Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.edu
It is still very early for the onset of downy mildew of cucumbers and other cucurbits in Delaware and Maryland. These diseases cannot overwinter here, and must be reintroduced to our area each year from cucurbit crops in the South. This year, the disease has started to moving north quickly and has already been reported on cucumber and watermelon in South Carolina. The watermelon occurrence is very early. Downy mildew specific fungicides do not need to be applied yet in Delaware or Maryland. However you should scout your cucurbits after this storm has passed and keep a look out for downy mildew in the coming weeks.
Images of downy mildew on watermelon. Note the dark spots and flecks that are surrounded by yellowing (chlorosis). The chlorosis in new infections is delimited by the veins.