Mass-screening of mutants resistant to Alternaria blotch from in vitro-cultured apple shoots irradiated with X-rays

We have established mass screening methods for producing mutants resistant to Alternaria blotch disease in several cultivars of apple (Malus domestica) by irradiation with X-rays. An in vitro assay system using chemically-synthesised AM-toxin I of Alternaria alternata (Fr.) Keissler is successful for screening the resistant mutants. The degree of resistance to the disease was assayed by the number of necrotic lesions induced by applying various concentrations of AM-toxin I to leaf disks of the first, third and fifth leaves from the shoot apex of plants. Following the establishment of the optimal screening conditions, we produced several disease-resistant mutants by irradiating in vitro shoots with various doses of X-rays. After resistant mutants were selected by the first and second screening tests with AM-toxin I, they were tested by spraying them with a spore suspension of the pathogen. The resistance to the disease was maintained for five years during field cultivation of the mutants, implying that the resistance was not due to environmental factors but due to genetic fixation. No differences were observed in the number of fruiting trees or colouration of fruits between mutants and the original plants. (author)