Cucurbit Downy Mildew in Haywood County, July 28, 2023

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Cucumber downy mildew (CDM) has been confirmed on cucumber plant samples from a research plot at the Mountain Research Station in Haywood County, NC. Only 1% of plants showed symptoms and only 5% of the leaf area was affected. Water-soaked lesions on the undersides of leaves were observed with signs of the pathogen   — gray to black spores on the underside of the leaf. Very light yellow spots or lesions were observed on the upper sides of leaves. At the moment, downy mildew has not been observed on any other cucurbit in western NC.

Cucumber downy mildew

Cucumber downy mildew

Cucumber downy mildew

Cucumber downy mildew

Cucumber downy mildew

Cucumber downy mildew: note light yellow spot on left and gray/black sporulation on right

Growers in western NC are advised to implement management strategies. Effective fungicides can be found in the Southeastern US Vegetable Crop Handbook. More information can be found on the cucurbit downy mildew disease fact sheet.

Spores (sporangia) of the cucurbit downy mildew pathogen

Spores (sporangia) of the cucurbit downy mildew pathogen

This pathogen can rapidly become resistant to fungicides so, it is important to rotate fungicide groups and use tank mixes with protectants to minimize the risk of fungicide resistance. Cucumbers and cantaloupes are generally more susceptible to the disease than squash, watermelon, and pumpkin.

This pathogen spreads rapidly in air currents and high humidity, warm temperatures, and rainfall are really conducive to this disease.

Growers in western North Carolina are encouraged to apply systemic fungicides for this disease on cucurbits.

You can track the spread of CDM and report outbreaks anonymously on the Cucurbit Downy Mildew IPM PIPE (CDM ipmPIPE). If you see symptoms of CDM in your cucurbits, please notify your local Extension Agent to assist in diagnosis and to help you send a sample to the Plant Disease and Insect Clinic.