Downy mildew on impatiens: disease detected in Florida

Know the downy mildew disease and don’t let it devastate your greenhouse’s impatiens.

Michigan growers first learned of downy mildew on impatiens in 2004 when this disease wreaked havoc in many of their greenhouses. After a couple of years, disease scouting and fungicide use appeared to pay off and downy mildew on impatiens no longer seemed to be a major threat. Today, downy mildew of impatiens has everyone’s attention, especially with the current outbreak in both greenhouses and the landscape in Florida.

Downy mildew can survive in the landscape

A specialized type of downy mildew spore or “seed” has been found inside of diseased impatiens stems in the U.S. landscape and can contaminate the plant beds. This spore type is called an “oospore” and its job is to survive for a long period of time through unfavorable and extreme temperatures, or when an impatiens crop is not available.

Historically, impatiens plantings have been relied on to provide a vibrant splash of color in landscape beds that are shady and not always suitable for a wide range of plant material. Landscapers and homeowners alike tend to designate specific areas for their impatiens. If those beds become contaminated with downy mildew oospores, impatiens planted into those beds could become diseased and be defoliated. Homeowners won’t have access to effective fungicides to protect their impatiens beds and landscape contractors would have to apply fungicides treatments each week during wet weather. It is likely that impatiens could lose their place in our landscapes if they cannot be grown reliably.

Downy mildew moves on the wind

The symptoms of downy mildew on impatiens include a slight to pronounced yellowing of the foliage that may appear mottled or stippled (Photo 1) or plant stunting (Photo 2). A whitish fuzz or powdery-like substance develops on the underside of the leaves (Photo 3). This white material is actually a type of downy mildew spore or seed called the sporangium (Photo 4). A sporangium is designed to move via air currents, especially when the weather is overcast and humid. This type of spore cannot survive for a long period of time or withstand harsh environmental conditions. However, the ability of these sporangia to move for miles on the air currents makes downy mildew a communicable disease. This means that if one greenhouse grower has downy mildew on their impatiens, then impatiens growing in a nearby greenhouse will also likely develop this disease. The sporangia can move readily out of one greenhouse into another via the vents. Impatiens growing in the landscape are vulnerable to sporangia that originate from diseased impatiens growing in greenhouses or in the landscape.

Downy mildew on flat of impatiens
Photo 1. Flat of impatiens infected with downy mildew.
Photo credit: Mary Hausbeck, MSU

Plant stunting caused by downy mildew
Photo 2. Plant stunting caused by downy mildew infection.
Photo credit: Mary Hausbeck, MSU

Downy mildew sporangia
Photo 3. Underside of an impatiens leaf covered with downy mildew sporangia.
Photo credit: Mary Hausbeck, MSU

Downy mildew sporangia on a stalk
Photo 4. Downy mildew sporangia on a stalk growing from infected impatiens.
Photo credit: Mary Hausbeck, MSU

Downy mildew reproduces like a rabbit

This pathogen can make an astronomical number of sporangia in a brief period of time when the weather is humid or wet. In 2004, some growers commented that when they watered their infected impatiens, it seemed to “snow” in the greenhouse. That “snow” was actually the sporangia of the downy mildew pathogen being dislodged from the leaves by the water. Individual sporangia are microscopic. Huge numbers of sporangia must be clumped together in order for them to be visible with the naked eye. That means that there is no time to waste! Underestimating the potential of this pathogen to reproduce and cause an epidemic in a brief period of time is a common mistake. Waiting too long to begin a fungicide program, choosing the wrong fungicide, or letting too many days pass between fungicides applications should be avoided.

Downy mildew can lay quiet in impatiens

Keeping the plant foliage and greenhouse environment dry and applying effective fungicides on a regular basis is a must. The downy mildew pathogen can infect impatiens and not show any noticeable symptoms right away, especially if the weather is hot and dry. Healthy-appearing impatiens may or may not be as healthy as they appear. Regardless, they still need to be protected from oospores that may be in the landscape bed into which they are being planted or from sporangia that may be moving on the air currents. Therefore, growers should consider protecting their impatiens from the time that they emerge as seedlings or arrive as plugs or as cuttings.

Downy mildew can adapt to fungicides

The downy mildew on impatiens is a specialized mold that is restricted to the impatiens group of plants. 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 changing on a genetic level so that they can overcome fungicides. Therefore, a fungicide program must be well thought out and utilize multiple fungicides that have proven activity against downy mildew. Using fungicides preventively, prior to the development of downy mildew, is also helpful in delaying resistance in the downy mildew pathogen. Initiating a fungicide program in the midst of a raging downy mildew epidemic increases the risk of the downy mildew pathogen developing resistance. Alternating fungicides and tank-mixing two fungicides with different ways of attacking the downy mildew can also be an important strategy in managing the disease and helping to prevent the development of fungicide resistance.

Downy mildew fungicide recommendations

At this point in time, this pathogen can be managed with fungicides if the program is initiated before the downy mildew develops. There are some newer fungicides that can be used for downy mildew – Adorn, Stature, Segway, and Fenstop are examples. A broad-spectrum fungicide such as Protect DF can be used as a tank-mix partner with other more specific downy mildew fungicides that have some systemic activity. A Subdue drench, which is often used for Pythium, can also provide downy mildew protection. Heritage and Insignia sprays can protect against downy mildew and Alternaria leaf blight. Phosphorus acid-based products may also prove to be helpful. In general, adjuvants are not needed and in our experience with other downy mildew pathogens, do not enhance downy mildew control. However, adjuvants may help reduce the appearance of fungicide residue.

Related MSU Extension article: Alternative choices if downy mildew infested your impatiens last year

Mary Hausbeck’s research at Michigan State University on ornamental downy mildew is funded in part by the Floriculture Nursery and 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.

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