PATHOGENESIS OF VENTURIA-INAEQUALIS ON SHOOT-TIP CULTURES AND ON GREENHOUSE-GROWN APPLE CULTIVARS

Four apple (Malus X domestica) cultivars with different levels of resi stance 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. Prepene tration 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 dif fered among cultivars and growing conditions. The resistant cultivar F reedom expressed a hypersensitive response 72 h after inoculation both in vitro and in the greenhouse, whereas the resistant cultivar Libert y showed reduced stroma formation with no sporulation. Typical stromat a and sporulating lesions developed on leaves of greenhouse-grown plan ts 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 o ccurred in vitro, the proliferation of secondary hyphae under the cuti cular membrane was greatly restricted during the first 10 days after i noculation. Sporulation was observed in vitro 20-30 days after inocula tion. The abnormal development of the fungus in shoot-tip cultures may be the result of a greater availability of nutrients on the leaf surf ace.