Impatiens Downy Mildew: 2013 And Beyond

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Nora Catlin, a floriculture specialist with Cornell Cooperative Extension of Suffolk County, is trialing shade alternatives to impatiens, including begonia, torenia, coleus and New Guinea impatiens, at the Long Island Horticultural Research & Extension Center of Cornell University. Margery Daughtrey is trialing coleus cultivars for their relative resistance to the coleus downy mildew. Coleus will not drop leaves when infected with their own (entirely different) downy mildew.

Between warnings from garden writers and the national media and extensive signage at garden centers and retail outlets, the public was well aware of a problem with Impatiens walleriana this year.

“Some people reported they couldn’t sell the Impatiens walleriana they grew because the press had been so alarming to gardeners that they weren’t purchasing them,” says Cornell’s Margery Daughtrey.

Growers were also aware of possible problems with impatiens downy mildew and made adjustments to their production. From what Daughtrey observed in the market, the amount growers cut back on producing Impatiens walleriana varied quite a bit in 2013. Some box stores asked growers to produce them, and that was probably the main channel for their distribution this year, she says. Other growers cut back by 40 to 100 percent.

“If they did not cut back extensively, some of them were left with plants that just didn’t sell,” she says. “Really no one told me they wished they’d grown more of them.”

The Landscape Of Confirmed Cases

Compared to last year when impatiens downy mildew was confirmed in 33 states, landscape reports for 2013 are still relatively few, Ball Horticultural’s Colleen Warfield says. Early season landscape reports of this disease came from Florida, Texas, Louisiana, California and Hawaii.

Impatiens downy mildew appeared in southern Florida in October 2012, though it usually doesn’t show up on more established crops like curcurbits until around Thanksgiving, and never went away. University of Florida’s Aaron Palmateer grew impatiens in a research shade house continually during this period and consistently saw downy mildew, even on young plants.

“This was when temperatures were in the 80s and low 90s, and it was not cool during the nights,” he says. “Typically, downy mildews like cooler temperatures. This one is surprising me, and that’s not good.”

Growers in South Florida substantially decreased the amounts of impatiens they produced this year.

“We don’t have any reason to think it will ever go away [in southern regions], because impatiens live all the time there,” Daughtrey says. “There’s continuation of the disease on the host all through the season. That’s an area where you have to raise your eyebrows at the future success of impatiens.”

Kentucky had an early report of impatiens downy mildew in May. It was seen in Arkansas in June. By late June, there were isolated cases in New York and Delaware, possibly from overwintering spores. In late July, impatiens downy mildew had been confirmed in landscapes in North Carolina, Tennessee, Missouri, Indiana, Ohio, Oregon, Washington and Vancouver, B.C. with a suspected case in Georgia and isolated reports in Colorado.

“Something to consider is that a landscape find does not necessarily mean the plant got infected in the landscape,” Warfield says. “Plants could have been asymptomatic (or, hopefully not, symptomatic) when planted, and did not develop symptoms until after being planted out. That’s important because the pathogen may not survive and spread in some areas under normal conditions.”

Many areas that were affected last year may not have been this year because of the weather. If it was not wet enough in the spring, the disease may not have established itself in a way that was noticeable enough for people to report it, Daughtrey adds.

“One thing is for certain, if growers are producing bedding impatiens, regardless of the region where they are growing, they need to be managing this disease preventively using a combination of cultural and chemical methods,” Warfield says. “Depending on when the disease develops in a given region, this season will influence whether or not consumers will buy impatiens next year. We still have a couple months to go in the current season before we’ll know the answer to that.”

At press time, Daughtrey and Warfield both agreed that the remaining fall season could still result in more reported cases of impatiens downy mildew.

Alternatives To Impatiens Walleriana

When looking for something that grows in the shade, Warfield and Daughtrey say that most growers and gardeners reached for New Guinea impatiens, begonias, torenia, coleus and caladiums as replacements.

Syngenta’s Brian Corr reports an increased interest in the breeder’s Florific New Guinea impatiens series from seed, newly introduced this year. “We’ve also seen strong demand across the rest of our shade-loving assortment, especially for Bada Bing, Bada Boom, Eureka and Volumia begonias,” he says.

Corr also adds that breeding companies are working on new genetic alternatives to impatiens for use in the shade. Palmateer is working with a breeder to develop varieties with resistance. Further research this fall will trial more products that are cost-effective and environmentally sound for homeowners and landscapers.

Interestingly, downy mildew was also spotted in Delaware and New York on a different species of impatiens — I. balsamina.

“What’s interesting is that it made it through the winter and showed up on those plants in the spring,” Daughtrey says. A reseeding annual, Impatiens balsamina isn’t killed by downy mildew, but leaves become spotted. Downy mildew, however, can move from I. balsamina to I. walleriana.

What Can You Do?

In the landscape, Palmateer says some phosphorous and phosphoric acid-containing products that are labeled for use as foliar fertilizers have now shown to be highly effective against impatiens downy mildew and may even have eradicative properties. A prime example is Actaphos, which is labeled for use in Florida as a 0-28-25 fertilizer. These products are cheaper than some fungicides and landscapers who are using them for fertility are seeing the added benefit of impatiens downy mildew control, although they aren’t labeled for use in all states.

“It’s not labeled as a fungicide, so I don’t dare recommend it as a fungicide, but my research has shown that it’s highly effective at controlling downy mildew on impatiens,” Palmateer says.

Palmateer says that while I. walleriana has no natural defenses to downy mildew, with these types of treatments, he is more optimistic about the future of impatiens in South Florida.

And although there are now steps landscapers can take to protect landscapes, and greenhouses have good chemistries for control, Palmateer still recommends growers in Southern Florida not rely completely on impatiens, but to also offer some alternatives.

“In Florida, my recommendation would be to have Impatiens walleriana, but I wouldn’t put all my eggs in one basket,” he says. “If [growers] deal with landscape professionals that are using some of the new information that’s coming out of the University of Florida, I think they can do it successfully.”

Corr agrees that there is a future for Impatiens walleriana in the American landscape and that what’s happened in Europe may be an indicator.

“Impatiens sales will likely be down, but it will not be the end of impatiens as we know them,” Corr says. “In the future, impatiens will likely reach some percentage of their current volume and maintain a steady level at that point.”

When will that future arrive? Daughtrey says that while many growers may be thinking about returning to heavier production of Impatiens walleriana in 2014, a wait-and-see mentality may still be appropriate for some time.

“If impatiens downy mildew was active in their area once before, the possibility is still there for it to be active in their area again,” Daughtrey says. “I think caution is a good message right now.”

If you do choose to grow impatiens in 2014, Daughtrey recommends a preventative program, including a Subdue/Adorn combination drench at the end of the production cycle, which will give protection in the landscape for about a month.

“As long as growers follow recommendations for growing a healthy crop, there is no reason that impatiens cannot remain an important part of the bedding mix,” Corr says. “The treatment recommendations from Syngenta include products such as Micora, Heritage and Subdue Maxx fungicides, which have been tested and will help provide several weeks of extended protection after the plants leave their facility.”

USDA is looking for a better understanding of impatiens downy mildew and other diseases and has started a program to collect samples from across the country. If you come across a plant infected with impatiens downy mildew (or boxwood blight, rudbeckia downy mildew, sunflower downy mildew or brown rust of mums), visit OrnamentalPathology.com to find out how to share a sample.

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  1. Pingback: Impatiens Downy Mildew – Identification and Substitutes | Walter Reeves: The Georgia Gardener