Cucurbit Downy Mildew

Cucurbit downy mildew 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. Cucurbit downy mildew affects plant leaves, and severe infections can result in plant death. This pathogen does not overwinter in Kentucky and must be blown north from overwintering locations further to the south. Thus, the date of introduction of cucurbit downy mildew varies from year to year. Monitoring disease risk and preventative management are critical to reducing infection and damage.   

Cucurbit Downy Mildew Facts

  • Leaf symptoms first appear as pale or bright yellow spots on the upper surfaces of leaves (Figure 1). Symptoms are often first observed in older leaves at the base of plants. Over time, spots become angular or “blocky” in appearance, developing into necrotic lesions (Figure 2). During periods of high humidity, lesions on the undersides of leaves may develop a dark gray to purple, downy like appearance (Figure 3). Severe infections can result in rapid defoliation and plant death in a short period of time.  
  • The disease does not infect cucurbit fruit.
  • Disease is favored by rainy, humid conditions.
  • Caused by the water mold (fungus-like organism) Pseudoperonospora cubensis.
  • The pathogen survives winter in warmer locations to the south of Kentucky. The pathogen travels north on wind currents.  
Figure 1: Symptoms of cucurbit downy mildew begin as pale- to bright-colored yellow spots on upper leaf surfaces. (Photo: Kenny Seebold, UK)
Figure 2: Spots develop an angular or “blocky” appearance and expand to become necrotic lesions. (Photo: Kenny Seebold, UK)
Figure 3: During periods of high humidity, lesions on the underside of leaves may develop a dark gray to purple, downy like appearance. (Photo: Kenny Seebold, UK)

Management Options

Disease monitoring

The movement of cucurbit downy mildew is monitored on the Cucurbit Downy Mildew Forecasting page of the IPM pipe website. Cucurbit growers can sign up for free e-mail or text alerts in order to be updated on the proximity of downy mildew to an individual farm. Additional tools on the website allows for growers to calculate disease risk.  Disease monitoring permits for additional preventative measures to be used only when risk for disease is high.

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Cultural disease management

  • Utilize disease tolerant cultivars.
  • Avoid overhead watering to reduce leaf wetness.
  • Plant in sunny areas with good airflow.
  • Use recommended plant spacing to facilitate air movement and leaf drying.
  • Remove and destroy heavily infected plants.

Chemical disease management

Application of fungicides before disease arrives (preventative fungicide) provides the greatest level of disease protection. Timing of application should be established based on disease risk. When applying chemicals, it is important to always read and follow all label instructions.

  • Commercial Growers should consult the publication Vegetable Production Guide for Commercial Growers (ID-36) or the Southeastern US Vegetable Crop Handbook (SEVEW). Contact a county extension agent regarding specific recommendations for cucurbit downy mildew management. Growers should take steps, such as fungicide rotations or tank mixing, to reduce the risk of fungicide resistance. When selecting fungicides, be sure to note pre-harvest interval restrictions.
  • Home gardeners should consult the publication Home Vegetable Gardening (ID-128) for fungicide information. Contact a county Extension agent for additional information and recommendations regarding fungicides.   

Additional Information

  • IPM Scouting Guide for Common Problems of Cucurbit Crops in Kentucky (ID-91)
  • Cucurbit Downy Mildew Forecasting (IPM pipe website)
  • Sustainable Disease Management of Cucurbit Crops in the Home Garden (PPFS-VG-19)
  • Home Vegetable Gardening (ID-128)
  • Vegetable Production Guide for Commercial Growers (ID-36)
  • Southeastern US Vegetable Crop Handbook (SEVEW)

By: Kim Leonberger, Plant Pathology Extension Associate, and Nicole Gauthier, Extension Plant Pathology Specialist

Posted in Vegetables
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