COINFECTION BY DIFFERENT ISOLATES OF ALTERNARIA-ALTERNATA IN SINGLE BLACK SPOT LESIONS OF JAPANESE PEAR LEAVES

Coinfection by different isolates of Alternaria alternata in single black spot lesions of Japanese pear leaves was detected by restriction fragment length polymorphism (RFLP) analysis of the nuclear ribosomal DNA (rDNA) for differentiation of individual fungal isolates. Mixed-rDNA isolates, whose total DNA contained two types of rDNA units, have been isolated from single black spot lesions of pear leaves in susceptible cultivar Nijisseiki plants. To examine whether mixed-rDNA isolates resulted from coinfection by different rDNA variants, single-spore isolates were prepared from nine mixed-rDNA isolates and subjected to the rDNA RFLP analysis. Among the nine mixed-rDNA isolates, five were identified as true mixed-rDNA types: All single-spore isolates carried two types of rDNA units corresponding to the respective parent isolate. However, the remaining four isolates originated from lesions coinfected by two variant rDNA isolates. All the single-spore isolates from the four mixed rDNA isolates contained only one type of rDNA unit. Their rDNA types were identical to one or the other of the two types carried by the respective parent isolate, and two rDNA variants were detected in single-spore isolates from each parent. This result indicated that coinfection by different isolates of A. alternata might occur frequently in single black spot lesions of pear leaves in the field. Pathogenicity tests of rDNA variants obtained from coinfected lesions showed that nonpathogenic isolates in conjunction with pathogenic isolates occasionally infected lesions.