Cucurbit Downy Mildew Creeping Closer to Kentucky

Cucurbit downy mildew was confirmed on winter squash in central Tennessee on July 24, so all Kentucky cucurbit growers should be on alert for this disease in their crops. Symptoms are shown in Figures 1 and 2, on cucumber and squash, respectively. Early symptoms are most likely to appear in the centers or on the edges of leaves, where water may persist for extended amounts of time.

Figure 1. Symptoms of cucurbit downy mildew on underside of cucumber leaf. (Photo: Kenny Seebold, UK).

Figure 2. Dark sporulation of Pseudoperonospora cubensis, the cucurbit downy mildew pathogen, between veins on underside of butternut squash leaf. (Photo: Emily Pfeufer, UK).

Kentucky producers with cucumber, melon, squash, or pumpkin crops should consider spraying with a downy mildew-specific fungicide before their next rain event. This is most important for those in counties that border Tennessee, and is more critical in crops that have more than 2 weeks of production season remaining. Suggested products, by FRAC group, are indicated in Table 1, with comments on optimizing their efficacy (if applicable). See University of Kentucky ID-36 for a more complete listing.

FRAC group

Example product(s)


49 + ___ Orondis Opti, Orondis Ultra Tank mix Ultra with chlorothalonil or mancozeb.
21 Ranman Tank mix with chlorothalonil or mancozeb. Use an appropriate surfactant for best results.
22 Elumin, Gavel, Zing Tank mix with chlorothalonil or mancozeb if not already included.
27 Curzate, Tanos, Ariston Tank mix with chlorothalonil or mancozeb if not already included.
28 Previcur Flex Tank mix with chlorothalonil or mancozeb.
40 + 45 Zampro Tank mix with chlorothalonil or mancozeb.

Since downy mildew has not yet been found in Kentucky, producers suspecting the presence of this disease in their cucurbits should contact their local county agent as soon as possible. The county agent will coordinate submitting a sample to the University of Kentucky Plant Disease Diagnostic Laboratories. Awareness of cucurbit downy mildew’s presence helps all cucurbit producers target their fungicide applications, reducing losses to this disease caused by an airborne plant pathogen.


By Emily Pfeufer, Plant Pathology Extension Specialist


Posted in Vegetables