Evaluation of apple genotypes and Malus species for resistance to Alternaria blotch caused by Alternaria alternata apple pathotype using detached-leaf method

P>Alternaria blotch, caused by Alternaria alternata apple pathotype, is an economically important disease of apple (Malus x domestica Borkh.). Information on the resistance level of apple cultivars to Alternaria blotch is the key to a strategy for integrated control of the disease, however, such information on new apples recently released or on scab-resistant apples is lacking. The aim of this study was to develop an advanced technique, which allowed one a reliable assessment of apples for resistance to Alternaria blotch, and to evaluate resistance levels of various apple genotypes, especially scab-resistant apples. Detached-leaves, youngest opened leaf (leaf 1), second (leaf 2), third (leaf 3), fourth (leaf 4) and fifth (leaf 5) at shoot tip removed from growing shoots of four cultivars, were inoculated with conidial suspension of A. alternata apple pathotype, and the responses were rated at 48-hour postinoculation; a modified six-class disease scale was established for the rating system of Alternaria blotch. Leaf 1-3 were suitable for the evaluation and discrimination of resistance level of apple genotypes to the disease. Reliability of assessment was demonstrated in successive 3-year inoculation tests with detached-leaves of leaf 2 on 16 cultivated apples; incidence of the disease caused by natural infection of A. alternata at a field was well correlated with the resistance levels determined by the detached-leaf method among 11 apples. Two-year inoculations using detached-leaf method on 41 apple genotypes revealed that 17 scab-resistant apples, 'Freedom', 'Orlovim', 'Orlovski Pioner, 'Pervinka, 'R12740-7A', 'Regrindis', 'Antonovka', 'Dayton', 'Florina-Querina', 'Galarina', 'Liberty', 'Redfree', 'Reanda', 'Remo', 'Priam', 'Regine' and 'Priscilla', showed no visible symptoms and were regarded as resistant (R) to A. alternata apple pathotype. In addition to the above genotypes, five scab-resistant genotypes, 'GoldRush', 'Prima', 'Renora', 'Rewena' and 'Retina', showed pit-type small spots on a few inoculated leaves and were also included in this category of resistance level. Three scab-resistant genotypes, 'Jonafree', 'Reka' and 'Co-op 25,' exhibited extended necrotic lesions in the inoculated leaves and were regarded as susceptible (S) to the disease. Among 38 cultivated apples and selections of Japanese origin, 27 genotypes were R; 'Kitaro', 'Kotaro', 'Santaro', 'Chinatsu', 'Shinano Sweet', 'Shinano Red', 'Shinano Gold', 'Aori 13', 'Akibae', were included in this category, seven genotypes were S; 'Beninomai', 'Natsumidori' and several selections were included in S category, and the remaining four genotypes were moderately resistant. Parentage information clearly indicated that source of the susceptibility in the cultivated apples and selections developed in Japan were 'Delicious' or 'Indo'. In contrast to the genotypic differences for levels of resistance to A. alternata apple pathotype in cultivated apples, all Malus spp. tested exhibited complete resistance to A. alternata.