Use of mefenoxam to control pink rot [Phytophthora erythroseptica] of potato in Prince Edward Island

Abstract

Pink rot of potato, caused by Phytophthora erythroseptica, causes yield losses in the field and in storage. Surveys were conducted in Prince Edward Island (P.E.I.) during 1999–2001 to determine the sensitivity of local isolates of the pathogen to mefenoxam (Ridomil®). In addition, field trials were established to test the efficacy of foliar applications of Ridomil to control pink rot in stored progeny tubers. Of 47, 24, and 4 samples of infected tubers collected from harvested fields or warehouses across P.E.I. in 1999, 2000, and 2001, respectively, 15 (32%), 2 (8%), and 2 (50%) contained tubers showing symptoms of pink rot. Tissues excised from infected tubers of samples showing symptoms of pink rot yielded 62, 7, and 8 field isolates of P. erythroseptica in 1999, 2000, and 2001, respectively. In 1999 and 2000, 97 and 17 single-zoospore isolates were also generated, respectively, for testing of sensitivity to mefenoxam. An assay on agar amended with mefenoxam revealed that all field and single-zoospore isolates collected from P.E.I. were highly sensitive to mefenoxam (EC50 < 0.5 µg mefenoxam/mL). By contrast, tester isolates of P. erythroseptica from Maine were either sensitive, moderately resistant, or highly resistant to mefenoxam. Tubers ('Russet Burbank' and 'Kennebec') harvested from field plots managed with foliar applications of mefenoxam were inoculated with selected isolates of P. erythroseptica. Isolates that were inhibited on agar medium amended with mefenoxam were also suppressed in tubers from plants treated with foliar applications of the chemical. One foliar application of mefenoxam at tuber initiation was often sufficient to provide a significant (P = 0.05) reduction in pink rot incidence and severity. In some instances, two foliar applications (at tuber initiation and 2 weeks thereafter) reduced disease incidence and severity significantly (P = 0.05) more than a single application. By contrast, tester isolates of P. erythroseptica from Maine designated as either moderately or highly resistant following the amended-agar assay, also infected tubers from plants treated with mefenoxam. Fungicide efficacy was found to vary between cultivars and among years of study; suppression of pink rot was greater in 'Kennebec' than 'Russet Burbank', and fungicide efficacy was reduced in drier years. Management of resistance to mefenoxam and application scheduling are discussed.