There's been an increase in downy mildew affecting Impatiens plants recently. The particular mildew affecting impatiens is Plasmopara abducens. It spreads rapidly, under the right conditions, and once infected, there is no cure. However, there are some preventative measures you can take. But first, here's some background to help you understand the disease.
What is Downy Mildew?
Downy mildew diseases are caused by oomycetes or water molds. They are fungus-like, but more closely related to algae.
There are 2 types of downy mildew spores. One type, zoospores, can be splashed up by water or spread by the wind. The other type, oospores, reside inside the plant tissue and can spread rapidly and over-winter. At this point, there is no evidence that this particular mildew affecting impatiens is doing that, but why take chances.
Downy mildew is more prevalent in the spring and fall when the cool, wet or humid weather provides ideal conditions.
Which Impatiens Varieties are Affected?
According to an e-Grow Alert by Nora Catlin, Floriculture Specialist, Cornell Cooperative Extension of Suffolk County, the following types of Impatiens are susceptible to downy mildew.
- All Impatiens waller IANA, including the double and mini types. These are the most common bedding impatiens sold in garden centers. (I. waller IANA hybrids, like the Fusion® series, are less prone, but still susceptible)
- Balsam Impatiens / Garden Balsam (I. balsam in a) These are taller plants than common Impatiens, with elongated, pointed leaves.
- Jewel Weed (I. capers is) and Yellow Jewel Weed (I. pallidum). Generally considered weeds, but if they are in the area, they can spread the disease to your landscape Impatiens.
Impatiens Not Affected
- Himalayan balsam / policeman’s helmet (Impatiens glanduliferous), a wild and sometimes invasive species that can reach heights of 6 - 10 ft. (2 - 3 m), is tolerant of the disease but can act as a host.
- New Guinea impatiens (Impatiens hawkers) and its hybrids, like SunPatiens®, have so far shown resistance.
There may be more plants affected or that act as hosts, but this is the information so far.
Symptoms of Downy Mildew on Impatiens
Early symptoms can be hard to spot and might be mistaken for a nutrient problem.
- Leaves stippled or chlorotic (yellowing) and looking like they need fertilizer.
- Floppy or downward curling leaves.
- Sometimes white fuzzy spots will form on the undersides of the leaves. These are spores and usually appear in cool, damp weather. (See photo on page 2.)
- As the disease progresses the plants may stop growing and look stunted, drop their leaves or completely collapse.
What to do about downy mildew on impatiens.
What to Do about Downy Mildew on Impatiens
Although there is no cure for Impatiens already infected with downy mildew, there are some steps you can take keep it from spreading.
Avoiding downy mildew entirely can be very difficult, because so much is weather dependent. High humidity and cool, damp weather cannot always be avoided. But there are a few growing conditions you can provide, so as not to make matters worse.
- Provide good air circulation. Don't overcrowd your plants.
- Water from below, trying not to wet the leaves when watering and water early in the day.
- Remove any suspect plants immediately.
Cultural management is your best bet. Once a plant is infected, there is no cure. If you want to try controlling the spread of the disease with a preventative fungicide, neem and copper sprays are recommended for homeowners.
Will it Over-Winter and Re-infect Plants?
So far, there is no sign that it infects seed, although other mildews do - so there is a chance. Some mildews can also over-winter in plant tissues. Although that does not seem to be happening with this downy mildew, don't take a chance by composting affected plants.
If downy mildew becomes a major problem in your garden, you should consider not planting them for a year or two. Shade plant alternatives to impatiens include New Guinea impatiens, begonias, and coleus.
Impatiens Plants Attacked by Downy Mildew. University of Massachusetts Center for Agriculture