A healthy patch of impatiens should look like this by mid-summer.
© George Weigel
A deadly new strain of downy mildew disease is wiping out impatiens in many parts of the United States this summer. This disease devastated impatiens to such a degree in Europe last year (2011), that growers and garden centers far and wide stopped stocking them. Last year it was reported in just a few parts of the United States, but in 2012 it has spread quickly throughout much of the North and East, and also parts of the Southeast. In this country it poses enough of a threat today that some garden centers are advising gardeners to avoid planting impatiens.
Downy mildew disease is thought to be capable of overwintering in the soil throughout most American climate zones, but growers are hoping that the disease won't be as widespread here because our climates vary so much from region to region. For instance we know that downy mildew doesn’t do as well in hot, dry regions as it does in cooler, damper ones.
Symptoms & Cause
Downy mildew is a disease that causes stunted growth, leaf drop and eventual wilting and death of entire impatiens plantings. The disease is caused by a fungus-like water mold (Plasmopara obducens) that spreads primarily on wind currents.
This patch of impatiens is wilting away as a result of downy mildew disease.
© George Weigel
Although downy mildew has been known for at least a century, it became a big problem for impatiens in the last few years as a more virulent strain developed. Gardeners usually first notice the disease as a general stunting and yellowing of the leaves, which they usually figure is either bug damage or a nutritional problem. Insecticides and fertilizer, however, don't help. As the disease progresses, the leaves usually curl down around the edges and start to drop. The tell-tale sign is a felt-like, white coating on the leaf undersides. Infected leaves carry the spores with them as they drop to the ground, and infect the soil. There is no effective and practical control for home gardeners. Growers are able to keep a lid on the disease by spraying weekly with fungicides that aren’t typically available to home gardeners.
What to do?
Once impatiens planted in the garden become infected, it’s difficult—if not impossible—for gardeners to control the disease. Plant experts advise gardeners to destroy infected plants and discard them with the trash. Do not put them on the compost pile or it will also become infected since composting may not kill the pathogen. Because the disease flourishes in damp weather and humid conditions, gardeners can slow the spread by avoiding overhead and overnight watering, although that alone isn’t likely to stop an outbreak, if spores are present and conditions are right.
The tell-tale sign of a downy-mildew infection on impatiens is the white felt-like coating on the leaf undersides.
© George Weigel
Even if downy mildew doesn't wipe impatiens off the entire U.S. map, it’s serious enough to knock the plant off its throne as America’s top-selling annual flower. Impatiens have become our go-to annual for shady settings. Until now, it’s been a plant that’s easy to grow and widely available in many colors at affordable prices. If you've got downy mildew on your impatiens or it shows up nearby, there’s a good chance it'll kill future plantings. That’s why some growers and garden centers in regions where the disease reared its ugly head this summer are already advising gardeners to avoid impatiens until mildew-resistant varieties or better controls are developed. Presently there are no downy mildew resistant varieties of bedding impatiens in the marketplace. Besides infecting common impatiens (Impatiens walleriana) sold in garden-center packs, downy mildew also infects naturalized, orange-flowered jewelweed or touch-me-not (I. capensis), a related species that serves as a host and disease-spreader.
New Guinea Impatiens is a different species thought to be highly resistant to downy mildew. The new SunPatiens® —a hybrid with New Guinea types as one of the parents—is considered mildew-resistant as well. These plants are more expensive than typical bedding impatiens and tend to be used in containers and window boxes.
Beyond that, gardeners can avoid the disease by switching from impatiens to other shade-tolerant annuals, such as wax and other begonias, coleus (Solenostemon, now Plectranthus), wish bone flower (Torenia), bush violet (Browallia), angel’s wings (Caladium) and polka-dot plants (Hypoestes). In light shade try sweet alyssum (Lobularia), calico plant (Alternanthera), petunias, dwarf spider flower (Cleome), flowering tobacco (Nicotiana), cup flower (Nierembergia), ivy geraniums, verbena and periwinkle (Vinca).