There is a new disease that is showing up in flower gardens all over the country that is attacking impatiens. Impatiens Downy Mildew was first seen in a few areas of the country but now seems to be fairly wide spread, including here in the Dakotas. This past year was the first time that I saw it in impatiens growing here at McCrory Gardens. The symptoms are not real noticeable at first, just some downward tipped leaves and maybe some grayish fuzzy growth on the undersides of the leaves. After a couple weeks it will cause major yellowing of the leaves and many of the plant's leaves and flowers will fall off. By late last summer, most of them were dead in one of our big shady garden areas. We didn't see the problem in other areas where we had impatiens but I suspect that it will become more widespread as time goes on. The disease is favored by cool wet weather, which certainly does not describe last summer, but we got it anyway. It is spread by splashing water, like from overhead irrigation, which we did do a lot last summer, and also by wind.
The good news is that this fungal disease mostly affects the typical garden impatiens, Impatiens walleriana. These are the plants that so many gardeners like to use in their shade gardens because they are easy to grow, have great flowering and look good all season long. Some other types, like the New Guinea Impatiens seem to tolerate or are immune to the disease. It can, however, affect some wild species and balsam, which we do like to grow here at McCrory.
If you are producing impatiens, know that the disease is primarily spread when cuttings are taken from infected plants. There is no evidence that the disease can be spread with seed propagation. But, that means that if you are growing impatiens in your greenhouse, separate the vegetatively propagated plants from those that you grow from seed. Also, carefully inspect and plugs or cuttings that you might buy from a wholesaler or distributor and look for symptoms. If you do a search for Impatiens Downy Mildew you will see many excellent articles on the disease, how to identify it, fungicide treatment strategies and other suggestions.
The bad news is that once the disease appears, there is not much you can do to control it. There are some protective fungicides that can be used but they must be applied repeatedly and it is best to switch to different fungicides to avoid problems with resistance. Cultural control strategies include increased plant spacing, improve air flow around plants and water early in the day so plants will dry off quickly. Do not water late in the day or at night which would allow for much faster disease development. Watch for infected plants and if you can, completely remove the plant, including the roots and get it out of the garden. We don't know for sure if the spores will be able to overwinter here in the Dakotas or not. Again, we had unusually warm weather last winter, which may have led to more plants getting infections from earlier plantings.
If you have shady gardens, it is probably not a good idea to rely on regular impatiens to be your primary flower this coming growing season. Rather, consider some alternatives, like the New Guinea or SunPatiens that are becoming more