Impatiens downy mildew: Outbreaks reported in Michigan and nearby states

Greenhouses in four Michigan counties report problems with impatiens downy mildew; growers in three nearby states also experience outbreaks.

Downy mildew on impatiens is a relatively new disease problem for greenhouse growers in the United States. The downy mildew pathogen is Plasmopara obducens and infects bedding impatiens, double impatiens and balsam; New Guinea impatiens, other flowering bedding plants and vegetables are not susceptible. Bedding impatiens are a favorite plant for use in shady areas where they are planted in mass for bright color in the landscape. Double impatiens are used in hanging pots, planters and decorative pouches.

Earlier this spring, growers in three nearby states reported outbreaks of downy mildew on impatiens in their greenhouses. In the last two weeks, downy mildew on impatiens has been confirmed in Michigan greenhouses spanning four counties. In each case, the disease was widespread within the greenhouse and the losses were nearly 100 percent. The diseased plants were disposed of immediately and were not purchased by consumers. The cold and rainy spring has slowed the movement of plant material from garden centers to homeowner’s landscapes.

It is imperative that only healthy impatiens be introduced to consumers’ landscapes. A type of downy mildew spore that is especially long-lasting may form in the stems and leaves of infected impatiens and is called an oospore. These long-lasting oospores are not readily visible without the aid of a microscope and were found in Michigan samples of infected impatiens gathered from the landscape in 2012 (Photo 2). If the impatiens plants with this long-lasting oospore are not promptly removed from the greenhouse or 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 oospores, it may become difficult to successfully grow impatiens in the same location in another year.

Although the 2012 summer weather was hot and dry in many parts of Michigan, some of the more northern areas of the state received adequate rainfall along with cooler temperatures. It was in these more northern areas of Michigan that impatiens downy mildew appeared to be more widespread in the landscape. However, by late summer, impatiens downy mildew was a common problem throughout many areas of the state.

Diseased impatiens may appear to be a bit off-color with a white mildew coating the underside of the leaves (Photo 1). However, green leaves may also have pathogen sporulation evident on the undersides of the leaves. As the infection continues, the leaves turn yellow and may fall off the plant, leaving only the stems behind (Photo 2). The white mildew coating on the undersides of the leafs (Photo 1, middle) is a type of spore known as a sporangium (Photo 1, bottom) that moves around the environment via air currents. When sporangia settle out of the air onto impatiens leafs, a new infection results if there is a period of leaf wetness.

Diseased and healthy leaf Underside of leaf Oval spores
Photo 1. (Left) Diseased impatiens leaf on the left, healthy leaf on the right. (Middle) Underside of an impatiens leaf covered with downy mildew sporangia. (Right) Oval spores produced on stalks that extend from the underside of the leaf. Photo credits: Mary Hausbeck, MSU

Downy mildew infected impatiens Landscape planting
Photo 2. (Left) Downy mildew-infected impatiens showing leaf abscission. (Right) Landscape planting of impatiens following an epidemic of downy mildew. Photo credits: Mary Hausbeck, MSU

Once the plant is infected, there is not a fungicide that can “cure” the plant. Many other crops also have their own specialized downy mildew and we’ve learned that as a group, the downy mildews are quite adept at overcoming fungicides. Therefore, a fungicide program must utilize multiple products that have proven activity and offer differing modes of action against this pathogen. According to Michigan State University Extension, using fungicides preventively, prior to the infection and development of downy mildew, is also important in delaying resistance in the downy mildew pathogen. Initiating a fungicide program in the midst of a raging downy mildew epidemic is not recommended as fungicides cannot “cure” the disease. Alternating fungicides and tank-mixing two fungicides that offer different means of halting the downy mildew can also be important strategies in managing the disease and helping to prevent the development of fungicide resistance in the pathogen.   

Greenhouse growers and professional landscapers have downy mildew fungicides available to them. It has been our experience through fungicide trials that the most effective fungicides must be used in an intensive application program to protect impatiens from this pathogen when the environmental conditions are favorable for disease development. Many of the recommended downy mildew fungicides are newer products and growers may not be readily familiar with them. Adorn (fluopicolide, Photo 3, top right is a photo of an impatiens leaf), Segway (cyazofamid), Micora (mandipropamid), Orvego (dimethomorph + ametoctradin), and FenStop (fenamidone) are examples of some of the newer fungicides that are labeled for downy mildew control.

Untreated leaf Infected leaf
Infected leaf Infected leaf

Photo 3. Greenhouse trial results. (Top left) Untreated control. (Top right) Adorn SC 2 fl oz drench. (Bottom left) Subdue MAXX EC 1 fl oz drench. (Bottom right) Heritage WG 4 oz + Capsil 4 fl oz spray. Photo credits: Mary Hausbeck, MSU

Michigan State University research conducted to date indicates that some fungicides are better when applied as a drench compared to application as a foliar spray. (NOTE: Many fungicides may not be labeled for use as a drench for downy mildew control. Please read the label carefully and contact the registrant with questions). A broad-spectrum fungicide such as Protect DF or Dithane (mancozeb) can be used alone or as a tank-mix partner with other more specific downy mildew fungicides that have some systemic activity. A Subdue MAXX (mefenoxam, Photo 3, bottom left) drench provides downy mildew protection and is often used for control of Pythium. Heritage (azoxystrobin, Photo 3, bottom right) and Insignia (pyraclostrobin) sprays can be helpful against both downy mildew and Alternaria leaf blight. Phosphorus acid-based products may also prove to be helpful when applied as a drench.

In general, adjuvants do not enhance downy mildew control, but may help reduce the appearance of fungicide residue and allow the leaf to dry faster.

LogoMary Hausbeck’s research at Michigan State University on ornamentals is funded in part by Project GREEEN, the Western Michigan Greenhouse Association, the Metro Detroit Flower Growers Association, and the Floriculture and Nursery Research Initiative of the Agricultural Research Service under Agreement #58-1907-0-096.