.

Four apple (Malus domestica) cultivars with different levels of resistance to Venturia inaequalis, the incitant of apple scab, were grown in the greenhouse and as shoot-tip cultures in vitro. Young, actively growing leaves were inoculated with a conidial suspension. Development of the fungus during the early events of pathogenesis and concurrent host responses were observed by light and electron microscopy. Prepenetration and penetration activities were indistinguishable in resistant and susceptible cultivars grown in vitro or in the greenhouse. After penetration, however, colonization and establishment of the fungus differed among cultivars and growing conditions. The resistant cultivar Freedom expressed a hypersensitive response 72 h after inoculation both in vitro and in the greenhouse, whereas the resistant cultivar Liberty showed reduced stroma formation with no sporulation. Typical stromata and sporulating lesions developed on leaves of greenhouse-grown plants of the susceptible cultivars Empire and McIntosh, but on shoot-tip cultures, the mycelia proliferated atypically above the cuticular leaf surface, and the number of appressoria increased significantly 5 days after inoculation. Although penetration and primary hypha formation occurred in vitro, the proliferation of secondary hyphae under the cuticular membrane was greatly restricted during the first 10 days after inoculation. Sporulation was observed in vitro 2030 days after inoculation. The abnormal development of the fungus in shoot-tip cultures may be the result of a greater availability of nutrients on the leaf surface.

Additional keywords: apple tissue culture, screening for disease resistance.