by Eileen Schilling, Horsford Gardens & Nursery
Impatiens Downy Mildew is a disease that is specific to Impatiens wallerianna. All varieties are affected. This is the most common species of Impatiens planted. It includes the Accent, Blitz, Double, Trailing, and Butterfly varieties.
Impatiens Downy Mildew was first reported in the United States in the 1940s but the reports were few and scattered. We did not have much of a plant plug industry at that time and we did not import plants the way we do today. Over the past few years the disease has become devastating to gardeners and growers in the United Kingdom and South Africa, the latter being one of the biggest growers of annuals in the world. In 2011 there were several reports of Impatiens Downy Mildew reported in the U.S. By 2012 the disease was confirmed in 32 states. Landscapes on both the East coast and the West coast saw it sweep through plantings both in the public and the private domain.
Impatiens Downy Mildew is a disease that spreads via spores that travel by air and by water. Wind can disperse the spores over hundreds of miles, which is what happened during the hurricane on the East coast in 2011. If the spores are on the ground or in leaf litter they will spread to the plant when water splashes up on them. The ideal conditions for the spore to thrive are moist conditions and cool nights. They like Vermont summers and spores can overwinter in garden soil.
The symptoms begin with the leaves of a healthy looking plant suddenly turning light green or yellowing. The edges of the leaves curl under. The underside of the leaves have a fine white coating. These are fungus spores. To the gardener it looks like the plants need some water and a shot of fertilizer. Nothing helps and in about 7 to 10 days the leaves drop off and the stems collapse. Once infected, plants do not recover.
I know this disease was in Vermont last summer because plantings in the shade garden at the Nursery exhibited exactly this series of events. We also had many customers come to the nursery in August and ask if we had any idea why their Impatiens suddenly died. They described the life cycle of the disease.
If you had the disease it is not recommended that you plant Impatiens wallerianna in the same spot in the garden since the spores can overwinter in your soil. If you do plant Impatiens wallerianna be on the lookout for the symptoms discussed above.
Infected plants should be removed—roots included—and disposed of either in the dumpster or burned. Do not compost infected plants. Cutting back the plants will not help because the disease is in the entire plant, not just living on its leaves. There are no effective fungicides available to the home gardener.
The production of Impatiens wallerianna is a multi-million dollar industry. As you can imagine, growers all over the world are working together to combat the disease. So far most of the effort is on controlling the disease. There is a good amount of information about the disease on the internet, particularly articles written by plant pathologist at Cornell and the University of Delaware.
Horsford’s Nursery will not be growing Impatiens wallerianna in their greenhouses this year, nor will they be brought in to sell from outside growers. We feel that the likelihood of getting infected plants is great. Since the fungus was present in gardens in the area last year, even if we begin with clean stock the plants can become infected from overwintered spores. We also believe that the best way to stop the disease is to do our best not to spread it.
There are a great many plants that we will be carrying as alternatives.
The New Guinea Impatiens (Impatiens hawkerii) is a whole different species and is not susceptible to Impatiens Downy Mildew. Coleus, Begonias, Fuschia, Lobelia, and Browallia are other great shade choices and the Nursery will have a good supply of these.
For assistance with your shade garden plantings talk to one of our greenhouse growers or staff gardeners.