P&PDL Picture of the Week for
September 2, 2013

Downy mildew of cucurbits

Dan Egel, Extension Plant Pathologist, SWPAC, Purdue University

Downy mildew of cucurbits has been reported in 3 counties in Indiana: Knox, Porter and Wayne. Symptoms of downy mildew include bright yellow lesions visible from the top of the leaves of plants in the cucumber family including cucumber, cantaloupe, pumpkin and watermelon. Lesions on a cucumber leaf are shown in figure 1. On the underside of the leaf, growth of the fungus-like organisms that causes downy mildew can be seen under moist conditions (figure 2). The current hot, dry weather over much of Indiana should slow down the spread of spores since downy mildew is favored by relatively cool, moist weather. Homeowners who want to control downy mildew should look for a product that lists both the plant host in their garden and the disease downy mildew. Products with the active ingredient chlorothalonil might help to slow the spread of the disease. Fortunately, the fungus-like organism that causes downy mildew (Pseudoperonospora cubensis) does not overwinter in Indiana; it must be blown in or otherwise brought into Indiana each year. Therefore, whether or not downy mildew will show up in Indiana next year is unknown.

Click image to enlarge

Figure 1. Lesions on cucumber leaf

Figure 2. Underside of cucumber leaf
Photo by Liz Maynard

Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service