Vegetable Disease Update – July 15, 2016

Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland;

Downy mildew on cucumber is now present in Sussex County, Delaware and in Cumberland County, New Jersey.

Several additional late blight outbreaks were confirmed in Maryland over the past week. Late blight is active in Queen Anne’s County, Dorchester County, and Somerset County, MD on the eastern shore. Garrett and St. Mary’s counties also have late blight outbreaks. Local rains continue to make our weather favorable, despite an increase in temperatures.

The pathogen genotype that has been identified in all these outbreaks is US-23. The US-23 genotype can cause disease on both potato and tomato and is sensitive to mefenoxam (the active ingredient in Ridomil). Therefore in addition to the fungicides that are listed in the commercial recommendation guide as effective (, Ridomil products that are labeled for use on potato and tomato late blight can be used.

Powdery mildew on cucurbits (squash, cucumber, pumpkin, etc.) is now present on Delmarva. Scout your fields for the presence powdery mildew. The threshold is one lesion per 50 old leaves. Once the threshold is reached in your field, targeted fungicides for powdery mildew management should be added to spray programs. Organic producers can apply fungicides that are approved by your certifying agent such as Cueva and Regalia.


Initial powdery mildew lesions will be small and white, and often only visible on the lower leaf surface.