This bed of old-fashioned, bedding impatiens thrived all season in 2015 despite the threat of downy mildew disease, that became rampant in 2012.
© George Weigel
Four summers ago, gardeners in at least 20 U.S. states watched their tried-and-true beds of impatiens mysteriously and suddenly die. The explanation turned out to be a virulent new disease called downy mildew that swooped in and killed our top-selling annual flower like a plague. But the worse news was that the disease sticks around in the soil, prompting plant pathologists to warn that most of us might have a hard time growing traditional impatiens (known botanically as Impatiens walleriana) from here on out.
Then a weird thing happened last summer. Many gardeners who insisted on planting old-fashioned impatiens anyway, despite the disease risk – including scores who experienced downy mildew first-hand – found that their impatiens performed reasonably well. So what gives? Is the mildew threat over? Did it peter out in the soil? And most importantly, does it mean we can go back to planting impatiens as in the good old days?
The short answer is, “No.” Like many diseases, impatiens downy mildew can fluctuate in intensity from year to year. One big factor is weather. The microscopic oospores (think of them as germs) that spread this disease via wind, water, and infected soil don’t do as well in hot, dry summers as they do when it’s cooler and wetter when they reproduce. The hot, dry weather that many of the mildew-prone states had from mid-summer on in 2015 probably helped keep a lid on infection. The back-to-back colder-than-usual winters that much of the Northeast and upper Midwest had in 2014 and 2015 also may have reduced overwintering oospore colonies there.
An early sign of downy mildew infection on impatiens is when a gray-white coating forms on the leaf undersides.
© George Weigel
But what may be having more of an impact than anything is that far, far fewer impatiens are in landscapes these days than prior to 2012. Fewer plants means fewer opportunities for an infected impatiens to pass mildew along to neighboring plants. Also slowing down infection is that growers are well aware of the disease and generally treat impatiens with fungicides to prevent the disease. That stops mildew in the greenhouse where the plants are raised, but once out in open ground, impatiens become vulnerable without continued spraying. This is why plant pathologists and growers are still not recommending widespread planting of impatiens in home gardens. Given favorable weather and the presence of fast-spreading oospores, downy mildew could easily return with a vengeance any year.
The first sign of infection is a gray-white coating on the underside of the leaves, followed by yellowing and stunting of the leaves, then wilting and plant death. Plants can go downhill in a matter of days, usually in late July to early August, just as they’re hitting prime. The prevailing advice is to go ahead and try a few impatiens if you like, but don’t be surprised if you get only a half-season out of them.
While downy mildew affects the popular bedding Impatiens walleriana, old-fashioned balsam impatiens (I. balsamina), and related native jewelweed (Impatiens capensis) it does not affect the larger – and more expensive – New Guinea impatiens. Those who garden in shade have switched to that type with success. Others have turned to newly introduced, disease-resistant hybrids of bedding and New Guinea impatiens called SunPatiens®, Bounce®, and Big Bounce®.These have performed well in most gardens and look a bit more like traditional bedding impatiens than the more pointed-leafed New Guinea types. However, they’re significantly more expensive than the previously cheap 4- and 6-packs of bedding impatiens.
These are SunPatiens growing fully and unaffected by the downy mildew disease that usually kills traditional bedding impatiens.
© George Weigel
If you take the risk and go with bedding impatiens, try these steps to maximize your odds:
- Ask the seller if he/she knows what kind of fungicide treatment the grower used. A greenhouse spraying protocol developed in Michigan can give disease protection for months in the field – enough to milk most of a season out of impatiens even if disease is floating around.
- Grow impatiens in beds where disease has not been a problem. Hope that nobody plants infected plants near you.
- Give your plants adequate space, to improve air movement which allows the foliage to dry more readily. Avoid over-watering; and if you must water overhead at all, do so in the morning when the leaves will dry quickly.
- Stick with containers or hanging baskets for your impatiens. At least you have good drainage there, and are starting with fresh, soilless, disease-free potting mix.