APS Homepage

Poster: Biology & Disease Mgmt: Chemical Control


Management of mefenoxam-resistant isolates of Pythium ultimum causing pythium leak on potato in the Pacific Northwest.
I. CARRILLO (1), P. Wharton (1), K. Fairchild (1), L. Porter (2), K. Frost (3), P. Hamm (3) (1) University of Idaho, U.S.A.; (2) USDA-ARS, U.S.A.; (3) Oregon State University, U.S.A.

Mefenoxam is a fungicide used to manage oomycete pathogens on potato. Resistance to mefenoxam has developed in Phytophthora infestans in all potato growing regions of the U.S, and is becoming more common in P. erythroseptica. In a 2006 survey, many fields in the Columbia Basin were found to have mefenoxam-resistant isolates of P. ultimum. The percentage of resistant isolates to sensitive isolates for these fields ranged from 0 to 92.5%. Despite the risk of resistance and the cost of mefenoxam, it is still widely used in potato production in the Pacific Northwest (PNW) due to its efficacy in managing pythium leak and the lack of effective alternative compounds. In the PNW, pythium leak is most often seen at harvest or during storage. However in recent years, particularly in fields with high levels of resistance, extensive damage has been found weeks ahead of harvest, suggesting a higher disease risk for resistant isolates than sensitive ones. In this study, a field survey was undertaken to evaluate the extent and persistence of mefenoxam resistance in populations of Pythium in the PNW and determine potential alternative fungicides that can be used for the control of mefenoxam resistant P. ultimum isolates. Results showed that the levels of mefenoxam resistance in PNW Pythium isolates had decreased since 2006. Several new fungicides were also identified which had good efficacy against P. ultimum.