|Photo courtesy of Michigan State University|
While there have been sporadic reports of this disease in production greenhouses in the United States since 2004, widespread regional outbreaks of impatiens downy mildew were observed for the first time in North American landscapes in 2011. The organism that causes downy mildew is a type of “water mold” or oomycete. Downy mildew can spread by two different types of spores. One type is easily airborne and remains viable for just a short time; and the other type, a zoospore, moves through a film of water.
The spores develop and infect impatiens when a film of water is present on the plant tissue, and the relative humidity in the air is high, during cool or warm periods. Sporulation and infection will not occur under hot or dry conditions.
|Photo courtesy of Palm Beach County Extension|
Remove and dispose of infected plants (roots included) immediately. Do not compost the infected plant material. Fungicide products available to homeowners provide little control. Avoid overhead irrigation (especially night-time irrigation) and any conditions that result in long periods of leaf wetness.
The disease can spread rapidly from one landscape planting to the next so that in the course of a summer one infected planting could take out many more over a large area. The spores can fly up to 400 miles. As the impatiens die of the disease, the fungus drops overwintering spores to the ground so impatiens planted there within the next 2-3 years will also become infected.
Ellen Egan’s advice to all gardeners and landscapers: If you use just a few impatiens in container plantings, not an amount that will break your budget or your back to replace if they get sick, go ahead and risk using impatiens as usual. However, if you plant large beds of impatiens, switch to something else for a couple of years at least, while we see what happens. The fewer host plants there are for the disease, the less it will be able to take hold in our area. Fewer beds of impatiens this year means fewer spores flying around, and fewer beds infected with overwintering spores next year.
Alternative plant choices:
Wax Begonias are a good substitute, since they handle shade well, are easy to grow, and are the same price as impatiens. The main drawback is the lack of color choices – no purples, oranges, or intense hot pinks. Also they don’t quite spread out the same way. But they are very nice once you make up your mind to like them.
New Guinea Impatiens have a great color range and shape, but tend to be less vigorous, not as easy to grow.
Coleus are a good shade-loving plant, and look great in big beds. The drawback is that they get their own strain of downy mildew, which makes it unsafe to plant them outside until the weather takes on real summer warmth. From then on it doesn’t bother them.