Assessing and managing metalaxyl-m resistant strains of the potato pink rot pathogen Phytophthora erythroseptica in Canada

Phytophthora erythroseptica Pethybr. is the causal agent of potato pink rot which results in a wet rot and complete breakdown of tubers leading to significant yield losses in field and storage settings. Traditionally, products containing metalaxyl and later its enantiomer metalaxyl-m have been used to manage P. erythroseptica. However, in recent years, metalaxyl-m resistant strains of P. erythroseptica have been recovered in the United States and Atlantic Canada. A national survey was conducted from 2013 to 2016 to assess the distribution of metalaxyl m-resistant populations in Canadian potato- producing regions. The survey recovered resistant isolates from Prince Edward Island, Nova Scotia, New Brunswick, Ontario, Manitoba, and Alberta. As a result of this survey, resistant isolates were identified for the first time in Ontario, Manitoba, and Alberta. Due to the increasing frequency of metalaxyl-m resistant populations there is a need for alternative management strategies. Therefore, registered and experimental fungicides were assessed for the capacity to control the pink rot pathogen in field and storage settings. The fungicides evaluated were metalaxyl-m, oxathiapiprolin, fluopicolide, phosphites, and a biological control agent Bacillus subtilis. The EC50 values (the fungicide concentration inhibiting mycelial growth of the pathogen by 50% relative to the 0 μg mL-1 control) for two strains (PE9913; metalaxyl-m sensitive and PE1204; metalaxyl-m resistant) of P. erythroseptica exposed to metalaxyl-m, oxathiapiprolin, fluopicolide, and phosphites were determined using an amended agar assay. The EC50 values were 0.057 and 842.41 ppm, respectively, for the metalaxyl-m sensitive and metalaxyl-m resistant isolates grown on metalaxyl-m amended media. The EC50 values of the metalaxyl-m sensitive and metalaxyl-m resistant isolates exposed to oxathiapiprolin, fluopicolide, or phosphite were