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by Elizabeth H. Roche, Nancy J. Taylor & Francesca Peduto Hand
Downy Mildews are some of the most destructive diseases of ornamental crops. They attack a wide variety of bedding ornamentals, including impatiens, pansy, snapdragon, salvia, as well as cut flowers, including scabiosa and stock. Downy mildew symptoms can greatly vary depending on the host where they develop as well as the stage of the disease (early vs. late infections), and may include:
- Small chlorotic spots on upper side of leaves (early infections – subtle, may pass unobserved)
- More extensive bronze to brown lesions on upper side of leaves (often angular-shaped)
- Leaf curl
- Flower drop
- Leaf drop
- Stem collapse
- White to gray, fuzzy growth on underside of leaves
For assistance in identification, contact the C. Wayne Ellett Plant and Pest Diagnostic Clinic.
Downy mildews are very difficult to control once established so every effort should be made to prevent the disease and its introduction into the greenhouse. Always purchase disease free plants from reputable sources. Inspect plants regularly for signs and symptoms of disease and remove suspected plants immediately to prevent pathogen spread. Infected plants, plant debris, and soil should be removed from the greenhouse by placing into closed containers, then buried or burned.
Keep relative humidity below 85% by venting and heating in order to stop germination of spores and decrease sporulation rate on infected plants. Reduce leaf wetness with proper plant spacing that ensures air circulation between and within plants. Because sporangia are disseminated via water splash, it is important not to use overhead irrigation.
Several fungicides are available to control the disease and should be applied preventively for maximum efficacy. Check with your State specialist or County Extension educator for the most updated information on products efficacy. Keep in mind that if sporulation is observed on the leaf underside, control will be difficult to impossible. Many fungicides that provide excellent control when applied preventively will not work curatively.