WSU Extension


Downy mildew 

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Caption: Downy mildew on snapdragon
Photo by: R.S. Byther
Snapdragon : Downy mildew
(revision date: 3/22/2021)

Affected plants show stunting and pale yellowish green foliage. Leaves curl in and droop. Leaf undersides may develop a white to purplish fungal growth. The growing point usually dies, and the plant may break off at the base. Downy mildew is caused by a fungus that is favored by cool, wet conditions and/or high humidity. Spores are produced in abundance on infected tissue. Seed transmission may occur, but remains as speculation only.
Management Options

Non-Chemical Management
  • Reduce greenhouse humidity by combination of heating, venting, and circulating air.
  • Plant in areas with adequate sunlight and air circulation.
  • Space plants to foster air flow.
  • Remove and destroy infected plants and plant debris.
  • Do not overhead irrigate.
Select non-chemical management options as your first choice!

Listed below are examples of pesticides that are legal in Washington. Always read and follow all label directions.
  • Bonide Fruit Tree & Plant Guard R-T-Spray
    Active ingredient: boscalid, lambda-cyhalothrin, pyraclostrobin  |  EPA reg no: 4-488
  • Organocide Plant Doctor
    Active ingredient: mono- and di-potassium salts of phosphorous acid  |  EPA reg no: 71962-1-70179
  • This list may not include all products registered for this use.

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Caption: Downy mildew on snapdragon
Photo by: R.S. Byther