Close inspection of Impatiens walleriana plants and immediate action this season may help slow the spread of impatiens downy mildew in North America, according to Ball Horticultural Co. Landscape contractors and homeowners are encouraged to take the following immediate steps for continued success in growing Impatiens walleriana in landscape beds:
• Inspect impatiens for evidence of downy mildew (see Symptoms below).
• Note site and location of infected beds.
• Promptly remove entire plants and leaf debris completely from beds and containers.
• Dispose of plant material offsite rather than incorporating it back into the bed. Composting is not recommended.
• If an infection was noted this year, replanting a bed with Impatiens walleriana next season is not recommended; use alternative plants for these beds.
“The occurrence of impatiens downy mildew in North American landscapes this season has prompted Ball Horticultural Co. to issue an alert to the landscape trade in an effort to slow the spread of impatiens downy mildew,” said Colleen Warfield, plant pathologist for Ball Horticultural Co. “Scouting impatiens beds for this disease, and promptly removing infected plants and leaf debris, can help reduce overwintering spores in the soil that may initiate new infections next season.”
The downy mildew that is infecting impatiens is specific to Impatiens walleriana (garden impatiens) and will not infect other plants. All seed and vegetative varieties of Impatiens walleriana are susceptible to downy mildew. However, New Guinea impatiens are highly tolerant of this disease.
Impatiens walleriana can become infected by short-lived aerial spores that are easily dispersed by wind currents and splashing water, or by oospores that may survive the winter in the soil. Cool temperatures, especially at night, are ideal for rapid disease development. Moist air, rainy weather or irrigation practices that extend the amount of time moisture remains on the leaves also encourage the development and expression of downy mildew.
Downy mildew symptoms on Impatiens walleriana typically start with a few leaves that appear slightly chlorotic or stippled, and become completely yellow over time. Some varieties will have subtle gray markings on the upper leaf surface. A white, downy-like growth may be present on the underside of primarily yellow leaves, but can also be found on the underside of green leaves. As the disease progresses, premature leaf drop results in bare, leafless stems. Eventually these stems can become soft and the plant collapses, similar to frost damage.