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Caption: Downy mildew on impatiens
Photo by: D. Ferrin, LSU Ag Center, bugwood.org
  
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Impatiens : Downy mildew
(revision date: 3/10/2017)


Biology
Downy mildew of impatiens is a disease affecting the standard garden impatiens, Impatiens walleriana, and its cultivars and hybrids. The disease is caused by a fungus-like organism which survives in plant debris and infested soils. The disease develops and spreads rapidly when conditions are favorable (wet foliage, cool temperatures, and humid air). Downy mildew symptoms may initially appear as mottling, stippling or yellowing of infected leaves, usually beginning on the youngest leaves and shoot tips. The margins of affected leaves may curl downward. Seedlings are especially susceptible to damage, and plants infected early in development may be stunted. Symptoms of downy mildew may be mistaken for other problems such as nutrient deficiency or root rot. During cool, humid conditions, a downy or moldy-looking white growth may develop on the underside of leaves. Infected leaves and flowers drop from the plants, often leaving only a bare stem with a few leaves or flowers remaining at the tip. As the disease progresses, plants will become completely defoliated, collapse, and die. The disease is spread easily from infected plants by air or water movement. While all varieties and hybrids of I. walleriana are susceptible, New Guinea impatiens (I. hawkeri) is highly tolerant of the disease.
Management Options

Non-Chemical Management
  • Do not replant garden impatiens in areas where the disease was present in previous years. Instead, choose resistant or tolerant plant species such as begonia, coleus, or New Guinea impatiens. Wait 2-4 years before replanting a site with susceptible plants.
  • Inspect bedding plants for symptoms to avoid introducing disease. Isolate newly purchased plants and monitor for disease symptoms for several days before planting in the landscape.
  • Space plantings to increase airflow and reduce leaf wetness and humidity. Avoid overhead watering whenever possible or limit to early morning hours so leaves have time to dry during the day.
  • If downy mildew is suspected, immediately bag and remove diseased plants (including roots) and fallen leaves and flowers. At the end of the season, remove as much plant debris as possible from the beds. Do not compost diseased plant material.
Select non-chemical management options as your first choice!

Chemical Management


Chemical management of impatiens downy mildew is not recommended for home landscapes. Manage disease with cultural techniques, good sanitation, and by planting disease-tolerant or resistant species in place of garden impatiens where this disease has been a problem in the past. If chemical control is necessary, applications must be made BEFORE infection occurs and repeated regularly throughout the growing season to maintain protection. Rotate between different active ingredients to prevent fungicide resistance.

Listed below are examples of pesticides that are legal in Washington. Always read and follow all label directions.
  • Agri-Fos Systemic Fungicide
    Active ingredient: mono- and di-potassium salts of phosphorous acid  |  EPA reg no: 71962-1-54705
  • Bonide Fruit Tree & Plant Guard Conc
    Active ingredient: boscalid, lambda-cyhalothrin, pyraclostrobin  |  EPA reg no: 4-488
  • Bonide Fruit Tree & Plant Guard R-T-Spray
    Active ingredient: boscalid, lambda-cyhalothrin, pyraclostrobin  |  EPA reg no: 4-488
  • This list may not include all products registered for this use.
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Caption: Downy mildew on impatiens
Photo by: D. Ferrin, LSU Ag Center, bugwood.org
Caption: Leaf drop on impatiens caused by downy mildew
Photo by: M.A. Hansen, Virgina Tech, bugwood.org