Downy mildew: A threat to the beloved impatiens
by Mary Tucker
April 10, 2014 09:50 PM | 1260 views | 0 0 comments | 21 21 recommendations | email to a friend | print
Impatiens (Impatiens walleriana) have long been one of the most reliable and floriferous of summer bedding annuals. They grow in shady spots, where a burst of color is always welcome, sporting a long-lasting succession of blooms in a wide variety of shades, including white, pink, lavender, red, and orange. There are also stunning double forms with flowers that look like tiny roses. For many gardeners, impatiens (also known as sultana or busy Lizzie) are a must-have for the summer shade garden or for containers and hanging baskets.

Now that I’ve told you how wonderful this plant is, I have the unhappy duty of informing you that you may not be able to grow this favorite annual much longer! The sad fact is that a fungal disease is threatening the lovely and beloved impatiens.

This disease is a form of downy mildew (Plasmopara obducens) that is specific to Impatiens walleriana. This pathogen has been around for years, but for some reason it has become a serious problem in the last few years and has been spreading rapidly, now affecting plants in many regions of the United States. In 2012, the disease was reported in 33 states, and it could become an even larger problem this year and in years to come.

Downy mildew thrives in damp, cool conditions, so Georgia’s summer heat may stave it off during much of our growing season. However, as cooler weather approaches, be on the lookout for this deadly disease, especially if we have damp weather conditions or if irrigation has kept your garden beds moist.

The disease can hit quickly, with plants seeming fine one day and sickly the next. The leaves will start to curl at the ends and show signs of yellowing. You are also likely to see white mildew that looks somewhat like flour on the underside of the leaves. The damaged leaves will drop off the plant, and then the plant as a whole will die.

The spores spread very easily and are dispersed when splashed with water or when carried on the wind. So if your neighbor’s plants have the disease, it’s likely yours will get it too. Once the disease hits, the spores overwinter in the soil, making it impossible to grow impatiens in that spot for years to come.

There is not much you can do to treat the disease once it affects your impatiens. Fungicides have little effect since the spores spread so easily. Your best remedy is to remove infected plants and dispose of them. Then plan to use an alternative bedding plant in future years.

Nurseries and research horticulturists are attempting to establish growing protocols to help mitigate the plants’ susceptibility to downy mildew. However, the disease has been so devastating in some areas of the country that many growers have severely curtailed their production of impatiens. After all, no one wants to sell a plant that is likely to die in the homeowner’s garden even if it was healthy while at the retailer’s nursery. Let’s hope breeders can eventually develop disease-resistant strains, but that solution is likely to be many years away. In the meantime, expect to see fewer and fewer impatiens in gardens or for sale in retail garden centers.

Though it’s hard to imagine summertime without impatiens, there are some excellent substitute annuals to provide that punch of color in shady spots, some of which make their impact through colorful foliage rather than blooms. One of the best substitutes for Impatiens walleriana are New Guinea impatiens (Impatiens hawkeri), which are not susceptible to downy mildew. Other options to consider are wishbone flower (Torenia fournieri), some of the many begonias (Begonia spp.), coleus (Coleus hybridus), and caladium (Caladium bicolor).

If you get through this year with no problems, don’t be afraid to try growing impatiens next year. There is no guarantee that downy mildew will be a problem in your area since the disease is relatively localized right now. Just carefully inspect plants before purchasing them to be sure that they have healthy leaves with no signs of mildew.

Let’s hope downy mildew finds our Georgia climate unattractive so we can keep growing our beloved impatiens.

Information about Extension Solutions for Homes and Gardens can be found on the UGA extension website, www.caes.uga.edu/extension/cherokee ; or contact the Cherokee County Extension Office, 1130 Bluffs Parkway, Suite G49, Canton, GA, 30114, 770-721-7803. The Georgia Master Gardener Extension Volunteer Program is a volunteer training program offered through county offices of the UGA extension. Follow Cherokee County Master Gardeners on facebook at www.facebook.com/cherokeemastergardeners for gardening tips as well as upcoming seminars.

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