Cucurbit Powdery Mildew

Powdery mildew is a common disease of all cucurbits, such as cucumbers, squashes, pumpkins, and watermelons. This disease can occur in greenhouses, high tunnels, commercial fields, and home gardens. Cultural management practices can reduce losses from this disease.  

Cucurbit Powdery Mildew Facts

  • Leaf symptoms first appear as spots with a white, powdery appearance on upper or lower leaf surfaces (Figure 1). Over time, spots expand to cover large portions or entire leaf surfaces. Severely infected leaves may become yellow or necrotic. As the disease progresses, affected leaves may die, and defoliation can occur. Note that some cucurbit cultivars may have an appearance that can be easily confused with powdery mildew.
  • Stems and leaf petioles can also be infected. Specifically, the stem where pumpkins attach to plants (“handles”) can become infected, resulting in reduced marketability as carving pumpkins.
  • Cucurbit powdery mildew does not infect fruit. However, fruit quality and yield may be impacted as a result of insufficient leaf material.
  • Disease is favored by high humidity, but does not require high levels of moisture for infection.
  • The powdery mildew pathogen infects during the reproductive stage of plant growth. Thus, flowering plants are at the greatest risk of disease.
  • Caused by the fungus Podosphaera xanthii.
  • The pathogen survives winter on infected plant debris.
Figure 1: Cucurbit powdery mildew symptoms begin as white, powdery spots on upper or lower leaf surfaces. (Photo: Kenny Seebold, University of Kentucky)

Management Options

Cultural disease management

  • Utilize powdery mildew resistant cultivars.
  • Plant in sunny areas with good airflow.
  • Use recommended plant spacing to facilitate air movement and leaf drying.
  • Apply a layer of mulch to reduce direct contact with soil surface.
  • Remove weeds that may serve as alternate hosts.
  • Remove and destroy infected plants.
  • Clean-up plant debris at the end of the growing season.
  • Rotate crops.

Chemical disease management

Application of fungicides before disease arrives (preventative fungicide) provides the greatest level of disease protection. 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

  • Foliar Diseases of Cucurbits (PPFS-VG-10)
  • IPM Scouting Guide for Common Problems of Cucurbit Crops in Kentucky (ID-91)
  • 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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