How to manage impatiens downy mildew in the landscape

Diseased impatiens may have been sold at retail.

Downy mildew has been reported on bedding impatiens at retail. Although this report has not been confirmed, home gardeners and the landscape industry should be watchful for this potentially devastating problem.

Look for these symptoms

Tell-tale symptoms include a white coating of spores covering the undersides of leaves (Photos 1-3). This can go unnoticed unless the leaves are turned over and inspected. Infected leaves may appear yellowish, small or cupped down (Photo 4). Advanced infections result in defoliation of the plant leaving bare, leafless stems.

diseased leaf
Photo 1. When you turn a diseased leaf over, you can see the
fuzzy, white spores of the downy mildew pathogen.
Photo credit: Mary Hausbeck, MSU

Magnified leaf
Photo 2. This is what the underside of a diseases leaf looks
like under magnification. Not the large clusters of downy
mildew spores. Photo credit: Mary Hausbeck, MSU

spores
Photo 3. Higher magnification shows the oval spores
produced on stalks that extend out from the surface of the
underside of the leaf. Photo credit: Mary Hausbeck, MSU

diseased leaf and healthy leaf
Photo 4. Note the smaller, yellowed, diseased-impatiens
leaf on the left, compared to the healthy leaf on the right.
Photo credit: Mary Hausbeck, MSU

Know which plants are at risk

Impatiens downy mildew infects standard bedding impatiens and double impatiens that are often used in hanging baskets. Impatiens downy mildew is different from cucurbit downy mildew. Impatiens downy mildew does not infect cucumbers or squash (or any vegetable) or any flowers besides standard bedding and double impatiens and balsam. New Guinea impatiens are resistant to downy mildew.

Select impatiens carefully

Inspect plants for disease symptoms (Photo 5) prior to purchase and before planting. Specifically, look at the undersides of the leaves carefully. Little, reddish spots are natural to the plant, but a coating of white indicates downy mildew disease. Moist, wet conditions and cool night temperatures favor the development of this disease. Impatiens downy mildew can be spread to new locations via shipment of infected plants. It also has a type of spore that can move long distances via wind, and can also be splashed by water to nearby plants. Healthy plants can become infected after planting into the landscape from airborne spores.

plant stunting
Photo 5. Plant stunting and absence of flower buds (see plant
on the right) is another symptom of impatiens downy mildew.
Photo credit: Mary Hausbeck, MSU

Michigan greenhouse growers have been proactive

Based on knowledge of impatiens and other types of downy mildew, MSU Extension provided recommendations and educational programs for greenhouse growers and Master Gardeners. Commercial growers have used newly-developed fungicides to protect impatiens from downy mildew, but these fungicides do not protect plants once they go out into the landscape.

Consider using other shade-loving plants

New Guinea impatiens, begonias and coleus thrive in shady areas and will not succumb to impatiens downy mildew. Specific information on some colorful choices for shady landscape beds can be found by reading Alternative choices if downy mildew infested your impatiens last year.

Dispose of diseased impatiens immediately

Impatiens downy mildew has a survival spore that may allow it to overwinter in the soils of Michigan landscape beds. Diseased impatiens should be removed from the landscape immediately, placed in plastic bags that are then closed and placed in the trash. It is important to gather all plant debris from the landscape beds. Diseased impatiens should not be composted. After removing infected impatiens from the landscape, do not replant impatiens.

Use MSU Diagnostic Services

For assistance in identifying impatiens downy mildew, go to the MSU Diagnostic Services website. This site contains a submittal form that can be downloaded as well as instructions on how to send in samples. Questions and digital images may be submitted to .(JavaScript must be enabled to view this email address).

For more information on impatiens downy mildew, see these articles

Mary Hausbeck’s research at Michigan State University on ornamental downy mildew is funded in part by the Floriculture and Nursery Research Initiative of the Agricultural Research Service under Agreement #58-1907-0-096 and by the American Floral Endowment.

Floriculture & Nursery Research Initiative  American Floral Endowment

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