In recent weeks, several alerts focusing on Impatiens Downy Mildew (Plasmopara obducens), an oomycete, or water mold disease, which is related to the pathogen commonly referred to as late blight of tomatoes and potatoes, have been circulating among Extension educators across the Northeast. Fortunately, Impatiens Downy Mildew (IDM) strikes garden impatiens and little else.
IDM is a destructive foliar disease, defined by a somewhat velvety growth of fine white spores, visible only on the underside of infected leaves, which often curl under, turn yellow, and drop from the plant. It can take as long as two weeks for the spores to become visible, so plants can appear healthy even after they have become diseased.
Flowers will drop, too, until eventually the flourishing mounds of rich green foliage and colorful blossoms have been reduced to small bunches of anemic buds and sickly yellow leaves hanging at the tips of unadorned stems. Sooner or later, the plant dies.
In the United States, IDM has existed in the wild since the late 1800s but, until this century, because it had never been seen in production markets, it had been considered more or less non-threatening. During the last decade, however, IDM has somehow transformed into a devastatingly aggressive and widespread bedding-plant disease.
The disease first appeared in England in 2003, with no prior history in that country. The following year, limited outbreaks were observed in the United States.
In 2008, occurrences were confirmed in several eastern states. Reports continued in 2009 and 2010, but remained sporadic until 2011 when outbreaks were unexpectedly verified in landscape and container plantings in California, Illinois, Indiana, Minnesota, Massachusetts and New York.
Last year, outbreaks of IDM suddenly became extraordinarily widespread. Plant pathologists confirmed its existence in 33 states, including Washington, Oregon, Michigan, Wisconsin, Indiana, Illinois, Tennessee, Kentucky, in every state on the Gulf Coast from Texas to Florida, in and every state on the eastern seaboard from Florida to Maine, including the District of Columbia. The disease has also been confirmed in other European countries, Australia and South Africa.