Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland
There have been new reports of cucumber downy mildew from Wicomico, Cecil, and St. Mary’s counties in Maryland. These reports add to previous reports from Caroline county, Maryland and Kent and Sussex counties in Delaware. Suffice to say – downy mildew on cucumber is widespread in our region. Downy mildew has not yet been reported on any other cucurbit crop (squash, pumpkin, watermelon or cantaloupe). Scouting should be frequent and aggressive on those cucurbits.
Recent research from Dr. Keinath in South Carolina indicates that some previously effective fungicides are no longer effective on cucumber downy mildew. Quadris, Revus, Presidio, and Forum performed very poorly in South Carolina. I would avoid those materials on cucumber, because the downy mildew pathogen that affects the Delaware/Maryland crop often originates from the southeastern US.
The materials that performed well in South Carolina were Ranman, Gavel, Tanos, Zampro, Bravo, and Previcur Flex. Before symptoms develop, use Bravo or Zampro, or a tank mix of a protectant fungicide plus Previcur Flex or Tanos. After symptoms appear, apply Gavel or a tank mix of a protectant fungicide plus Ranman. (Please note that this information relates only to the strain of Pseudoperonospora cubensis that causes downy mildew on cucumber – and that came from the same population that exists in coastal South Carolina).