|Nelson, M - WASHINGTON STATE UNIV|
|Grove, G - WASHINGTON STATE UNIV|
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 7, 2007
Publication Date: March 31, 2008
Citation: Gent, D.H., Nelson, M.E., Grove, G.G. 2008. Persistence of Phenylamide Insensitivity in Pseudoperonospora humuli. Plant Disease.92(3):463-468. Interpretive Summary: Downy mildew is an important disease of hop that can reduce marketable yield of the crop by 100% during certain years. Resistance to the class of fungicides known as phenyamides developed quickly in the pathogen population follow use of these fungicides for disease control. Little is known about the persistence of resistance to these fungicides in the downy mildew pathogen. In this study, the prevalence of phenylamide resistance among isolates of the pathogen was quantified. Resistance was found to be widespread in hop yards sample in Oregon and Idaho, despite the near non-use of this fungicide in the past 15 to 20 years. We also detected resistance to phenylamide fungicides in Washington State, which is the first time this has been documented in the downy mildew pathogen. Management of downy mildew should rely on measures other than these fungicides in yards and/or regions where phenylamide insensitivity is prevalent, which is now known to include certain hop yards in Idaho, Oregon, and Washington State.
Technical Abstract: Downy mildew, caused by Pseudoperonospora humuli is an important disease of hop in the U.S. Insensitivity to phenylamide fungicides was detected in isolates of the pathogen in production regions in Oregon and Idaho in 1992, and has since been used on a very limited basis. The prevalence of phenylamide insensitivity among isolates of P. humuli collected from 2005 to 2007 in the Northwestern U.S. was quantified using inoculum derived from 201 diseased shoots (basal spikes) and 42 monosporic isolates. Insensitivity to mefenoxam was detected in 31 of 74 (41.9%) basal spikes collected from 13 hop yards. Insensitivity to the related compound metalaxyl was detected in 52 of 80 (65%) spikes collected from 9 hop yards, including four hop yards in Washington State. Log ED50 values ranged from -2.25 to 2.67 for basal spike isolates and -2.27 to 2.98 for monosporic isolates and had a similar distribution. Log ED50 values for monosporic isolates and entire basal spike isolates were significantly associated. However, the slope of the regression line was <1, indicating that the log ED50 values obtained from entire basal spike isolates were greater than the corresponding log ED50 values obtained from a monosporic isolate obtained from that spike. This research suggests that insensitivity to phenylamide fungicides is a stable phenotype in P. humuli. Management of downy mildew should rely on measures other than these fungicides in yards and/or regions where phenylamide insensitivity is prevalent, which is now known to include certain hop yards in Idaho, Oregon, and Washington State.