The extended cool wet weather present throughout much of Minnesota has brought out the symptoms of impatiens downy mildew early this year. Plants infected with impatiens downy mildew drop their blossoms and have yellow stippled leaves that curl under at the edges. As the disease progresses, infected leaves fall off, leaving barren green stalks, which flop to the soil like cooked spaghetti noodles with time. Plants that are infected very young often remain small and yellow, dropping leaves and collapsing before they can ever grow to maturity.
The pathogen responsible for impatiens downy mildew is Plasmopara obducens, a water mold that thrives in cool wet conditions. Spores of the pathogen can be seen as fluffy white growth on the lower surface of infected leaves. These spores are easily spread on wind and splashing rain.
M.Grabowski, UMN Extension
Photo 2: Sporulation of downy mildew on the lower surface of an infected leaf
Gardeners observing symptoms of downy mildew on impatiens should remove plants immediately. Once spores of the pathogen have been found on the plant, there is no way to prevent its eventual decline from disease. Impatiens downy mildew only infects some species of impatiens. All varieties of Impatiens walleriana, standard impatiens, are highly susceptible to the disease. In gardens where the disease has been found, other shade annuals like begonia, coleus, and caladium are good choices for replacement plants. New Guinea impatiens, Impatiens hawkeri, are resistant to the disease and can also be planted in beds with a history of disease.
Some fungicides can protect healthy impatiens from infection with downy mildew but most are unavailable to home gardeners. Gardeners interested in protecting impatiens from downy mildew should contact a landscape professional with a pesticide applicators license to treat plants. More information about impatiens downy mildew can be found at the UMN Extension Garden webpage.