Update as of 2020:
This is still useful information for impatiens-lovers, but fortunately it’s not as necessary.
There are now some resistant varieties, the ‘Beacons’ series. There’s not yet a full color selection, but there are some of the most important ones: white, purple, deep rose, orange and red. I hope for pink and lavender soon.
Now, on to the original message, which is still need by those growing any but Beacon varieties.
Impatiens Downy Mildew is a fungus disease that attacks “regular” impatiens, i.e. Impatiens walleriana, the ones we most often plant. Those also include the double-flowered “rosebud” impatiens, and the Fusion Exotic impatiens. It does not affect New Guinea and Sunpatiens. It spreads by spores moving through the air or in splashing water. Wind can carry spores up to 400 miles. It may be able to overwinter in the soil in beds where it has infected impatiens the previous summer. It thrives in cool, damp conditions, but is still dangerous even in warm dry summer weather.
Why haven’t I heard of it before?
It is a new disease in the world, a new genetic variation of some other form of downy mildew. (There are many different strains of downy mildew, most of which infect just a few types or only one type of plant) It started in Europe in around 2009, where it devastated impatiens plantings in several countries. It spread to the eastern USA in 2011, and made it to Oregon in 2012.
What will it do?
It will kill impatiens, sometimes slowly, sometimes surprisingly quickly, depending on conditions. The first early symptoms are yellowing of the leaves, which might just look like they need fertilizer. The next step is leaves curling downward, and flowers decreasing in number and size. By that time you may be able to see velvety (downy) white stuff on the undersides of the leaves. You will not see the white powdery look of Powdery Mildew on top of the leaves. The next stage is leaves falling off, until only bare green stems are left. At last even the stems collapse.
What can I do?
There are fungicides that we professional growers can use to produce clean, healthy crops of impatiens, but their cost and the complexity of their use is too much to make sense for home gardeners to try to use. The best things to do are:
1. Consider limiting impatiens plantings to planters and hanging baskets, where they will be less susceptible since they are away from cool damp soil.
2. Keep your impatiens well fed and watered since healthy plants are generally more resistant to disease than hungry plants.
3. Watch for signs of disease and quickly remove any plants you are pretty sure have it. If leaves have fallen, pick them up, too. Seal everything in a ziplock bag and put in the trash – do not compost.
Should I plant any impatiens?
If you are willing to risk a certain amount of money and effort, go ahead. Downy mildew will probably hit some areas and not others – you might be lucky. Wind patterns may be on your side and spores won’t blow your way. If you have a beach house, there will probably be no problem there because the winds off the ocean will be spore-free. And, as mentioned above, container plantings are safest, and they are also easiest to replace.
What else can I plant instead?
Sunpatiens, which are also good in shade, are a very good substitute, and almost have the same color range, just so far lacking a good true pink. The plants are larger than regular impatiens, but they’ll give you the same nice spreading shape. The main drawback is they will cost you more, because they cost me more to grow. But if they were spread farther apart, so fewer were used, it might not be too big a cost difference.
New Guinea impatiens have a great color range and shape. Some people find them to be less vigorous, not as easy to grow well, others have good success with them. It probably depends on the conditions in your yard. They also are higher priced than regular impatiens.
Wax Begonias are a good substitute, since they handle full or partial shade well, are easy to grow, with low maintenance. They are the same price as impatiens, and are available in jumbo packs like 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.
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 is not deadly in their case, but it makes it better to wait to plant them outside until the weather takes on real summer warmth. From then on it doesn’t bother them. They are very colorful and low maintenance. There are inexpensive, impatiens-priced varieties available in 4 inch pots and jumbo packs, and more expensive varieties.
Torenia are a snapdragon-like plant – great for people who love blue and purple. The seed-grown bushy type will grow fairly similarly to impatiens and be the same price as impatiens. The more vigorous trailing kind will be flatter if planted in beds and will cost more.
Polka-dot Plant, like coleus, has colorful foliage that looks great in the shade. The color range is limited to pink, white and soft red, but they’re pretty, easy to grow, and low-priced.
Tuberous Begonias, such as the Nonstop variety, get big, lush and gorgeous.
Hiemalis Begonias, including the Solenia types, make wonderful masses of flowers in a huge range of colors.
Will the disease ever get better or go away?
It is too soon to know yet. Last year in Europe where the disease struck first, it wasn’t as bad as in previous years. It’s unlikely that it could ever completely go away, though. What’s more likely is that plant breeders will develop new hybrids of the various species of Impatiens that will grow like our old favorite but have the disease resistance of another species. Let’s hope they come up with some good ones soon. In the meantime, providing fewer impatiens for the mildew to prey on is a good way to limit its spread.