Alert: Downy Mildew on Impatiens This Year
Posted: August 20, 2012
Callers note that affected plants are stunted, yellow and that they look like sticks, with a few yellow leaves hanging on.
Downy mildew of standard impatiens (Impatiens walleriana) is a relatively new disease, although it has been known in the United States since 2004. Yellowing and stunting of affected plants is usually the first symptom that growers notice. Close examination of those plants reveals a fuzzy white growth on the undersides of the leaves. Symptoms on the upper leaf surface often begin as discrete yellow spots. Infected plants are reduced to barren stalks as they lose their leaves. The causal organism, Plasmopara obducens, is more closely related to water molds such as Phytophthora than true fungi. Infection is favored by cool temperatures in the 58- to 72-degree range and humidity above 85 percent.
Downy mildew showed up early in 2012
Greenhouse growers reported problems with downy mildew on a variety of crops quite early this year, likely as a result of the very mild winter and early onset of spring. Although growers scout and treat crops preventatively for downy mildew, those fungicides only provide protection for so long. Once impatiens are moved through retail outlets and planted in landscapes, little protection is left.
How Did It Wind Up in My Garden?
If you had problems with down mildew on impatiens in 2011 and planted them in the same place in 2012, the disease may have survived the winter on plant debris in the bed. Traditionally, downy mildew does not overwinter well in colder climates. It overwinters in warmer areas of the southern United States and is blown north on the wind and with storm currents. Transplants can become infected after they have been planted out in the landscape, so do not be in a hurry to blame your supplier.
How Do I Control It?
Fungicide applications are not as useful once impatiens have become infected in the landscape. Control focuses on sanitation and planting resistant species. Remove and bag infected plants as soon as they are noticed to reduce the spread to healthy plants. Allow adequate space between plants at planting to permit good air circulation, and avoid overhead irrigation, especially during cool weather.
All standard impatiens – Impatiens walleriana – are susceptible to impatiens downy mildew, including doubles, minis and interspecific hybrids such as Fusion® impatiens. If downy mildew has spoiled impatiens plantings this year, avoid trying to grow them in the same area next year. Annuals that are resistant include New Guinea impatiens (Impatiens hawkeri), begonias and coleus.
Other species of downy mildew are commonly associated with cucurbits, roses and grapes. But those plants are not infecting impatiens, and impatiens are not infecting them.