Phthalides produced by Coccomyces strobi (Rhytismataceae, Rhytismatales) isolated from needles of Pinus strobus

Abstract

As part of a broad survey of the metabolites produced by foliar endophytes of conifers, a strain was isolated from surface-sterilized needles collected from eastern white pine on the north shore of Lake Huron, Ontario. The isolate was sterile in culture. Species identity was not resolved by ITS sequence because of the absence of corresponding sequences in public sequence databases, however the strain was placed in Rhytismataceae (Rhytismatales). Field collections targeting Rhytismataceae species on Pinus strobus were conducted enabling the isolate to be identified as Coccomyces strobi. An extract of the culture filtrate demonstrated moderate antifungal activity. Two new phthalides, (±)-strobides A-B (1-2) and the known phytotoxin cyclopaldic acid (3) were characterized from C. strobi DAOMC 251937. This is the first record of the occurrence of this species as a foliar endophyte. Previous reports have suggested that it is a cambium endophyte associated with small cankers on branches. The production of the phytotoxin cyclopaldic acid suggests that C. strobi is a weak opportunistic pathogen, switching from an endophytic to a pathogenic phase when the host tissue becomes weakened.