Remember that little rumor about impatiens? That there is something out there bent on tracking down, torturing, and destroying all traces of the genus? Well, not exactly, but there is something that’s keeping some of our more commonly used colorful little friends from blooming like they should be. It’s called Impatiens Downy Mildew, and it is a fungus-like pathogen that begins by attacking the leaves of the infected plants. Early symptoms include yellowing, curling, and stippling of the leaves, as well as white, downy spores on their undersides during wet, or humid weather. Hence the name, Downy Mildew. As the infection progresses, plants will bloom less, lose their leaves, and eventually become pretty much a bare stick before succumbing to the final stages of the disease. Spread occurs through soil, wind borne spores, or water splashing from infected nearby plants.
Thankfully, not all impatiens are susceptible, but the common Busy Lizzie (Impatiens walleriana) has been pretty well decimated by the disease in the past few years. Unfortunately, this is probably THE most common species of impatiens utilized. They are the ‘Go-To’ shade plant for vibrant swathes of color. Well, they were anyway. New Guinea Impatiens (Impatiens Hawkeri), however, are highly resistant the to fungal disease (meaning they are very, very rarely infected).
New Guinea Impatiens
Research efforts to create a more resistant variety of Impatiens walleriana are currently underway, but have not been successful as of yet. For this reason, we strongly suggest the use of other highly colorful bedding plants for use in full to partial shade.
Some great substitutes are begonias, torenias, petunias, and coleus. Some people also suggest the more resistant Impatiens hawkeri mentioned earlier, but these tend not to flower as well in deep shade. A more detailed and inclusive list can be found at the American Floral Endowment, which breaks down possible substitutions by region.
If substitution really isn’t an option, then the application of preventative fungicides has been met with some success. Fungicide regimens including sprays like Adorn, Subdue Maxx, Fenstop, Heritage, Protect, and Pageanthave shown to be effective in many cases. A possible preventative regimen can be found here. However, the addition of fungicides after the plant has already contracted IDM, lowers the effectiveness considerably. There is no cure for IDM post-infection.
Consider also, that frequent scouting and inspection of your plants is important for early detection and the prevention of disease spread. Once you find an infected plant, it is important to remove, bag, and seal the plant, and any leaves and petals, before carrying them out to your trash pile. These should NOT be composted, as the infected spores are likely to overwinter and affect the surrounding soil. Similarly, affected areas should be rested from growing Impatiens walleriana for at least a year or more to avoid recontamination.
And, remember, happy planting with whichever route you choose!