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Impatiens Downy Mildew: A New Disease in Vermont

Last fall the University of Vermont Plant Diagnostic Clinic in Burlington identified a new destructive disease infecting impatiens called downy mildew. The fungus-like pathogen (Plasmopora obducens) is very host specific and attacks only common garden impatiens (Impatiens walleriana), causing rapid defoliation during wet, cool weather. The New Guinea impatiens (I. hawkeri) is resistant to the disease as are all other bedding plants.

If your impatiens died early last year, it is likely that this new disease was the cause. The first symptoms of the disease are yellowing in the leaves, often with the leaf margins curling downward. An obvious white downy-like coating will appear on the leaf undersides when conditions are cool (60 to 73 degrees F) and moist or humid.

This coating contains sporangia of the fungus, which are easily dislodged and can be moved by splashing rain and for longer distances, sometimes hundreds of miles, by wind. When sporangia germinate, they release thousands of swimming spores called zoospores. These spores can swim in moisture on the leaf surface and infect tissue.

If conditions are dry, new infections will not occur. The pathogen also can cause plant stunting and flower drop, leaving bare stems.

Impatiens downy mildew has been found in U.S. greenhouses since 2004, but it was not until the summer of 2011 that the disease was found in landscape plantings. It was widespread in Florida in winter of 2012 and by the end of that season, the disease had been confirmed in 33 states, including four new England states--Vermont, New Hampshire, Massachusetts and Maine.

The pathogen has the ability to withstand cold winters by producing an overwintering structure called an oospore. As the infected plants decompose in the landscape, the oospore is released into the soil. It germinates in the spring, producing more sporangia and leading to a reoccurrence of the disease when conditions are cool and moist. It is important to clean up and destroy infected impatiens so these overwintering structures are not introduced into the soil.

Although the disease can be managed with fungicides in the greenhouse, this is neither practical nor recommended in home landscapes. Avoid replanting impatiens where they were planted the previous season.

Due to the pathogen's ability to produce sporangia that travel long distances, the best management strategy is to avoid the disease entirely by choosing plants that are resistant to downy mildew. Several excellent choices are available including New Guinea impatiens, begonias and coleus. Your local garden center or greenhouse can recommend suitable plants for your area.

For more information on the disease and photos, go to http://www.uvm.edu/mastergardener.

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