Impatiens Downy Mildew
The Impatiens Downy Mildew Issue Persists
Impatiens are a staple of our shade gardens. The loss of these bright and beautiful shade bloomers felt absolutely devastating to many gardeners, not to mention the lost revenue for growers and nurseries.
The problem surfaced before 2004 and had spread intensely by the spring of 2013 and continues to be a serious problem for growers and gardeners. Impatiens Downy Mildew, IDM, is caused by Plasmopara obducens, a fungal disease. Initially we all hoped it would be limited to the walleriana species and be quickly controlled. Fortunately it remains limited to the walleriana species but the problem persists and became widespread. Researchers continue to investigate why a centuries old pathogen to suddenly explode into a devastating disease.
Many of us have continued to purchase the afflicted impatiens if we can find them and if they look healthy. There is no other alternative that can match the performance, appearance and color of Impatiens walleriana so we risked it. In fact, some years they performed well all season. More than once, however, I have lost every plant. If the plants are infected, there is absolutely nothing that can be done and the Impatiens will die.
That risk we have taken in the past was unknowingly a mistake. We have learned as more research was done that we have likely contaminated our soil for years to come. The fungus spores can spread by wind and watering splash, these are short lived spores but can spread to other plants rapidly. There is also a spore that can survive for years in the soil, even through winter. These resistant spores seem to be released into the soil when the plant dies. Whether that can be prevented by removing plants immediately when symptoms appear is unknown but unlikely. Nonetheless, plants diagnosed with IDM should be completely removed including roots as quickly as possible and destroyed. Never compost the infected plant material. Fungicides used by growers can help to prevent the infection but will do nothing to cure and infections.
The good news is growers keep working to control the fungus by preventing it in the first place. Gardeners are looking for evidence of the fungus when selecting bedding plants and watching for it to develop in their gardens. If the fungus has been found to be a current problem in your region it is likely that you will not even find impatiens for sale. It is very important to the growers and nurseries to protect their future crops by destroying infected plants rather than selling them. So, yes, I have been buying impatiens again with a watchful eye.
If you do continue to plant impatiens, you need to know what to look for so that infected plants can be removed quickly. And we have probably all been fooled in the past by the initial symptoms. The leaves may start to look a little pale so we fertilize, wait and watch. Next the leaves get very yellow, and we may now hope beyond hope that they need even more fertilizer. Please don't do that, remove those plants immediately. Next the leaves all drop very quickly, the blossoms will all be dead and you are left with stick-like stems. You may never even see any powdery mildew like substance under the leaves, but if you do there is no doubt your plants are infected by Impatiens Downy Mildew.
If you have planted infected impatiens your best bet is to plant an entirely different shade plant for the next several years to avoid constant re-infection from the soil. New Guinea Impatiens are much less susceptible to IDM, but since they can also be infected an entirely different plant genus would be advised for a while. As a preventive measure where you have never planted infected Impatiens, New Guinea Impatiens would be a good choice.
Do consider some alternatives to Impatiens walleriana until there is better control for IDM:
Bounce impatiens have a similar look with excellent blooming for sunny and light shade only.
Sunpatiens have large colorful blooms that grow in full sun and good tolerance to heat.
New Guinea Impatiens is probably our best alternative to Impatiens walleriana with big colorful blooms in both shade and sun. The plants are overall larger.
Begonia are colorful and do well in shade or sun.
Caladium are a very colorful large leaf plant that do very well in shade.
Coleus, like Caladium are also very colorful leafy plants. Some are most colorful in shade, some are most colorful in sun.
Heuchera Coral Bells offer many options for colorful foliage, many of them prefer shade.
And for the best news, a new high resistant PanAmerican Impatiens by breeder Ruud Brinkkemper is looking very promising. It is a walleriana originating from Super Elfin series impatiens that has been phenotyped and genotyped for visual characteristics from an Impatiens mutation (we don't know if it was a natural mutation or identified through gene sequencing) to resist the fungus. If all goes well, we will know soon enough. The plan is to launch the seed in 2020.