Metalaxyl sensitivity of isolates of Phytophthora erythroseptica in PrinceEdward Island

Isolates of Phytophthora erythroseptica, causal agent of pink rot of potato , were obtained from diseased tubers collected in Prince Edward Island (PEI ), Canada. In an in vitro assay of field and single-zoospore isolates, all isolates of the pathogen from PEI were sensitive to metalaxyl (EC50 < 0.5 < mu>g mL(-1)), unlike moderately or highly resistant reference isolates obta ined from Maine, USA. Allozyme-banding patterns at the glucose-6-phosphate isomerase (Gpi) locus were identical (91/91) for all isolates of P, erythro septica examined from PEI and Maine, but could be used to distinguish isola tes of P. erythroseptica from local isolates of Phytophthora infestans. Ino culation of potato tubers from plants treated with foliar applications of m etalaxyl in the field indicated that compounds inhibitory to metalaxyl-sens itive isolates of P. erythroseptica were present in the periderm, even afte r 4 months' storage. By contrast, moderately or highly resistant isolates o f the pathogen caused significantly (P less than or equal to 0.05) more dis ease than sensitive isolates in tubers from plants treated with metalaxyl. The effectiveness of metalaxyl in controlling pink rot in a particular regi on will depend on resistance levels of local populations of P. erythrosepti ca. Preventing the development and spread of resistant strains of P. erythr oseptica will be critical in maintaining metalaxyl as an effective chemical for control of this pathogen.