Hello, I planted impatiens in window boxes for a shady balcony. I changed the soil to new potting soil, thinking the Downy Mildew fuss was over. They started growing OK, but now I am getting speckled blooms, some withering and lack of new buds. Attached is a photo. Can you please tell me if this is Downy Mildew, or Powdery Mildew, and if it can be treated? Many thanks.
Thank-you for contacting Toronto Master Gardeners with your question about Impatiens. The effects of downy mildew (caused by Plasmopara obducens) on Impatiens walleriana, beginning in 2011 were devastating. Much has been done since then to combat the problem through breeding disease resistant varieties and the development of effective fungicides. I am including links to information about the fungus and the current state of science on the topic (at the end of my answer).
I am not sure, however, that your plants are showing symptoms of downy mildew. Normally, some spotting would occur on the leaves, and some curling. Within a short period of time the tell-tale fuzzy growth of mildew on the leaf surface would be visible. If you cannot see any fuzzy spots on the under sides of the leaves, the spotting on the flower petals may be attributable to another fungal disease, botrytis blight (caused by botrytis cinerea). Overcrowding and a lack of air circulation can lead to this condition, especially with an onset of cool, wet weather such as we have experienced recently. It is treatable by fungicide, if you catch it early enough. You can obtain fungicide at your local garden centre. It is also important to clean up any dead or dying foliage around the plants.
The following link includes a useful table to help identify diseases in impatiens:
The following link is to an answer to a similar previous question regarding downy mildew on impatiens:
The nest links are to articles with photos and detailed descriptions of downy mildew on impatiens:
I hope you are able to keep your lovely impatiens plants growing well!