Disease Alert: Impatiens Downy Mildew
Last year and this, Toronto Master Gardeners have been asked why impatiens are so hard to find in garden centres and why, if you can find and plant them, they are dying.
The answer is the epidemic of impatiens downy mildew, which the Wall Street Journal recently dubbed “the Great Impatiens Blight”. It began to appear regularly in 2011, spread widely in 2012 and in 2013, most retailers have reduced their stock of Impatiens walleriana drastically because of the prevalence of the disease.
As described by the Purdue University Department of Botany and Plant Pathology, early symptoms of impatiens downy mildew are difficult to detect: Leaves may be slightly yellow (chlorotic) with stippling and may mimic nutritional deficiency or spider mite damage. Leaves will droop. Under cool, wet, humid conditions, white-colored sporulation may be visible on the undersides of the leaves. Sporulation, however, might not always be visible, especially if conditions are warm and dry. As the disease progresses, plants appear stunted and foliage drops, resulting in green, leafless stems.
Moisture, key to the disease, is necessary for the pathogen to produce spores and cause new infections; this spring and early summer in Toronto has had ideal conditions for it to spread.
There is no cure.
The plants should be removed and put in the garbage, not your compost. Thoroughly clean any containers in which the plants were grown, then soak them in a 10% bleach solution. Similarly, decontaminate any tools that you used to work with the impatiens plantings.
For a more detailed discussion of the disease, see the UW-Madison/Extension Plant Disease Diagnostics Clinic article here.
The good news is that other kinds of impatiens, such as the brightly coloured New Guinea Impatiens (Impatiens x hawkeri) do not seem to be affected by this epidemic. Until a cure has been discovered, Impatiens walleriana is best avoided. Luckily, there are many other great annuals for shady locations. The Annuals for Shade Gardening Guide on this site lists several. Just click here to see it.