Ask the plant pathologist about impatiens downy mildew: Part III – Landscaping

An interview with MSU’s floriculture pathologist about potential landscaping issues due to impatiens downy mildew.

Editor's note: On Jan. 21, 2013, Part II of this series has been removed from the MSU Extension website.

This is the last article in a series about impatiens downy mildew for the commercial grower. In the first article we asked Michigan State University Extension’s ornamental plant pathology expert, Mary Hausbeck, some questions about the biology of this disease. In this  final article, we ask Dr. Hausbeck questions about how impatiens will perform in the landscape and what homeowner or landscaper options for impatiens downy mildew may be available.

Question #1: What can homeowners do to protect themselves from impatiens downy mildew?

Answer: Using preventive fungicides in the greenhouse insures that a healthy plant is provided to the homeowner. Fungicide sprays can be continued in the landscape by professional lawn care companies as they have access to commercial fungicides. In contrast, homeowners do not have access to the fungicides that are most effective against downy mildew.

Question #2: Are the overwintering dormant spores (oospores) of impatiens downy mildew able to survive in the Michigan landscape?

Answer: There is some evidence from the northeastern United States that these oospores will indeed survive the winter. We don’t know the length of time that these spores will survive, or if we will have 100 percent survival of all the spores over a winter or period of winters. But, you only need a few spores to survive to have a problem.

Question #3: Is it possible to chemically treat the soil of a landscape bed that previously had diseased impatiens prior to planting healthy impatiens back in that same bed this coming spring?

Answer: No chemical product currently on the market will kill dormant spores in the soil. What about fumigation or sterilization? Fumigants are no longer widely available and must be applied by a certified applicator. Since impatiens are planted in shady areas, fumigants could damage the root systems of the woody trees and shrubs near the impatiens bed.

Question #4: I visited a large outdoor planting of impatiens in the landscape this past summer. Most of the impatiens were showing symptoms of impatiens downy mildew, but there were some that were completely unaffected. Is this evidence that there are some genetic individuals that may be resistant to impatiens downy mildew?

Answer: No. As in any disease, there are always some individuals that seem to escape the disease. Think of the latest meningitis scare from steroid injections. Not everyone who received the tainted steroid injections came down with meningitis. We call these escapes. The same is true with impatiens downy mildew. There are always going to be some individual plants that seem to escape the disease pressure when it is all around them. But, trials in the United States and trials overseas have shown time and time again that there is no cultivar of impatiens that is less susceptible to impatiens downy mildew.

Question #5: A landscape friend of mine insists that copper can protect impatiens from impatiens downy mildew because his plantings of impatiens that were near a copper fountain didn’t get impatiens downy mildew this year while all his other nearby impatiens plants did. Is that true?

Answer: No. Copper does not adequately protect impatiens from impatiens downy mildew.

Question #6: What if a homeowner grows impatiens in containers and had the disease last year. Can they dump the soil from those containers and start with fresh soil and plant again next year?

Answer: It’s a great start, but it depends on what is near those hanging baskets this coming season and the environment. Impatiens downy mildew sporangia can spread via air currents. While it is possible that the homeowner may be able to get through most of the season without impatiens downy mildew, growing healthy impatiens in clean soil is not a guarantee that the impatiens will stay healthy for the entire summer. The weather conditions and the health of nearby impatiens in the landscape are the key factors influencing whether container-grown impatiens can remain beautiful and free of impatiens downy mildew.

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

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