The ever so popular bedding plant Impatiens walleriana has become a staple of shade gardens everywhere because of their ability to produce brightly colored flowers from spring until frost. A few years ago we noticed a slight decline in their performance, what started as a patch or two has now escalated to entire displays being wiped out across the city. Click here for more information from the BC Ministry of Agriculture.
A pathogen in a form of downy mildew called Plasmopara obducens. It first appears as a white layer on the underside of the leaves which then turn yellow, the flowers then fall off leaving only bare branches. The stems eventually completely collapse and the plant dies. It first 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 there any chemical controls that can be used?
There are no fungicides available to gardeners for the control of this disease.
Experts recommend waiting 3-5 years before attempting to plant Impatiens again. The downy mildew pathogen has the potential to lie dormant in soil until the following season when it can infect newly planted impatiens.
Cost increases to flower displays:
Our options are limited for shade tolerant annuals. We experimented last year with the use of New Guinea Impatiens in these low light conditions and were pleased with the outcome. Our old favorite Impatiens walleriana is an inexpensive bedding annual, unfortunately replacing them with New Guinea Impatiens or other alternatives such as Coleus or Fuchsias which are double the cost will result in an increase to our Spring seasonal flower displays.