A Nonpathogenic Mutant of Colletotrichum magna Is Deficient in Extracellular Secretion of Pectate Lyase. Castro Wattad . (1) Department of Postharvest Science of Fresh Produce, The Volcani Center, Bet Dagan 50250, Israel; (2) Department of Plant Pathology and Microbiology, The Hebrew University of Jerusalem, Faculty of Agriculture, Rehovot 76100, Israel. Stanley Freeman(3), Amos Dinoor(2), and Dov Prusky(1). (1) Department of Postharvest Science of Fresh Produce, The Volcani Center, Bet Dagan 50250, Israel; (2) Department of Plant Pathology and Microbiology, The Hebrew University of Jerusalem, Faculty of Agriculture, Rehovot 76100, Israel; and (3) Department of Plant Pathology, The Volcani Center Bet Dagan 50250, Israel. MPMI 8:621-626. Accepted 28 April 1995. Copyright 1995 The American Phytopathological Society.

A nonpathogenic mutant strain (path-1) of Colletotrichum magna (teleomorph Glomerella magna) was isolated and grew endophytically in cucurbit seedlings. Decay development on the pericarp of avocado fruits was significantly delayed in the wild-type (wt) isolate of C. magna compared with C. gloeosporioides isolate Cg-14, but both isolates colonized mesocarp tissue equally well. However, path-1 did not cause disease symptoms in either the pericarp or the mesocarp. Electrofocusing of culture filtrates of Colletotrichum strains revealed the presence of pectate lyase (PL) and polygalacturonase (PG) at pls of 7.9 and 4.8, respectively, but no significant activity from path-1 was detected. Western blots detected PL in avocado fruit tissue infected with Cg-14 and wild-type C. magna. DNA of the three Colletotrichum isolates hybridized to a pel probe from C. gloeosoporioides indicating that at the DNA level no deletion in the pel gene was evident in path-1. Similar levels of pel mRNA expression were also found for wild-type and path-1 of C. magna, and Cg-14 of C. gloeosporioides. However, when PL antibodies were cross-reacted with cell lysate of Colletotrichum strains, PL accumulated in the hyphae of path-1 of C. magna, but was not detected in Cg-14 cell lysate. This suggests that the malfunction in extracellular secretion of PL proteins results in differential pathogenicity of path-1 isolate on avocado.