Occurrence of Fungicide-resistant Strains of Alternaria mali in Tokusa, Ato-cho, Yamaguchi Prefecture, Japan

Occurence of polyoxin- or iprodion-resistant strains of Alternaria mali Roberts, the causal fungus of Alternaria blotch of apple, was investigated by the minimum inhibitory concentration method at two orchard (Orchards A and B) in Tokusa, Ato-cho, Yamaguchi Prefecture, located at one of the warmer districts among the apple-growing areas in Japan, from 1984 to 1988. In Orchard A the, the frequency of polyoxin-resistant strains was 60.3% in 1984, and decreased gradually to 46.9% in 1986. It quickly decreased to 27.0% in 1987 and to 8.9% in 1988. This last value reached to a level at which the reapplication of polyoxin in Orchard A was available. In Orchard B, on the other hand, the frequency was 51.5% in 1984, and decreased slowly to 37.5% in 1988. This last value proved that the reapplication of polyoxin in Orchard B should be forbidden. Decrease in the frequncy at both orchards was considerably slow compared with that reported at apple-growing orchards in northern Japan. The overwintered causal fugus on apple plants was detected by isolating from every portions of fruiting branches in April of 1987 and 1988． The frequency of the fungus was highest on the barks of nodes, and also high on barks of internodes and on scaly leaves. Polyoxin-resistant strains as well as susceptible strains at Tokusa may abundantly overwinter under the relatively warmer condition than that in northern Japan. This may cause the slow decrease in the frequency of polyoxin-resistant strains at Tokusa. Frequency of iprodion-resistant strains in both orchards kept a low level of value from 1984 to 1988. Therefore, the iprodion-resistant strains of A. mali at Tokusa were suggested to havenot been affected to increase their frequency by the present controlling system in this district in which iprodion has been applicated one time a year.