SEM INVESTIGATIONS OF INFECTION PATTERNS OF VENTURIA-INAEQUALIS (COOKE) ADERH ON LEAVES OF DIFFERENTLY RESISTANT APPLE CULTIVARS

Primary infections of terminal and ontogenicly resistant leaves of sus ceptible apple cvs. 'Golden Delicious' and 'Weisser Klarapfel' as well as chose of the resistant cv. 'Sir Prize' by Venturia inaequalis were observed by scanning electron microscopy. Conidia germination and for mation of appressoria were identical in all cultivars and on leaves of different age. Symptoms of incompatibility become visible by renewed conidia germination, branching of germ tubes and hyphal growth out of the appressoria. This could be observed after the formation of primary appressoria on the third leaf of susceptible cultivars as well as on the first true leaf of the resistant cultivar. About 24 h after inocul ation and formation of the first appressoria on the third leaf of 'Gol den Delicious', a stop of hyphal growth occurred. First stroma formati on on the first true leaves of 'Golden Delicious' occurred about 48 h after inoculation, on those of 'Sir Prize' about 72 h after inoculatio n, but seemingly inhibited. The second and the third leaf of the susce ptible cultivars showed stroma formation about 96 h after inoculation, the second leaf of 'Sir Prize' after 120 h, the third leaf showing no ne. inhibited hyphal growth and reduced stroma formation in second lea ves of susceptible cultivars seemed to be identical with chose in the first true leaves of the resistant cultivar.