Downy mildew causing problems on impatiens in the landscape

Clean downy mildew up now so it won’t stay this winter.

Downy mildew on impatiens is causing problems in some U.S. landscapes this fall. Downy mildew is a pathogen of both vegetative and seed-produced Impatiens walleriana (the garden impatiens) throughout the world and has been reported in the United States, Canada, Asia, Europe and India. In the United States, downy mildew on Impatiens walleriana was first reported in 1942. Since that time, downy mildew on Impatiens walleriana has occurred sporadically throughout the United States

Recently, the disease has been confirmed in coastal southern California, northeastern Illinois, northern Indiana, Cape Cod in Massachusetts, Long Island and upstate New York. Michigan gardeners are encouraged to check their yards for downy mildew symptoms on their impatiens. Suspect plants should be sent to MSU Diagnostic Services for confirmation of downy mildew.

The downy mildew that infects Impatiens walleriana is caused by a fungal-like microscopic organism. The downy mildew on snapdragons cannot blight roses, nor can the downy mildew on impatiens spread to any plant besidesImpatiens walleriana.

Downy mildew checklist

  • Yellowish or pale-green foliage
  • Downward curling of the leaves
  • Distortion of the leaves
  • White to light-gray fuzz on the undersides of the leaves
  • Emerging leaves that are small or discolored (yellow or pale green)
  • Flower buds may fail to form
  • Plants may be stunted

The most obvious sign of downy mildew is the white, grayish fuzz that develops on the underside of the leaves. This can go unnoticed because homeowners are not accustomed to turning over the leaves to look for plant problems. Perhaps other than an especially severe case of Botrytis, no other disease causes such obvious gray fuzz especially on the underside of the leaf. This disease should not be confused with powdery mildew, which affects some floriculture crops and has white, fuzzy growth.

Diseased plants should not be placed in a cull or compost pile. Healthy-appearing plants that are adjacent to the diseased plants should also be discarded.

Mary Hausbeck’s research at Michigan State University on ornamental downy mildew is funded in part by the Floriculture Nursery and Research Initiative of the Agricultural Research Service under Agreement #59-1907-5-553 and by the American Floral Endowment.

Agricultural Research Service and American Floral Endowment 

For more information, visit Hausbeck’s webpage on ornamental research.

Downy mildew on impatien
Downy mildew can cause stunting of impatiens.

Infected impatiens leaf
The underside of an infected impatiens leaf can have white-light gray fuzz.

Dr. Hausbeck’s work is funded in part by MSU’s AgBioResearch.

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