Not Tickled Pink

This article appears in the April 2017 issue of Potato Grower.

Pink rot, caused by Phytophthora erythroseptica, is a common disease of potatoes that results in tuber breakdown and significant yield loss in field and storage settings. Tuber infection usually occurs in wet, low-lying areas prior to harvest. Superficial infections are asymptomatic but can later lead to tuber breakdown in storage or inoculate new areas via seed tubers.

Traditionally, pink rot has been managed with metalaxyl-based products, such as Ridomil Gold. However, in recent years, metalaxyl-resistant strains of the pathogen have developed in the U.S. and eastern Canada, signaling the need for alternative management strategies as resistant populations spread. A national survey was established to assess the distribution of resistant strains in Canadian potato-growing regions, and trials were conducted to assess alternative fungicides to inhibit infection under field conditions.

The research was funded by Syngenta, the Prince Edward Island Potato Board, Potato Growers of Alberta, and Keystone Potato Producers Association. Research teams from Agriculture and Agri-Food Canada also provided valuable assistance.

National Survey

Pure cultures of P. erythroseptica were isolated from tubers with pink rot submitted by industry, provincial, and federal agricultural organizations in Canada. The isolates were plated onto V8 agar amended with 0, 1, 10 or 100 micrograms of metalaxyl-m mL-1. The isolates were rated as metalaxyl sensitive, moderately metalaxyl resistant, or highly metalaxyl-resistant based on the metalaxyl concentration needed to suppress pathogen growth by 50 percent relative to the control.

From 2013 to 2015, metalaxyl-resistant strains were recovered from Prince Edward Island, New Brunswick, Nova Scotia, Ontario, Manitoba and Alberta. Most fungicide-resistant strains were concentrated in eastern provinces (Ontario and the Maritimes). However, a few fungicide resistant strains were recovered from western Canada, indicating an expansion in range and distribution of metalaxyl-m resistant isolates of P. erythroseptica in Canada.

Inoculated Field Trials

Inoculated field trials were conducted in Harrington, Prince Edward Island from 2014 to 2016 to assess the ability of six treatment programs to suppress infection by a metalaxyl-sensitive or a metalaxyl-resistant strain of P. erythroseptica. Plot rows planted with 10 seed pieces of the Shepody cultivar received an agar slurry pathogen inoculum applied in-furrow prior to planting.

The treatments included in-furrow applications of Ridomil Gold 480SL, Serenade SOIL, Presidio, Phostrol, Orondis, and a foliar treatment of Phostrol (five applications at two-week intervals during the course of the season). The treatments were replicated four times. Treatments were assessed for their ability to limit infection in daughter tubers.

Harvested tubers from each plot were rated for number and mass of infected (diseased) and non-infected tubers for each treatment. Inoculated diseased controls yielded between 25 and 35 percent diseased tubers at harvest; no disease was found in non-inoculated healthy controls.

Ridomil Gold provided excellent disease suppression against the metalaxyl sensitive isolate but not the resistant isolate. The foliar treatment of Phostrol provided excellent disease suppression; however, in-furrow Phostrol application did not provide adequate disease control. In-furrow treatments of Orondis or Presidio provided significant suppression of infection by both strains in 2014; Serenade SOIL provided some suppression of disease caused by the metalaxyl resistant strain.

Summary

The expansion in range and distribution of metalaxyl-m-resistant isolates of P. erythroseptica in Canada further signifies a need for alternative management strategies. Further testing is needed to continue to assess the efficacy of these treatments to manage and inhibit pink rot development in field settings. The results of these trials suggest that some of these treatments alone or in combination will play a role in mitigating pink rot infections in future. This ongoing work will facilitate new product registration and provide growers with viable integrated strategies to manage pink rot.

Contributing authors of this study are Bennett Crane, Rick D. Peters, Aaron Mills, Adam Foster, Kathryn A. Drake, Dorothy Gregory, Ian Macdonald and Anne MacPhail with Agriculture and Agri-Food Canada’s Charlottetown R&D Centre; Larry M. Kawchuk with Agriculture and Agri-Food Canada’s Lethbridge R&D Centre; and Larry Hale and Christian Lacroix from the University of Prince Edward Island.   

Summary of national metalaxyl-m sensitivity survey for Phytophtora erythroseptica collected from 2013 to 2015 in Canada: