Impatiens & Downy Mildew

So for years and years you've loved growing impatiens in your shady gardens... but last year you noticed they didn't do so well, or perhaps they died off all together.  Chances are  they had downy mildew.

Impatiens downy mildew is caused by an organism called Plasmopara obducens. It appeared several years ago in Europe, showing up in North America in 2011 in Florida, the Mid-Atlantic and the Northeastern United States. It is now showing up in Southern Ontario.

Downy mildew is spread by spores located on the underside of infected leaves. The spores can be spread by wind or rain as it splashes onto the leaves. The organism is a water mould requiring moist conditions to develop spores and cause new infections.
Downy mildew attacks only Impatiens walleriana (the bedding variety). Other types, such as New Guinea and SunPatiens, are not affected.

There are no fungicides available to gardeners for the control of this disease.

To dispose of infected plants, Do not place infected plants in home composters in case the spores overwinter. Instead, bag and put them at the curb to be composted in a municipal facility (where the heat is high enough to destroy the organism). Alternatively, you can burn or bury the affected plants deeper than 50 centimetres (20 inches).
 Experts advise against planting Impatiens walleriana where infected plants once grew.  If you want to grow impatiens in a container that previously held infected plants, thoroughly wash the container with soapy water and a drop of bleach. Be sure to use fresh planting soil mix. Because the disease is specific only to Impatiens walleriana, you can grow any other bedding plants without any risk.

Should you avoid buying bedding impatiens (Impatiens walleriana) for your garden this year?
Because downy mildew is a relatively new problem, whether it becomes more serious or not remains to be seen.  Perhaps the extreme cold this past winter has destroyed some of the spores, but only time will tell, that so watch for it.  The spores live in the soil they are planted in, and spread in the air.

Seed companies are working on breeding seed varieties that are mildew resistant, however, that may be 4-5 years away.

Downy Mildew - A curse or an opportunity?   read this article from MSU