Downy mildew on impatiens is a relatively new disease problem for greenhouse growers and now is being observed widely in Michigan landscapes. The downy mildew pathogen that infects bedding and double impatiens does not infect New Guinea impatiens, other flowering bedding plants or vegetables. Bedding impatiens are typically used in shady landscape areas and may be used in hanging pots. Double impatiens are typically used in hanging pots or other container plantings.
Infected impatiens may appear to be a bit off color with a white mildew coating the underside of the leaves. As the infection continues, the leaves may fall off of the plant, leaving only the stems behind. The white mildew coating on the undersides of the leaves is a type of spore (e.g., pathogen seed) that moves around the environment via air currents. When that spore settles out of the air and onto impatiens leaves, a new infection results.
Another type of spore that is especially long-lasting may form in the stems of infected impatiens. These long-lasting spores are not readily visible without the aid of a microscope. If the impatiens plants with this long-lasting spore are not promptly removed from the garden and disposed of, the garden soil may become contaminated with the downy mildew pathogen. Once the garden soil is contaminated with these long lasting downy mildew spores, it may become difficult to successfully grow impatiens in the same location in another year.
Professional landscapers have some fungicide choices available to them. These include Adorn (fluopicolide; apply as a spray or drench), Alude (phosphorous acid; apply as a spray or drench), Protect (mancozeb; apply as a spray) and Subdue MAXX (apply as a spray or drench). Greenhouse trials have indicated that Adorn, Alude and Subdue MAXX applications appear to be especially effective when applied as drenches prior to downy mildew infection. Once the plant is infected, there is not a fungicide that can “cure” the plant.
Infected plants should be removed entirely from the landscape bed. No visible plant debris should be left behind in the landscape. All plant debris should be placed in a trash bag, closed and disposed of in the trash.
-- by Mary Hausbeck, Michigan State University Extension, Department of Plant, Soil and Microbial Sciences. Dr. Hausbeck’s work is funded in part by MSU‘s AgBioResearch.
This article was published by MSU Extension. For more information, visit http://www.msue.msu.edu. To contact an expert in your area, visit http://expert.msue.msu.edu, or call 888-MSUE4MI (888-678-3464).