A Nonpathogenic Mutant Strain of Colletotrichum magna Induces Resistance to C. gloeosporioides in Avocado Fruits. Dov Prusky. Department of Plant Pathology, University of California, Riverside, CA 92521 U.S.A. Rusty J. Rodriguez, Noel T. Keen. Department of Plant Pathology, University of California, Riverside, CA 92521 U.S.A. MPMI 7:326-333. Accepted 18 January 1994. Copyright 1994 The American Phytopathological Society.

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 rtype formed two- to threefold more appressoria on avocado peel than path-1. Cell wall elicitors from all three Colletotrichum oisolates induced higher levels of phenylalanine ammonia lyase activity an mRNA in avocado cell supension cultures than untreated controls. Path-1 and path-1 cp-inoculated with Cg-14 also induced higher levels of phenylaannine ammonia lyase in freshly harvested fruits than Cg-14 inoculated and untreated contols. 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 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 anti-fungal diene.