Submitted to: American Journal of Potato Research
Publication Type: Abstract Only
Publication Acceptance Date: 8/12/2007
Publication Date: 2/1/2008
Citation: Hamm, P.B., Porter, L., David, N.L., Gieck, S.L., Miller, J.S., Gundersen, B., Inglis, D.A. 2008. Occurrence of mefenoxam resistant isolates of Pythium spp. in the Pacific Northwest. American Journal of Potato Research (Abstr.) 85: 13. Interpretive Summary:
Technical Abstract: Several Pythium spp. can cause leak on potato, and are commonly controlled by mefenoxam, a single-site mode of action fungicide. Previous research has shown that mefenoxam-resistant isolates of Pythium spp. are present in potato production areas of the U.S., but detailed information on occurrence is lacking for the Pacific Northwest (PNW). A three-year (2004 – 2006) study determined the incidence of mefenoxam-resistant isolates of Pythium spp. in soils commonly cropped to potato in the PNW. A total of 576 soil samples were collected in Idaho (141), Oregon (153), and Washington (282). Soils were diluted (10-1) in 0.10% water agar and plated onto clarified V-8 agar amended with 0 or µg/ml of mefenoxam. Numbers of Pythium colonies/gram of dry soil were determined. Pythium isolates were recovered from 90% of the soils. Of these, 31% grew on the mefenoxam-amended plates. Resistant isolates were present in less than 2% of the Idaho soils, but were recovered from 51% and 35% of the Columbia Basin soils from Oregon and Washington, respectively. Resistant isolates were not found in northwestern Washington or central and southern Oregon samples. While eight species of Pythium were recovered during this study, P. ultimum was the most common. Isolated cases of severe potato leak damage due to resistant isolates of P. ultimum have occurred in the Columbia Basin and could be expected to increase if there is further spread of mefenoxam-resistant isolates. Alternative chemicals and/or cultural practices that reduce inoculum need to be identified. (oral, Plant Protection, PAA membership 1805).