Selection of mutants resistant to Alternaria blotch from in vitro-culturedapple shoots irradiated with X- and gamma-rays

We produced mutants resistant to Alternaria blotch disease in several culti vars of apple (Malus x domestica Borkh.) by irradiation with X- or gamma -r ays. An efficient in vitro assay method was established using chemically-sy nthesized AM-toxin I of Alternaria alternata (Fr.) Keissler to screen for m utants resistant to Alternaria blotch disease. The frequency of necrotic le sions was investigated by applying various concentrations of AM-toxin I to leaf discs of the first, third, and fifth leaves from the shoot apex of sev eral apple cultivars, including Jonathan, Fuji, Oorin, and Indo. In vitro-g rown apple shoots of susceptible cultivars were then treated with various d oses of X- or gamma -ray irradiation. Several mutants resistant to AM-toxin I were obtained by combining the techniques for tissue culture of apple sh oots with the AM-toxin I screening method. Following a repeat second screen ing test with AM-toxin I, mutant plants were sprayed with a spore suspensio n of A. alternata and found resistant to be the fungal pathogen. These muta nts showed normal phenotypic appearance, and so far, no difference has been observed between the original plants and mutants except for the susceptibi lity to Alternaria blotch.