Downy Mildew Spreads Across 34 States

Impatiens walleriana are most susceptible to the mildew, while New Guinea impatiens are tolerant; however, the latter strain is less tolerant of heat and are more prone to root rot. If discovered, infected plants, fallen leaves and plants in the immediate vicinity should be uprooted and destroyed, as symptoms may not be visible for up to two weeks.

The impatiens that usually brighten up gardens in the coming months will be fewer and far between this year. Downy mildew, a disease deadly to the impatiens walleriana common to spring landscaping, is reaching into landscapes nationwide.

In 2011, the disease was confirmed in 11 states. In 2012, it was in 34. Experts say that landscapers who plant impatiens will be taking a serious risk. Once the plant can be infected by the mildew spores, which are found in soil, water and winds from as far away as 100 miles. Small yellow spots appear on the tops of the leaves and fluffy white-gray growths on the undersides.

"The [fungus has] figured it out. During dormancy, they form a very thick-walled spore that's resistant to cold and flooding and drought," Pennsylvania State University plant pathologist Gary Moorman told the Pittsburgh Tribune-Review.

There's no cure, and no affordable, foolproof way to forestall the end. The plant shrivels and dies in a matter of days or weeks, while the offending spores can live on in the soil for a year or more.

Cool temperatures, high humidity, and moisture from rain and overhead sprinklers or irrigation systems fuel the spread of the disease. And spores in the ground can survive the winter.

Moorman believes that the downy mildew problem could cripple impatiens production and sales for years.

h/t: Pittsburgh Tribune-Review