Janice Peterson: Impatiens Downy Mildew—a devastating disease for a garden favorite
One of the basic tenets of good gardening is crop rotation. Home vegetable gardeners know they shouldn't plant tomatoes, potatoes or peppers (all members of the Nightshade family) in the same spot year after year because of the potential for disease build up in the soil. It's good practice to alternate regularly with a completely different crop like squash or green beans. The same is true for annual flowers—we really shouldn't be growing the same ones in the same spot every year.
I know what I'm supposed to do, but it's easy to get into comfortable habits especially when dealing with a shade garden workhorse like impatiens. I've been planting a couple of flats of impatiens in my shady borders for the last 20 years. They give me continuous color the entire growing season (until the first frost). So far I've been lucky but my luck may soon run out with the recent spread of Impatiens Downy Mildew, a new and pretty devastating disease of bedding impatiens (Impatiens walleriana).
Impatiens Downy Mildew is a fungal disease that strikes when the weather is cool and wet. Impatiens will quickly show symptoms—leaf yellowing and curling, fuzzy white coating on the underside of leaves, and eventual defoliation, leaving only bare stems. It affects single and double impatiens, but not New Guinea impatiens or most other plants. Once the disease is present impatiens can't be grown in that soil for several years. I haven't seen this disease yet but from what I've read it's just a matter of time.
There are other shade tolerant annuals that deliver lots of color, depending on the level of shade. New Guinea impatiens, begonias, coleus, and fuchsias are all good options. Rotary Botanical Gardens has several shade gardens that display many beautiful annual alternatives to impatiens. I really like begonias for their continuous bloom, availability and easy care. Michigan State University offers a great list of alternatives to impatiens.
I'm trying to keep a positive attitude and think of this as a gardening opportunity to try new plants—although I'm sure to sneak in a few more impatiens as long as I can get away with it!