Monitoring downy mildew on impatiens

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By Judith Adam

Fortunately, New Guinea impatiens is resistant to downy mildew.  (Photo from Veseys.com)

For years every garden centre and corner grocery store has been flaunting flats of bright impatiens seedlings. It would be fair to say impatiens is overplanted, but their vivid colours and generous floral displays are hard to resist. Unfortunately, impatiens is being threatened by a devastating mildew disease that destroys every leaf, flower and bud, regardless of whether it’s grown in a container or in the ground.

I’ve seen impatiens afflicted with downy mildew over the years, but never the full destruction of plants as recently reported. Infections in the United States are widespread, and the disease pathogen (Plasmopara obducens) has also been positively identified in Ontario, Quebec and British Columbia. (An Internet search for “impatiens and downy mildew” will turn up lots of information.) This strain of downy mildew is specific to both single and double garden impatiens (Impatiens walleriana), or busy lizzies as you might call them, but doesn’t infect other kinds of impatiens. New Guinea or sun impatiens (I. hawkeri) is resistant to it.

Downy mildew is encouraged by excessive watering, high air humidity and relatively low temperatures, between 15 and 23°C. Midsummer drought conditions with low humidity and high temperatures may help to stave off mildew infections, while moist spring and late summer weather will encourage the disease. Once disease spores are present, it’s almost impossible to prevent infection from spreading. Downy mildew on impatiens is spread two ways: short-lived dispersal spores produced in the white powdery coating on the undersides of leaves are dispersed by wind and water splashes; and survival spores (called oospores) that are produced inside stems and petioles, and deposited in the soil. Spores in the soil can overwinter, and will infect new plants the following season.

Initial symptoms of downy mildew on impatiens usually occur in moist conditions with cool nights, beginning with the youngest foliage, causing leaves to appear yellowed, slightly chlorotic or stippled, and often with a thin, white downy coating on the leaf undersides. The white powder can also be found on the undersides of leaves still green. Affected leaves fall off, leaving bare stems.

Fungus infections are difficult to control, and the only effective treatment is preventative. A preventative spray program every two weeks from the day of planting until the end of the growing season is required to provide protection before fungus spores become established on foliar tissue. A mixture of one-tablespoon (15 mL) light horticultural oil (available at garden centres) combined with one tablespoon of baking soda per gallon (4L) of water, sprayed on all sides of the leaves and stems every two weeks, is effective in preventing infection.

Impatiens plants that are produced in greenhouses are likely disease free when they arrive at the market. However, once planted in the open garden, weather conditions may trigger the deadly downy mildew spores. What should we do about this? Will it be necessary to ban impatiens from the garden? Well, I’m still going to plant them, and certainly hope for the best. But unlike other years when I’ve given little regard to planting conditions and still had gorgeous flower displays, I’ll now give more consideration to how I plant my impatiens.

I can’t control air temperature, but perhaps I can do something about water and humidity. This year I won’t set impatiens plants so close together as I have in the past. Leaving more space between individual plants will help to lower humidity and increase air circulation. I won’t water impatiens from overhead, but instead will flood the ground and avoid wetting the foliage. Any sick plants will be immediately removed and disposed of in garbage; and during fall cleanup, all impatiens plant parts – stems, flowers, and root balls – will be lifted, but not composted (mildew spores can live in compost).

This is something like treating impatiens with all the fuss of hybrid tea roses. But I’m willing to keep growing impatiens, just so long as they’re healthy and happy in my garden.

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