NONPATHOGENIC MUTANT STRAIN OF COLLETOTRICHUM-MAGNA INDUCES RESISTANCE TO C-GLOEOSPORIOIDES IN AVOCADO FRUITS

A mutant strain (path-1) of Colletotrichum magna (teleomorph Glomerella magna) that grew endophytically in watermelon plants was recently isolated. Unlike the wild-type isolate, the mutant did not cause disease symptoms and protected plants from disease caused by the wild-type fungus. We observed that inoculation of freshly harvested avocado fruit with path-1 inhibited subsequent decay development by the normal pathogen Colletotrichum gloeosporioides. C. gloeosporioides isolate Cg-14 and the wild-type and mutant strains of C. magna germinated and formed appressoria on fruit peel, but Cg-14 and the wild type formed two- to threefold more appressoria on avocado peel than path-1. Cell wall elicitors from all three Colletotrichum isolates induced higher levels of phenylalanine ammonia lyase activity and mRNA in avocado cell suspension cultures than untreated controls. Path-1 and path-1 co-inoculated with Cg-14 also induced higher levels of phenylalanine ammonia lyase in freshly harvested fruits than Cg-14-inoculated and untreated controls. The isolates differed, however, in the level and extent of induction of epicatechin and an antifungal diene in the peel of ripening fruits. Path-1 induced higher levels of epicatechin that lasted in ripening fruits, whereas Cg-14 induced epicatechin levels for 1 day only. It is therefore suggested that the enhanced resistance of avocado fruits to C. gloeosporioides by path-1 results from the induction of epicatechin, a phenol that inhibits oxidation of the antifungal diene.