Tuesday, August 13, 2013

Impatiens Downy Mildew; New Disease to Colorado by Mary Small

Photo courtesy Purdue.edu

 Two Plant Diagnostic Clinics in Colorado have recently received samples of impatiens downy mildew. This was a large problem last year in many eastern states but not here.
Early symptoms include leaf chlorosis and a stippled appearance similar to spider mite feeding. Leaves may curl under slightly at the edges. Eventually leaves drop and “plants” are merely a grouping of stems. Finally, the stems die too. You will find a dense white sporulation on the leaf undersides.
High plant density, overhead watering (especially at night) and high humidity all contribute to the development of the disease. Once plants are infected, they should be pulled out. There is no rescue treatment fungicide, only preventatives. It will overwinter in our cli- mate, unfortunately.
July rainfall and humid periods (think July 12-15) contributed to the development of the disease. If conditions are dry and air circulation is good, the disease doesn’t develop.
The disease does not seem to develop on impatiens grown from seed, but does on impatiens grown out from cuttings.
Tamla Blunt at the Colorado State University Plant Diagnostic Clinic is tracking the disease’s locations and spread in Colorado. For more information, see this web page: http://www.endowment.org/afe-news/press-releases/221- controlling-downy-mildew-on-impatiens.html
Photo courtesy Palm Beach County Extension