A funguslike plant killer found in garden beds and greenhouses along the East coast over the past few years is expected to impact common garden impatiens once again this year, and plant pathologists say there is little that gardeners can do to stop the disease from spreading.
According to a review written by Nora Catlin of the Cornell Cooperative Extension of Suffolk County, a pathogen known as Plasmopara obducens has caused a funguslike disease known as impatiens downy mildew. This fungus was first noted as a problem in the United States in 2004, where sporadic cases were showing up in greenhouses. By 2011, downy mildew had been found to be affecting common garden impatiens, or Impatiens walleriana, across the nation.
“In some cases the symptoms were dramatic, with the disease causing complete collapse of large plantings,” wrote Catlin.
Last year, impatiens downy mildew was seen much earlier in the planting season.
According to Cheryl Smith, plant health specialist for the University of New Hampshire Cooperative Extension program and the director of the university’s plant diagnostic lab, signs that plants are infected include stunted growth, off-color leaves, flower drop, and stem collapse. Smith noted that a telltale sign of the fungus is white, downy growth on the underside of leaves.
Smith and her colleagues have been studying Impatiens downy mildew over the past year. Although she admits the effects are devastating on garden beds, Smith stressed that research has shown this particular downy mildew only impacts impatiens walleriana in such a dramatic way.
As for why the increased spread of impatiens downy mildew in the U.S. has occurred in the past five or so years, Smith said she and her colleagues are stumped.
“I think it’s been here for 20 or 30 years, but it never really caused much of an issue,” she said. “I’m not sure anyone really knows the answer right now as to why it has become a problem in the past few years.”
Impatiens downy mildew thrives in wet, shady conditions. Smith said the increase of cases across the U.S. could also be attributed to fluctuating weather patterns.
Although the fungus can sometimes be spread through water, Smith said impatiens downy mildew regularly spreads through wind. It produces a large number of spores, which thrive on the stem and leaves of Impatiens and are easily carried long distances in the wind.
“All we need is some good, windy conditions and some wet rainy weather and we’ll have ideal conditions for this to spread,” said Smith.
Trying to prevent impatiens downy mildew from impacting his nursery, Bryan Wentworth, co-owner of Wentworth Greenhouse in Rollinsford, said he reduced the number of impatiens he grew for this year’s planting season.
“We made the decision last fall after seeing the amount of gardens affected by downy mildew to cut our production of impatiens by two-thirds,” said Wentworth.
He noted that impatiens are typically sought after by gardeners because of their easy upkeep and relatively cheap initial purchase cost, and that common impatiens are the number one bedding plant sold each year.
Wentworth and his staff have increased the number of alternative plants offered to customers who were looking to purchase impatiens. Wentworth highlighted the greenhouse’s offerings of new guinea impatiens, a variety not affected by downy mildew, begonias, colius and other types of plants that do well in conditions similar to common impatiens.
“There’s no real great substitute that does what impatiens do, which is flower like crazy,” said Wentworth.
Jon Appleby, co-owner of Salmon Falls Nursery in Berwick, Maine, says he is not too worried about the impatiens his nursery is selling becoming infected.
“It hasn’t hit Maine too badly yet, so we’re still going to sell impatiens like we normally would,” said Appleby.
He purchases impatiens from growers in Canada in an area he said has not had downy mildew.
Smith said there is little local gardeners can do to save their impatiens once they are infected.
“Once a home garden has impatiens downy mildew, the best thing really to do is pull the impatiens, including the roots, get them in a bag and put them in the garbage,” said Smith.
She said once plants are showing signs of being infected, including white downy spores on leaves and collapsing, it is too late.
Smith said symptoms will occur in June or July.
“You can plant them and enjoy them, but you should expect them to die,” said Smith.
She cautioned gardeners to avoid planting impatiens walleriana at all in an effort to prevent the fungus from spreading not only to their own gardens but to their neighbors.
There is no cure-all fungicide for impatiens.
“My colleagues have tested fungicides, and they just don’t work well,” Smith said.
She noted that certain active ingredients in fungicides help prevent impatiens downy mildew from spreading rapidly, but stressed that application needs to occur quite often. Smith explained that the leaves of impatiens must be coated front and back for the fungicide to work, and must be applied at least once a week.
“This might keep the fungus from growing as fast, but it’s not the magic bullet,” she said.
Wentworth said he and his staff are doing their best to spread the word.
“I’ve definitely seen a slow increase in the number of people who have heard about downy mildew, especially after last fall when it seemed to be showing up in garden beds around this area,” said Wentworth.
Wentworth Greenhouse now offers a tip sheet for those interested in learning about downy mildew.
“In 2012 many home gardens were infected and research indicates that we will only see conditions worsen,” reads the information sheet.
It says that impatiens walleriana, double impatiens, fusion series and balsam impatiens are affected by this particular strain of downy mildew. New Guinea impatiens and lobelia are not affected.
“Although garden impatiens may seem irreplaceable, there are actually a number of cool plants that will thrive in that shady spot,” reads the information sheet.
It highlights 17 alternative plants, including begonias, caladium, diamond frost euphorbia, iresine bloodleaf and sweet potato vine.
For updated information about impatiens downy mildew, visit the UNH Plant Diagnostic Lab’s website at www.unhpdl.org.