Q. I planted three flats of impatiens this season; by themselves, in mixed displays, in the backyard, in the front yard, in beds, in pots, in deep shade, partial shade and semi-full sun. They were fed naturally (no chemical fertilizers) and watered. And they all lost their leaves and died. I was told by a grower that there has been a wide-spread problem with Downy Mildew this year...
- ---Jack in the German town section of Philadelphia
My impatiens started out really nice, then began disappearing little by little. Flowers first, then leaves, then stems. I looked for bugs but could see none. What's up?
- ---Danny in Reading, PA
A. What's up is something that would have been headline news all summer if it had attacked a food crop like tomatoes or sweet corn. But because it instead went after an ornamental annual flower—and only one type of that ornamental annual flower—it flew under a lot of people's radar, including mine. Only after a farmer friend asked why I hadn't mentioned it did I start making calls. And what I discovered was astonishing.
Although the Impatiens family is huge—around 850 species in the genus—there are basically only two members commonly used as bedding plants, explained Marge Daughtrey, a Senior Extension Associate in the Department of Plant Pathology at Cornell University's Long Island Horticultural Research & Extension Center: The common shade-loving bedding plant known scientifically as "I. walleriana" and the taller, more sun-tolerant "New Guinea" hybrids (scientific name I. hawkeri). The former—the ones everybody just calls 'impatiens'—are toast; virtually wiped out over a single season by a specific downy mildew.
New Guinea impatiens are, so far, totally unaffected by this disease. Affected only slightly is our native American member of the family, the legendary and ubiquitous Jewel Weed; it just gets a little spotting on some of its leaves when it encounters the disease—so little you wouldn't notice anything was wrong unless you looked real close.
But there's nothing 'slight' about the effect of this mildew on garden variety impatiens. It strikes fast, explains Cornell's Daughtrey. First the flowers seem to vanish, then the leaves disappear, leaving only stems—a visual that looked so much like slug damage, I assumed that slimmers had visited my little patch. (Yes, my impatiens were hit this summer as well; apparently, few gardens were spared.) In some neighborhoods, chipmunks took the blame; in others it was deer.)
It turns out, weirdly enough, that the disease that actually did this (Plafmopara obducens) is nothing new. Cornell's Daughtrey explains that it was first identified in the United States (on Jewel Weed) way back in the 1800s, and has just kind of been hanging around ever since. It's one of a number of pathogens grouped under the umbrella term 'downy mildew'—a designation so broad it includes a number of different genera ('families') as well as many species (family members). All, she explains are 'water molds', having primitive "swimming spores" that can multiply rapidly in wet situations.
No one knows why this one, now termed "impatiens downy mildew" or IDM for short, suddenly developed a taste for garden impatiens. What we do know, explains Daughtrey, is that it wiped out small groups of them in England in 2003, appeared in greenhouses in NY, Tennessee and California in 2004, and then only accounted for a couple of reported cases a year—until 2009, when it wiped out most of the impatiens plantings in Saratoga Springs (NY). Then it hit parts of southeastern New York state "real bad" last Fall—but so late in the season that most gardeners assumed it was just frost damage. (Impatiens are real drama queens about cold weather, and are typically the first plants to join the Choir Invisible when light frosts are even mentioned.)
This year it hit hard almost everywhere except the Dakotas and Montana. And it hit early; Daughtrey found it on Long Island impatiens on June 4th—that's barely two weeks after safe planting-out time.
When asked what we can do next season, Daughtrey simply said, "say bye-bye to walleriana impatiens. The disease has suddenly become widespread, fungicides aren't very effective, and even if they were, you'd have to spray the plants every week.
"There may be few to no walleriana impatiens even offered for sale next season," she adds. "Growers are still talking about what to do, but nobody wants to sell plants that are going to shrivel up and die before the 4th of July; and the smart move would be to just take them out of cultivation for a number of years. There are lots of nice bedding plant alternatives, like New Guinea impatiens, annual begonias, coleus—and Torenia, 'the wishbone plant'—an under-used and very attractive shade-loving annual.
"If regular impatiens are available next season, at the very least don't plant them in the same spot where this year's plants collapsed and dropped downy mildew spores into the soil. And if you do plant them and they start to disappear, leave it be; you'd just wear yourself out in a futile attempt to save them by spraying."
Torenia sounds better by the minute….