Downy mildew observed on impatiens
Contractors who were frustrated with impatiens dying out in the landscape this summer can learn from a statement just released from Ball Horticultural Company. "The occurrence of impatiens downy mildew in North American landscapes this season has prompted Ball Horticultural Company to issue an alert to the landscape trade in an effort to slow the spread of impatiens downy mildew," says Dr. Colleen Warfield, company plant pathologist. Impatiens walleriana can become infected by short-lived aerial spores that are easily dispersed by wind currents and splashing water, or by oospores that may survive the winter in the soil.

To slow the spread of disease, the company is advising landscape contractors to closely inspect beds of Impatiens walleriana, promptly remove infected plants, and to plant alternative flowers and foliage for the next two seasons in beds with a history of this disease.

The downy mildew infecting impatiens is specific to Impatiens walleriana (garden impatiens) and will not infect other plants. All seed and vegetative varieties of Impatiens walleriana are susceptible to downy mildew. However, New Guinea impatiens is highly tolerant of this disease. Cool temperatures, especially at night, are ideal for rapid disease development. Moist air, rainy weather or irrigation practices that extend the amount of time moisture remains on the leaves also encourage the development and expression of downy mildew.

Downy mildew symptoms on Impatiens walleriana typically start with a few leaves that appear slightly chlorotic or stippled, and become completely yellow over time. Some varieties will have subtle gray markings on the upper leaf surface. A white, downy-like growth may be present on the underside of primarily yellow leaves, but can also be found on the underside of green leaves. As the disease progresses, premature leaf drop results in bare, leafless stems. Eventually these stems can become soft and the plant collapses, similar to frost damage.

To help contractors, Ball has posted a list of frequently asked questions and suggested alternative plants on its website.