Impatiens Downy Mildew threatens spring gardens; Here are the facts

Written by Doug Oster on .

Every garden season brings with it new challenges. Last year, gardeners struggled with impatiens downy mildew (Plasmopara obducens), a fungal disease that infects Impatiens walleriana, the most popular impatiens in home gardens.

The disease often starts slowly, causing leaves to curl downward and the plant to look thirsty or in need of fertilization. Sometimes white moldy spores can be seen on the undersides of leaves but not always. Eventually, flowers and foliage fall and the plant collapses.

Local nurseries and garden centers are concerned about how the disease will affect impatiens this season, and many are cutting back on the number of plants they will stock. They will also be carrying alternative shade-lovers to plant as substitutes.

Colleen Warfield, a plant pathologist for Ball Horticultural Co., has studied the disease and is trying to figure out why the pathogen, which was discovered in the 1800s, has recently become so virulent. She saw the disease infect impatiens two years ago in her own Illinois community, killing many of the plants late in the season. Even after diagnosing the problem for her neighbors, she watched as they planted impatiens the next season. The disease struck again, even later in the season. Most gardeners enjoyed a full season of flowers, which mirrors what gardeners experienced in Western Pennsylvania last year.

oster caladium replaceI love growing caladiums and they thrive anywhere impatiens do.The disease is transmitted through both soil and air. It's possible for the spores to overwinter in our soil, but that is not likely the cause of repeated outbreaks, she said in a phone interview. It's more likely that aerial spores are overwintering on plants in warmer climates like Florida and working their way north during the season. That's why we're seeing the disease at the end of the season, she said.

Weather also plays into the spread of the disease. Warm, humid conditions provide the perfect environment for impatiens downy mildew to thrive. Ms. Warfield has trialed many fungicides offered to home gardeners and hasn't found one that is very effective against the disease.

"Most of the time, the fungicide costs more than the plants. It's hard to justify spending $26 to treat a $1.25 six-pack of plants," she said.

Professionals do have more powerful fungicides that could treat the plants over the course of a season. Growers will be especially vigilant this spring in treating impatiens so they arrive disease-free, "but the retailer is still in the position where they can sell a perfectly healthy plant and it can become infected in the landscape," Ms. Warfield saids.

Most important, she added, don't take infected plants back to the nursery or garden center where you bought them. You'll be carrying the disease with you and could infect what's being sold. The plants should be bagged and put out to the trash to stop the spread of spores.

There's a long list of other plants gardeners can try, starting with coleus. There's a certain irony in coleus as an impatiens alternative because a different strain of downy mildew was devastating coleus in 2005. Growers wondered if it might be doomed as a bedding plant. But because they now preventively apply fungicides to the plant, coleus is a reliable replacement. Ms. Warfield hopes impatiens will follow the same course and in time can be planted without risk.

Randy Soergel of Soegel Orchard and Garden Center in Franklin Park became aware of impatiens downy mildew two seasons ago.

"I'm concerned that some people are going to run so scared that they are going to be afraid to plant anything in their yard thinking they can't come up with alternative plants," he said.

Downy mildew only affects specific species of impatiens. Varieties such as New Guinea impatiens and Sunpatiens are not at risk. Even though both of those are bred for sun, Mr. Soergel says they will bloom in part shade, too.

He'll carry fewer impatiens this season but cautions gardeners not to panic. There are still questions on what role last season's weather conditions played in spreading the disease. "If we have a regular season, there might not be the issues," he said.

Mr. Soergel sees this as an opportunity for gardeners to mix things up and experiment. Instead of planting a bed of just impatiens, mix in begonias and coleus. He said he is ordering more SunPatiens, which are a little more expensive than regular impatiens but cover a wider area. He's expecting growers to watch carefully for signs of the disease on plants being shipped ,and most will be treated with a fungicide before they arrive.

Jaclyn Pardini, a public relations specialist for Lowe's, said the large chain will continue to stock many flats of impatiens this year. "We only buy impatiens from growers who follow strict disease-control protocols."

She suggested adding caladiums to beds of impatiens. Ms. Pardini also had some cultural suggestions to help the plants fight off the fungal disease. She suggested planting a little later in the season, watering plants in the morning and using drip irrigation instead of overhead watering.

Kathy McGregor, owner of Sylvannia Natives, doesn't sell impatiens. She has lots of native perennials for shade. "First of all, you don't have to replant every year. You can keep adding to your plantings with perennials and then you begin to have a real showy understory in shade," she said.

The trick to filling a bed with perennials is mixing plants with different bloom times, she said. Choices for spring bloomers include wild blue phlox, tiarella or foam flower, yellow wood poppy and wild geraniums. For summer, try downy skullcap, great blue lobelia, black cohosh and cardinal flower. Choices for fall include white wood aster, heart-leafed aster and wreath goldenrod.

Darcy Kennedy, manager of Penn Hills Lawn and Garden, said she is carrying 75 percent fewer impatiens this season due to concerns over downy mildew. As alternatives, she recommended most of the shade lovers already discussed. The nursery also will be carrying varieties of torenia, often called wishbone flower. Last season, downy mildew was a big problem for her customers.

"We don't want to put them in that situation where they are having the same problems again. We're going to make everyone aware of what's happening and let them make the decision."

oster begonia caladium2This planter is filled with 'Gryphon' begonia, caladiums and some sweet potato vine.

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