impatiens downy mildew forecast: too soon to tell
I PRE-ORDER CERTAIN ANNUALS—reserving whole flats of favorites with my local garden center to make sure I won’t get shut out. Normally that early commitment includes a favorite peachy-colored impatiens, but this year, I’m not so sure. Impatiens downy mildew ravaged the popular bedding plant in many parts of the country last year, so I asked Margery Daughtrey, a plant pathologist and senior Extension associate with Cornell University, what the early line is from where she sits in her Suffolk County, Long Island, lab–and some substitutes I might consider for that annual order of mine.
First, a recap of what Impatiens downy mildew (the fungus-like Plasmopara obducens) looks like:
Did your impatiens seem to collapse in 2012? Early signs of infection may have been leaves that looked yellowish, as if the plant needed feeding, or foliage that curled under or seemed to wilt. Sometimes, a white material (the downy mildew) is visible on the undersides of leaves. Eventually, plants may defoliate, drop their flowers, and basically collapse. The fungus loves moist conditions and cool nights in particular. Particularly disturbing: symptoms happened earlier last year than before—as early as June.
The disease, which attacks Impatiens walleriana (the species our common shade-garden choice is bred from) showed up in 2012 first in Florida and later in the Mid-Atlantic, the Northeast, and even Utah and the West Coast. By Daughtrey’s count, 35 states had at least one case of the disease in 2012, and some Canadian provinces as well.
my q&a with cornell’s margery daughtrey
Q. I’ve been watching the trade publication “Greenhouse Grower” for news on impatiens downy mildew, and in a December article, you said that growers are of course well aware of the issues, and were making plans for 2013. Is there anything specific that’s “new” since?
A. Not really much change. The “Philadelphia Inquirer” did a story in February, but I noted that there were still impatiens being used as bedding plants at the Philly Flower Show! “Southampton Press” [on Long Island] focused on the impatiens downy mildew in their Hampton Gardener column this week.
Stories are springing up here and there to educate the public about this disease, but until we actually get into the spring retail season for bedding plants, we won’t know whether growers gambled correctly on how many impatiens to grow for this year. In some areas demand may be the same as before, and in others landscape gardeners and home gardeners may both be very skittish about planting impatiens.
Q. How are growers planning to offset the cutbacks in impatiens production?
A. Growers who know that the disease has been devastating in their area have cut back how many Impatiens walleriana they are growing this season, and are offering various begonias, coleus, New Guinea impatiens, torenias, etc. as alternatives.
Home gardeners should take this opportunity to look into exciting new annuals that may be new to them. Many work very well in shade. One of the most confusing things will be for them to realize that New Guinea impatiens do not get this particular disease—and that they do well in shade, as well as in sun.
Q. If gardeners had problems last year, is it possible that spores have overwintered? I read on Michigan State University’s site that this can happen.
A. Yes. Anyone who saw impatiens downy mildew last year really should not try to plant impatiens into that same flower bed this year. The impatiens makes several kinds of spores, and one of them is designed for overwintering the pathogen.
Q. Europe has a widespread problem that began a few years before ours, correct? Have they managed to get past it, or is impatiens still a problem crop there?
A. The disease was seen in Europe in 2003; then in the U.S. in California, New York and Tennessee in 2004, but only as a minor problem that appeared during greenhouse production—it did not create problems in the landscape. It has been seen occasionally in the States since.
The first landscape problem with impatiens downy mildew that I am aware of appeared in Saratoga Springs, New York, in 2009, and returned in 2010, 2011 and 2012 there. Southeastern New York had dramatic landscape problems in the fall of 2011 that returned beginning in June 2012. South Florida saw collapse of impatiens in the landscape in the winter of 2011, and this phenomenon returned in the winter of 2012.
As far as England and Europe: A recent column by Chris Beytes of “Grower Talks” magazine has the most recent info that I’ve seen. There is certainly still a problem there, but they are probably not as gaga about impatiens as Americans have been for the past 40-50 years.
Q. Are there web pages we can consult to keep track of how the season, and the disease, develop?
A. Lots of good things, all to be found here and edited recently. I’ve called out the most helpful portions below:
- Impatiens Downy Mildew Fact Sheet for Landscapers
- Impatiens Downy Mildew in Home Garden
- Alternatives Plants to Garden Impatiens
- Video of an Impatiens Plant from Exposure to Plant Collapse
- Pictures of Symptoms of Impatiens Downy Mildew
begonias, coleus and ‘annual color’ options
I’M TAKING A GOOD LOOK at begonias and coleus, in particular, along with flowering plants that Margery Daughtrey recommended such as torenia, to add color to semi-shady spots where I place containers. Some things I like for “annual color” (often from their foliage alone) in such conditions: