More information on downy mildew on impatiens

Overcast and humid weather favors impatiens downy mildew. Growers should apply fungicides preventively.

Downy mildew has been confirmed in Michigan greenhouses on bedding and double impatiens. Disease has been severe with a white mildew obvious on the undersides of the leaves. Some of the plants had yellowish leaves with white mildew on the underside, but the mildew could also be seen on the undersides of green leaves. Downy mildew can seem to appear overnight. This is because the early stages of the disease are microscopic, allowing the pathogen to advance unnoticed until it explodes with a white mildew on the leaf underside. The white mold that forms on the undersides of the impatiens leaves is the result of the production of a type of spores called sporangia.

Downy mildew symptoms
Symptoms of downy mildew infection include plant stunting.

Downy mildew on leaf
The upper side of an infected leaf may appear normal and green.

Downy mildew on leaf
But when you look underneath, it is covered with sporangia.

Sporangia can be readily dislodged from infected plants via air currents, overhead watering, or moving infected plants. Growers have noted that when infected impatiens are watered, a white cloud of sporangia can be seen. Once the sporangia have been released into the atmosphere, they may be transported for either short or long distances (i.e., within the same greenhouse or greenhouses a few miles away). If the sporangia settle out of the air onto an impatiens plant, these seed-like sporangia will germinate under wet and humid conditions.

Downy mildew sporangia
Sporangia grow on stalks that emerge from the surface
of the infected tissue.

Within several hours of the sporangia landing on an impatiens leaf, the downy mildew pathogen will have successfully invaded that plant. Once the pathogen has invaded the leaf, it will begin to ramify through the tissue with its thread-like growth, or it may lay quiet within the plant’s tissue for a few days or longer. These “quiet” infections are invisible to growers. There are no tests available to determine whether healthy-appearing plants have these “quiet” downy mildew infections. When there is an environmental trigger, such as consecutive days of damp, overcast weather, this “quiet” infection becomes activated. Within a day or so of this infection becoming active, the grower sees the white mildew that has seemingly developed “overnight.” At this point, the plant cannot be salvaged and must be removed from the greenhouse immediately via plastic bags and destroyed.

Not all sporangia that are released into the atmosphere will find an impatiens plant to land onto and these sporangia cannot survive indefinitely. Sporangia are a type of spore that cannot survive readily on greenhouse benches or floors. Perhaps sporangia will remain viable for a day or so on greenhouse surfaces or in the air during transport when the weather is overcast and humid, but the sporangia will not remain viable for more than a few hours when the conditions are bright and dry. Growers need not be terribly concerned about the downy mildew surviving long term in their greenhouses if all infected plants are removed. The oospore is a type of spore that was found in landscape beds last fall in some regions of the United States and that spore type can survive in landscape beds for potentially a long period of time. Some of the plants that I’ve seen in the greenhouse this spring appeared to be newly infected and did not have downy mildew disease advanced to the point of oospores developing. Other situations where the downy mildew is more advanced may offer a different scenario, but if the diseased plant along with soil and pot are all removed from the greenhouse and destroyed, the potential for any carryover should be minimized. There should never be volunteer impatiens growing under the plant benches. Do not leave any “leftover” impatiens in the greenhouse once the greenhouse sales season is finished.

Since sporangia can move readily on air currents and infect nearby impatiens, if one greenhouse has infected impatiens, it is likely that the downy mildew sporangia have contaminated all other impatiens located on the premises. If downy mildew is identified within a flat of impatiens, all plants must be disposed of immediately. In some instances, hundreds of impatiens flats have needed to be disposed of because they were in the same greenhouse as the heavily diseased impatiens. Diseased impatiens cannot be cured with any fungicide program. Rather, the plants should be immediately placed in plastic bags and removed from the growing premises. If the plants are not placed in plastic bags but put into a dumpster, the dumpster must have a lid that is kept closed and it should be taken offsite immediately once the plants are removed from the greenhouse. Geraniums, petunias and other floriculture plants that are growing in the same greenhouse as diseased impatiens are not likely to serve as a carrier for the downy mildew. To ease any anxiety regarding other floriculture plants potentially carrying hitchhiking downy mildew sporangia around, these plants could be sprayed with one of the recommended downy mildew fungicides (see label for a specific instructions and list of crops).

Fungicides must be applied preventively to all impatiens bedding plants and double impatiens. All hanging baskets and impatiens pouches must be included. Only selected fungicides proven to be effective against downy mildews must be used exclusively and frequently. Fungicides that make claims to control downy mildew, but are unproven, should not be used. Some fungicides can be applied as a drench (Subdue and Adorn are examples) whereas others should be applied as a spray to glisten. Good spray coverage of the plants is needed, but specifically targeting the undersides of the leaves is not necessary as the infection is likely occurring via the top surface of the leaves. Many of the fungicides are locally systemic and the active ingredient will be moved from the topside of the leaf to the bottom surface of the leaf.

For details on impatiens downy mildew spray programs, read a previously published MSU Extension News article, Impatiens downy mildew prevention and management.

More resources on impatiens downy mildew can be found at the American Floral Endowment website. American Floral Endowment

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

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