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High levels of resistance to phosphonate fungicides in the hop downy mildew pathogen, Pseudoperonospora humuli

David Gent: US Department of Agriculture, Agricultural Research Service

<div>Phosphonate fungicides are an important component of disease management programs for downy mildew on hop (caused by <em>Pseudoperonospora humuli</em>). In two fungicide trials conducted in commercial hop yards in Oregon in 2016, weekly applications of the highest labeled rate of a phosphorous acid fungicide provided no suppression of downy mildew at one location and approximately 50% disease suppression at the second location. These trials suggested potential resistance to phosphorous acid in the population of <em>P</em>. <em>humuli</em>. Isolates of the pathogen collected from a farm reporting disease control failures following use of phosphonate fungicides were assayed for sensitivity to phosphorous acid. The median concentration of phosphorous acid needed to reduce the incidence of infection by 50% was 1.9 times the maximum labeled rated of the fungicide FUNGI-PHITE. Further, there was a linear relationship between sensitivity to phosphorous acid and fosetyl-Al, indicating cross resistance between these compounds. Follow up field trials found that neither the highest labeled rates of a phosphorous acid product or fosetyl-Al significantly reduced downy mildew as compared to nontreated plots where resistant isolates were present, whereas fungicides with different modes of action reduced disease to varying levels. Further sampling is planned to determine how widespread resistance to phosphonate compounds may be in Oregon</div>