With the arrival of the spring planting season in much of the country, the common garden impatiens is an uncommonly hot topic of conversation.
That’s because a disease that has plagued Impatiens walleriana for the past several years in England, Europe, Africa, Australia and Asia was found in landscape plantings in the upper Midwest, the Northeast and coastal Southern California late last fall. Now infected plants have been discovered this year in residential and commercial landscapes in Florida.
The disease, impatiens downy mildew, is caused by a fungus-like microorganism called Plasmopara obducens. Warning signs of infection are stunted growth, off-color, light green leaves, leaf and flower drop, and, eventually, stem collapse, said Mary Hausbeck, a plant pathologist at Michigan State University.
“A tell-tale sign of the disease,” she said, “is a white, downy growth on the underside of the leaves.”
The disease is not new. “It showed up in a significant way in 2004 in the greenhouse industry,” Hausbeck said. “Prior to that, you have to go back a long way to find even a small report about it.” From 2006 until last year, it didn’t seem to be a big problem, she said.
Now, it’s a global problem and causing concern close to home in communities across the country.
After the disease started showing up in several regions in the United States in late August and early September last year, Ball Horticultural Company in West Chicago, one of the nation’s largest producers of plants for the U.S. horticultural trade, issued a nationwide alert.
“We wanted to make landscapers aware of the problem so they could pull any infected plants out of the ground before they collapsed,” said Colleen Warfield, corporate plant pathologist for Ball.
One of the mysteries about the disease is how it got to Florida. Plant pathologists aren’t sure, Warfield said.
It’s been suggested that “snowbirds” going to Florida homes for the winter could have unwittingly taken infected plants with them, she said. She also pointed out that the airborne spores could have hop-scotched their way to Florida on wind currents from infections in the Upper Midwest. “Spores of another downy mildew species have been projected to travel as much as 600 miles in 48 hours given the right environmental conditions,” Warfield said.
Regardless of how the fungus got to Florida and despite the possibility the airborne spores could drift from infected landscape plants to nearby commercial growing ranges, Warfield offered words of comfort to gardeners who will be looking for impatiens in nurseries this spring.
“There is no evidence the integrity of the supply chain has been affected,” she said.
That’s because plant producers have very rigorous management programs and U.S. suppliers and finish growers are now better informed and have been putting preventative programs in place since last year’s discovery that the disease had appeared in a significant way in landscapes in the United States, Warfield said.
Warfield emphasized that she does not believe any infected plants are knowingly being shipped for sale to either commercial landscape installers or to the general public.
Aaron Palmateer, an assistant professor of plant pathology at the University of Florida, agreed with Hausbeck.
“I was in two large production facilities in Miami-Dade County looking for diseased material to study in the lab,” Palmateer said. “But I didn’t see any. The only diseased material we found was in the landscape.”
“I don’t foresee any shortfall of clean plant material (of Impatiens walleriana) this year,” he added.
Hausbeck offered a similar forecast for Michigan’s bedding industry, saying she thinks growers there will have a good production year for Impatiens walleriana when spring planting season arrives. In some places it’s still too cold to plant. “The high here today is just 39 F,” she said recently from East Lansing, Michigan.
Warfield said it will help put gardeners’ minds at ease if they to understand a few characteristics about the disease. Those are:
It is specific to Impatiens walleriana. It does not affect New Guinea impatiens or SunPatiens.
The pathogen cannot infect other garden plants.
There’s no evidence the pathogen is transmitted to the plant by infected seed.
The pathogen also produces survival spores that can persist in the soil. They are released into the soil when infected leaves drop or stems collapse, which is one reason why it is important to promptly remove infected plants from landscape beds..
Because of the possibility of survival spores in the soil, it’s a good idea to not plant impatiens walleriana in a bed this year where infected or suspicious-looking plants of impatiens walleriana were planted last year.
Plant pathologists have no idea how long the survival spores could live in the soil. “Those spores could survive at least five to eight years in the soil based on studies of other downy mildews,” Warfield said.
Geography plays a role in where the disease takes hold. “It doesn’t like hot temperatures or dry conditions,” Warfield said. Favorable conditions usually occur in early spring or late fall when temperatures drop at night and there is excessive moisture from rain or humidity. These are the conditions that existed in the upper Midwest when the disease was detected in the United States last fall, she said.
For those who grow Impatiens walleriana, Hausbeck suggested a simple precautionary step: “Keep garden tools visibly clean to the eye,” she urged. “Get the soil off. This is what harbors the survival spores.”
Garden sanitation steps she recommends include:
Making potentially infected beds the last beds you work on during garden work days.
Washing tools with a strong stream of water.
Cleaning tools with soap and water and … cleaning them again.
Disinfecting tools with a solution of 10 percent Chlorox and water (if you want to be extra careful).
Gardeners who suspect they have infected plants should put them in sealed bags and dispose of them through municipal trash collections. These plants should not be composted.
Gardeners should also not try to cure an infected plant, Palmateer advised. “The disease is too aggressive,” he said. “You have to throw diseased plants out.”
For those who might decide not to plant Impatiens walleriana this year, some alternative shade-loving plants that would also provide summer color are:
New Guinea impatiens
Have other tips for growing impatiens? Leave us a note in the comments below.