Infected impatiens

A bed of impatiens after downy mildew hit.

Millions of flowers are prime targets this season for downy mildew, a cousin of pythium that’s ravaging landscapes across the country.

Colleen Warfield, corporate plant pathologist for Ball Horticulture, is one of the leading researchers focused on downy mildew. She says the disease, while devastating in the areas it impacts, can be managed by sharp-eyed contractors.

What it is. Downy mildew is a fungus-like organism or water mold, Warfield says, related to pythium and phytopthra that attacks Impatiens walleriana and interspecific hybrids with an I. walleriana parent as well as a few types of wild impatiens.

The disease has been identified in 32 states as of press time. Most are clustered on the eastern seaboard, from Maine to Florida. In the late summer and early fall, outbreaks occurred throughout the Midwest and in Oregon and Washington.

When impatiens started dying late last year, many contractors thought it was just frost damage, or a fluke. “Everyone knew there was a problem but no one thought this was a disease,” Warfield says. “Now we’ve got it in all these states.”

What it does. Infected plants can look healthy – it takes up to 14 days for the velvety-white spores to grow big enough on the undersides of leaves to be seen with the naked eye, Warfield says. But by then, it’s too late. Once it arrives, it hits hard, turning once-lush mounds of pink and white flowers into a tangled mess of gnarled stems.

“Anyplace it’s been introduced,” Warfield says, “the picture I just sent you, that’s the end result.”

The downy mildew pathogen has been in the United States since the mid-1800s, but scientists don’t know where this latest outbreak started, or what’s causing it to spread so quickly. Warfield says fungicides with the active ingredient fluopicolide have been effective in treating the disease, and that she hasn’t seen it build a resistance. But, she cautions landscapers to always rotate their products as often as possible.

Researchers don’t know if spores can live in landscape beds that were infected. To be safe, Warfield recommends not replanting with susceptible plants and instead rotating in alternatives like New Guinea impatiens, which have shown high levels of resistance, or other appropriate varieties.

When it happens. The pathogen seems to tolerate warm days, but when evening temperatures drop into the upper 50s or lower 60s, that’s an ideal time for the disease to develop, Warfield says. Combine that with “free moisture” – a rainstorm, or a few overcast days of drizzle or just a good layer of fog – and you’ll see the disease develop. All it takes is four hours of leaf wetness for the spores to infect the plant.

The tell-tale white spores on the underside of foliage.

“You will never know when spores have landed on the plant,” Warfield says. Studies on similar spores have shown that they can travel hundreds of miles through the air. “There’s no reason to think downy mildew can’t” do the same, she says.

And two plantings on one property could be affected differently. A tight grouping of impatiens in the shade, with little air movement, could be devastated, while the same variety planted 20 yards away in the sun perform as advertised.

What now? If you have infected impatiens on their hands, remove the offending plant material as soon as possible, then bury or burn it. Don’t replant the same species, and start thinking about alternative plantings.

“Don’t leave it in the bed longer than absolutely necessary,” Warfield says.

Also, be sure to verify your plants are preventively treated at the greenhouse. Warfield stresses that fungicides applied at the greenhouse won’t give season-long control of infected plants.

What the future holds. Warfield says that, while impatiens downy mildew is bad news for many landscapers and growers, it’s not the end of the world. A similar pathogen – coleus downy mildew – hit in 2005 “and didn’t kill the industry,” she says. Careful planning and aggressive treatment can prevent widespread damage.

Researchers continue to study the disease, and its effects on the landscape. Warfield says it can be managed, but contractors have to remain vigilant; there is little margin of error.

“It is a sporadic thing, it’s really hard to predict,” Warfield says. “If they’re located in a region of the country where it’s been seen, it’s a big threat.”
 

The author is editor and associate publisher of Lawn & Landscape. Email him at cbowen@gie.net.