Recently, I attended a workshop with plant disease diagnosticians representing many areas of the United States. This disease of impatiens was discussed briefly and may become a problem for some of our growers.
Impatiens downy mildew is a new disease to this country. It was reported as a problem in the United Kingdom in 2003 and was found in greenhouse production areas in the United States starting in the spring of 2004. The pathogen is Plasmopara obducens. Symptoms include stunted plants with yellow leaves. The leaves may be curled or distorted. Wilted leaves may be present among normal leaves. Often, the leaves have a yellow, speckled appearance. Margery Daughtrey, plant pathologist at the Long Island horticultural research and Extension center, states that infected plants appear similar to those with a spider mite infestation, so look closely for spider mites before you treat with a miticide. The downy mildew pathogen produces a white, fuzzy growth on the underside of leaves as a result of the development of sporangia. Diagnostic labs can view these sporangia and make a positive diagnosis on the spot.
This Web site by Daughtrey at Cornell University discusses the problem and includes a photo of the upper leaf surface of an infected impatiens plant: http://www.hort.cornell.edu/greenhouse/Ralstonia3.8.04.html. Another Web site by Mary Hausbeck at Michigan State University shows photos of the underside of infected leaves: http://www.plantpathology.msu.edu/labs/hausbeck/hausbeckDownyMildew.htm. Both sites should be helpful.