WEDNESDAY, JANUARY 20, 2013
By TOM ATWELL
A new pest is likely to force impatiens out of the landscape in Maine — and probably in all gardens east of the Mississippi.
Impatiens is an annual in Maine and a mainstay of flower beds, cemetery boxes and hanging baskets all through the summer. Although impatiens are not as popular as geraniums, it is the go-to annual if you want a continually blooming plant in shady conditions.
The pest is impatiens downy mildew, a fungus closely related to late blight, which kills tomatoes and potatoes.
“We had one confirmed case of impatiens downy mildew last year and a lot of rumors,” Carole Neil, an assistant horticulturist with the Maine Department of Agriculture, told greenhouse growers at a professional conference earlier this month at Longfellow’s Greenhouses in Manchester. The scientific name of the fungus is Plasmopara obducens.
The first signs of the disease are a slight yellowing and then a drooping of the leaves. Then a light gray, fluffy growth will appear on the underside of the leaves. In time, most of the plant leaves will drop off.
Neil said greenhouse growers can treat impatiens for the downy mildew, and there are chemicals that professional landscapers licensed to spray pesticides in the field can use.
But Cheryl Smith, director of the plant diagnostic lab at the University of New Hampshire, said the pesticides require complete leaf coverage. She believes that few landscapers are going to cover the underside of the leaves on a plant that grows as close to the ground as impatiens does.
The pesticides that work on impatiens downy mildew are not available to homeowners.
Neil said the disease is systemic, which means that it is taken into the cells of the plant, which is part of the reason it is so difficult to treat.
The disease is spread by the wind, and the winds can carry it a great distance, as well as by water runoff and plant movement. The disease does best in cool and moist areas. Since most people use impatiens because they do well in the shade, they will often be planted in areas that are ideal for the disease.
Neil said that it is estimated that healthy impatiens planted outdoors will probably live until around June before they pick up the disease outdoors. But she said many nurseries may choose not to sell them this year, because they know they are going to die eventually and will not want to be blamed by their customers.
The disease affects all Impatiens walleriana, which are the ones most commonly for sale. It also affects some Impatiens balsamina, known as balsam impatiens. Some wild impatiens — pallida and capensis — are affected, but they are known as jewel weed and few gardeners will miss them.
New Guinea impatiens, which are a different species called Impatiens hawkeri, and some hybrids such as SunPatiens are not affected, according to an e-Gro alert from Purdue University.
Neil and Smith said that when people find infected impatiens, they should bag them up and put them in the trash. Do not compost plants that have the disease. And you should not immediately replant impatiens in that infected area. The downy mildew will survive the winter in plant material and weeds left in the garden.
Smith said research has shown that the downy mildew will survive the winter in soil in Zone 5, which is coastal and southern Maine. She said they are doing tests in gardens in Zone 4b and Zone 3, so they will know soon if it will survive in colder regions.
Alternatives that Maine gardeners can use for shady areas, in addition to New Guinea impatiens, include coleus, caladium and begonia.
Tom Atwell has been writing the Maine Gardener column since 2004. He is a freelance writer gardening in Cape Elizabeth and can be contacted at 767-2297 or at: