Submitted to: Plant Pathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/6/2006
Publication Date: 7/1/2007
Citation: Porter, L.D., Miller, J.S., Nolte, P., Price, W.J. 2007. In vitro somatic growth and reproduction of phenylamide-resistant and -sensitive isolates of Phytophthora erythroseptica from infected potato tubers in Idaho. Plant Pathology. 56: 492-499. Interpretive Summary: Pink rot caused by Phytophthora erythroseptica is a major concern of potato growers in southern Idaho particularly since this pathogen has developed resistance to mefenoxam and metalaxyl, the active ingredients in Ridomil labeled products. Ridomil has been used since 1987 to manage this disease in potato fields. People in the potato industry were interested in how widespread the mefenoxam-resistant strain of the pathogen was in Idaho and whether the resistant strains were more fit than the sensitive strains. Knowing the presence or absence of the resistant strains in their fields would impact management decisions of potato growers. Knowing whether the resistant strains were more fit was important in determining the likelihood of the resistant type becoming permanently established and potential replacing the mefenoxam-sensitive strains in their fields. The present study identified that mefenoxam-resistant strains of the pink rot pathogen were present in 7 of 9 counties and 37 of 44 fields sampled in southern Idaho, indicating that the mefenoxam-resistant type of the pathogen is widespread. The mefenoxam-resistant strain of the pathogen also grows faster, and is capable of producing more survival spores than the mefenoxam-sensitive strain under laboratory conditions. This may indicate it is more fit and more likely to survive and reproduce and increase soil inoculum levels in potato fields where it is found. However, the resistant and sensitive strains did not significantly differ in their capability of infecting potatoes at six different storage temperatures. Growers in fields where only mefenoxam-sensitive strain of the pathogen were found are still encouraged to use Ridomil, but growers with resistant strains are advised to use phosphoric-acid-based fungicides to manage the problem.
Technical Abstract: Pink rot of potato is a major field and post-harvest problem in southern Idaho, particularly since 1998 when isolates resistant to the fungicide mefenoxam were detected. Isolates of P. erythroseptica were collected from 2001 to 2002 in nine Idaho counties and tested for resistance to mefenoxam on amended agar. Resistant isolates were identified in seven of the nine counties; 163 isolates were highly resistant, 7 moderately resistant and 57 sensitive to mefenoxam with mean EC50 values of 182, 23 and 0.5 ppm of mefenoxam, respectively. Growth and oospore production in V8 agar were assessed for 20 sensitive and 21 resistant isolates at 10, 15, 20, 25 and 30 degrees C. Optimal temperature for mycelial growth was 25 degrees C for both resistant and sensitive isolates. Optimal temperature for oospore production was 20 degrees C for sensitive and 15 degrees C for resistant isolates. Mycelial growth of resistant isolates was 2.5 to 3.1 times greater, and oospore production 6.8 to 20.5 times greater (P < 0.0001) for resistant isolates than sensitive isolates at temperatures from 10 to 25 degrees C. Zoospore production/mm of colony circumference was significantly greater for sensitive than resistant isolates, 14,191 and 9,959, respectively (P = 0.0109). The percentage of germinating zoospores was not significantly different for resistant and sensitive isolates (P = 0.9598). Incidence of pink rot tuber infection did not significantly differ (P = 0.05) for sensitive and resistant isolates at six storage temperatures. Optimal temperature(s) for pink rot infection was 20 degrees C for sensitive isolates and 15 and 20 degrees C were co-optimal for resistant isolates. Sexual fitness of mefenoxam-resistant isolates may be greater than sensitive isolates, but sensitive isolates may be more asexually fit based on the fitness parameters studied.