Impatiens Downy Mildew (IDM)

by Thayer Nursery

I am sure that more than a few of you depend on impatiens to provide season-long color in shady containers and beds. The latest news concerning them may leave you feeling a bit frantic! Impatiens Downy Mildew (IDM) was diagnosed on flowering impatiens in Massachusetts for the first time in 2011 and then devastated plantings in 2012.

Development of IDM is highly influenced by the weather, as it is a water mold that requires moisture to cause new infections. Wet foliage, cool temperatures (especially at night) and moist air are the ideal conditions for disease development. Plants in densely planted beds, heavily shaded areas or where their leaves stay wet for extended periods of time will generally have a higher incidence and severity of the disease.

The first sign of IDM is leaves that are slightly yellow or off-color. As the disease progresses, the undersides of the leaves will have white-colored, powdery-like spores. Eventually the leaves and flowers will drop off of the plant, leaving stems with only a few tiny, yellow leaves remaining. IDM can be spread long distances by wind currents, water splash (overhead irrigation included) or by the movement of infected plants.

Infected plants should be pulled, bagged and disposed of; never composted! It is expected that IDM will be a recurring problem, as its spores overwinter in the soil and can infect impatiens planted next year.

Remember the disease triangle: a susceptible host, the presence of a pathogen and certain environmental conditions are needed for the disease to develop. Infected areas should not be replanted with impatiens, but can be replanted with other plants. This could be a great opportunity to experiment with new plants and expand your gardening knowledge! Shade annual plants like New Guinea impatiens, torenia, lobelia, and other annuals with colorful foliage such as caladium, begonias, coleus, and hypoestes are safe to reset in affected areas.

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