Cucurbit downy mildew (CDM) is the most economically important disease of cucurbits in Kentucky. All cucurbits, such as cucumber, cantaloupe, summer and winter squashes, pumpkin, and watermelon are susceptible to this water mold (fungus-like organism/oomycete) pathogen. Since the CDM pathogen does not overwinter in Kentucky, each summer it must be blown in from overwintering locations. Once CDM is present, growers rely heavily on fungicides for management of this disease.
The dependence on fungicides for CDM management increases the risk of the pathogen developing resistance. As a way to monitor the efficacy of fungicides against CDM, county Extension agents in nine locations throughout Kentucky applied fungicides labeled for CDM management after the disease had been documented in their area. Ratings for the incidence and severity of disease were collected.
As of the week of August 6, 2018, CDM has been confirmed in East/Central Kentucky. The data shown below (Figure 1) represents results from two replicates of the experiment conducted at the Pulaski county Extension office. Disease incidence and severity were rated on a scale from 0 to 4, where 0 = no disease, 1 = 1-25% CDM, 2 = 25-50% CDM, 3 = 50-75% CDM, and 4 = 75%+ CDM. All products reduced CDM to the levels presented in Figure 1. Commercial growers are encouraged to use this information to potentially modify their spray programs. Cucurbit downy mildew has not been confirmed in areas of central or in western Kentucky. Growers in these regions are advised to continue their preventative spray programs.
Figure 1: Cucurbit downy mildew sporulation covering the bottom of the leaf ratings, averaged by treatment. Scale: 0 = no disease, 1 = 1-25% CDM, 2 = 25-50% CDM, 3 = 50-75% CDM, and 4 = 75%+ CDM. (Image: Emily Pfeufer, UK)
By Emily Pfeufer, Extension Plant Pathologist
Kentucky Pest News 