We have come to rely on impatiens (Impatiens walleriana) as the “go-to” annual plant that will brighten and add colour to the shady nooks in our gardens and add interest to our container plantings. Perhaps no other bedding plant has ever enjoyed such popularity.
However, last summer, you may have noticed strange things happening to your impatiens, such as yellowing leaves or a “downy” greyish film on the underside of the leaves. These are the first symptoms of downy mildew. After those first signs, the flowers and leaves will drop until only the stems are left, leading to the plant’s collapse. Here is some information you can arm yourself with before you head to the nursery:
What is Impatiens downy mildew?
Impatiens downy mildew is caused by an organism called Plasmopara obducens. It appeared several years ago in Europe, showing up in North America in 2011 in Florida, the Mid-Atlantic and the Northeastern United States. Now it is in parts of Canada.
How does the disease spread?
Downy mildew is spread by spores located on the underside of infected leaves. The spores can be spread by wind or rain as it splashes onto the leaves. The organism is a water mould requiring moist conditions to develop spores and cause new infections. Areas of deep shade where foliage stays wet for a long time tends to have a higher incidence of the disease.
Are all impatiens susceptible to downy mildew?
No. Downy mildew attacks only Impatiens walleriana. Other types, such as New Guinea and SunPatiens, are not affected.
Are there any chemical controls that can be used?
There are no fungicides available to gardeners for the control of this disease.
How should I dispose of infected plants?
Do not place infected plants in home composters in case the spores overwinter. Instead, bag and put them at the curb to be composted in a municipal facility (where the heat is high enough to destroy the organism). Alternatively, you can burn or bury the affected plants deeper than 50 centimetres (20 inches).
Should I plant impatiens next year in soil that once grew infected plants?
Experts advise against planting Impatiens walleriana where infected plants once grew. If you want to grow impatiens in a container that previously held infected plants, thoroughly wash the container with soapy water and a drop of bleach. Be sure to use fresh planting mix. Because the disease is specific only to Impatiens walleriana, you can grow any other bedding plants without any risk.