2012 and 2013 UPDATE: Many of our customers saw Downy Mildew on their Impatiens in 2011 and 2012 – if you decide to plant them again in 2013 it will be at your own risk.
Last summer many of our Cape Cod customers were shocked when the common impatiens they’ve relied on for years seemed to just melt away. This happened in small window boxes and large estate bedding areas and was seen all over the Northeast. It struck plantings on some properties as early as late-June, and others in September. Homeowners and landscapers alike watched what was normally a sea of color turn into a forest of bare, ugly stems.
The culprit was impatiens downy mildew disease. This is a fungus-like water mold with the scientific name Plasmopara obducens. The spores swim though water so this disease spreads quickly in moist or wet conditions. Spores are also carried by the wind over great distances; this explains why some people saw damage early in the summer while others didn’t lose their plants until fall. Despite having a similar name, this problem isn’t related to powdery mildew.
The question on everyone’s mind, of course, is “Can we safely plant impatiens this summer?” Unfortunately, the answer isn’t clear.
We know that spores of this disease can over-winter in the soil. We don’t know if or how the mild winter will affect this. Also unknown is if future weather conditions will promote or hinder disease problems. Most experts in the field recommend being safe and not planting common bedding impatiens (Impatiens walleriana) where there was a problem in 2011.
Fortunately, New Guinea impatiens and all types of begonias are not affected by downy mildew. Those who want to avoid a recurrence of the problem should plant these or other shade/part shade alternatives.
Is there any control of downy mildew and is it safe to replant this annual this year? If you or your customers want to plant Impatiens walleriana as usual, you can drench the soil with Actinovate and before planting and to spray the same product on weekly, particularly the underside of the foliage. You can also use another fungicide that is labeled for downy mildew. This treatment has been shown to at least slow the disease down in warmer, drier conditions, but if the summer is cool and damp you’re likely to have your plants affected anyway. There is no “silver bullet” cure for Impatiens downy mildew. If you want to be sure to have flowers, plant begonias or other shade-lovers instead. Download a pdf sheet listing good alternatives for common impatiens from our handouts page on the website.