A puzzling plant disease may dethrone one of the most popular and reliable flowerbed plants in North America, the garden impatiens.
A relatively benign condition known as impatiens downy mildew has recently turned ugly, for reasons under debate. For decades, U.S. gardeners rarely noticed downy mildew on their impatiens. But in the last two years, the disease has ravaged flower beds in some of the more humid parts of the country. After rain or fog followed by balmy nights, the disease can turn a lush flower border into a straggle of bare stalks that eventually collapse and die.
In recent years, aggressive impatiens downy mildew has flared up during disease-friendly weather in parts of Europe, South Africa and Australia. But the United States hadn’t seen more than a few scattered reports until widespread outbreaks began in 2011. By the end of 2012, pathologists had confirmed the disease in 33 states and Washington, D.C.
Since weather affects outbreaks, it’s hard to predict what 2013 will bring. But impatiens downy mildew was already active in Florida when the year began, says plant pathologist Colleen Warfield of Ball Horticultural Company, a Chicago-based company that breeds and distributes plants, including impatiens.
The disease is unlikely to eradicate the plants, but in some areas of the country, the risk can change a gardener’s mind about what to plant. Impatiens downy mildew “thrives in our coastal climate,” says plant pathologist Nancy Gregory of the University of Delaware cooperative extension program in Newark. In advice that would have been shocking a decade ago, she suggests gardeners skip impatiens unless willing to cope with the risk of an unsightly die-off.
In New York, another state that has seen two years of mildew, floriculturist Nora Catlin of Suffolk County frets over giving such advice. “I hate to tell people not to plant a plant,” she says, but this may be an opportunity to try growing something new.
Preventing an outbreak would require diligent pesticide treatment several times a month on the supposedly easy-care impatiens. Once the disease shows up in a plant, there’s no cure.
The pathogen can waft along on air currents, swim and perhaps survive the winter in soil. Gregory warns landscapers in her hard-hit state that even if they’re careful, “chances are someone down the block or around the corner could have some infected plant material that could spread to your nice clean garden full of impatiens.”
Ned Chapman, owner of Sunnyside Gardens in Saratoga Springs, N.Y., says he and most other growers he talks with have either stopped growing garden impatiens or cut back by 75 percent or so. (In 2007, the most recent year for which data are available, growers sold $134 million of impatiens at wholesale.)
In Oregon, the disease has appeared only sporadically, says plant pathologist Melodie Putnam, who directs the Oregon State University Plant Clinic in Corvallis. The state does not have the summer humidity that encourages the mildew’s quick spread.
One peculiar aspect of this outbreak is that the pathogen that is thought to cause the disease has been reported in North America since the 19th century. Called Plasmopara obducens, it’s one of the oomycetes, or water molds, a group that includes the pathogen that caused the Irish potato famine and the one wreaking havoc in California and Oregon with sudden oak death.
Until recently, P. obducens seemed relatively mild mannered, says Marco Thines, an evolutionary biologist at the Biodiversity and Climate Research Centre in Frankfurt, Germany, who has studied oomycetes. Cells of the pathogen whip through a film of water by lashing their propeller-like flagella. They colonize plant tissues by punching through cell walls and, once inside, poking nutrient-harvesting nubbins against the cell membrane like fingers squeezing a balloon.
Downy mildews can often attack only a restricted range of species, says plant pathologist Phil Jennings of the United Kingdom’s Food and Environment Research Agency. P. obducens strikes garden impatiens, Impatiens walleriana, and some cousins. It doesn’t destroy a related blooming annual, the New Guinea impatiens, I. hawkeri.
Delving into the genetics of the pathogen may clarify how the disease has changed to become more virulent. What has been considered one species may actually be several, and some variant may have evolved into an alarming form.
“I don’t think this is our grandfather’s Plasmopara obducens,” says plant pathologist and former U.S. national mycologist Joe Bischoff of the American Nursery & Landscape Association.
Preliminary analyses show at least three distinct genetic groups within P. obducens populations, Warfield says. In the lab, all three “are equally destructive,” she reports. Each can wreak havoc in at least some gardens, but whether they survive equally well in varied environments remains to be seen.
Garden impatiens may come in an array of colors, but all commercial varieties of Impatiens walleriana and interspecific hybrids with an I. walleriana parent are susceptible to the disease, Warfield says. “Developing a downy mildew–resistant garden impatiens is likely to take many years, and it may not even look like the garden impatiens of today.”