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Cornell University looking to find new downy mildew resistant impatiens
Impatiens Downy mildew is an extreme disease that demands attention. This disease on Impatiens walleriana is particularly thorough, systematically impacting each area of the plant from the flower to the roots until nothing remains.
“There are some viruses that create nice patterns that you can appreciate aesthetically, but you can’t appreciate this disease in any way—especially since it takes the flower out first. That’s a real low blow,” said Margery Daughtrey, Senior Extension Associate, Cornell University.
Although this disease was originally identified in 1897 on wild impatiens in Vermont, this pathogen didn't really resurface significantly until 2011, when there were outbreaks in several states. In 2012, impatiens downy mildew became widespread, impacting both growers and gardeners alike. One interesting aspect of downy mildew is that in the balsam impatiens, which is also susceptible to the disease, the leaves get very large spots but don’t fall off. However, the pathogen is easily transmitted from the balsam species to the walleriana one.
“If there are a lot of balsam plants in people’s gardens, the disease might come back courtesy of the balsam. And that same fungus that’s causing leaf spots on the balsam can completely destroy Impatiens walleriana,” said Margery.
This particular downy mildew appears to be extremely durable, with evidence of the disease being able to overwinter. One of the challenges is that gardeners don’t have the tools available to protect their garden from this disease, and so, Margery is investigating the downy mildew’s ability to make oospores (which is how the pathogen likely survives in the soil). Currently, the research is looking into how these oospores are formed and whether their formation can be blocked. The challenge is discovering what allows this pathogen to survive in the soil, with the long-term hope of freeing the industry from this disease.
One of the main challenges is protecting the plant after it leaves the security of the greenhouse environment. This is the reason why Margery recommends applying a preventative fungicide before the plant leaves the greenhouse environment.
“It’s a national problem, or actually, an international problem. Its severity has to do with the climate in the production area and it also has to do with the climate in the garden,” said Margery. “As we watch the disease unfold from year to year, we’ll learn something more about how it will vary from year to year and how many treatments with fungicides we need to prevent it. Those kinds of things will become clearer as we have more experience.”
Prevention is the Key
At the moment, there’s no cure for impatiens downy mildew. Margery emphasizes that usually by the time fungicides are applied to treat visible symptoms, it’s too late. However, there are ways to protect your impatiens from this disease.
• Monitor the humidity level in your growing environment; high humidity accelerates its growth.
• Use fungicides as a preventative measure to keep the disease controlled in the greenhouse.
• Plant your Impatiens walleriana crop from seed (to avoid possible inoculum from others’ greenhouse crops).
Future Research
At Cornell University, fellow Cornell researcher Mark Bridgen’s graduate student is looking into breeding new varieties of impatiens with the hope of finding a type resistant to downy mildew. Margery believes that the answer may reside in nature. She believes that in order to understand the pathogen better, it’s best to observe it in its natural setting, where the pathogen acts much less harmfully with wild impatiens.
“I would like to watch it from year to year and see how it operates there,” said Margery.
Future studies will also include manipulating the oospores to see how they react to different fungicides. Ideally, Margery would like to find a fungicide that would block the oospore production, which would offer a strategy that gardeners would be able to readily apply.
“Prevention is much more important than ‘treatment’ for downy mildew because I think it’s probably a pipe dream to think you can stop it in your greenhouse if you see even a little bit of it,” said Margery.
Researcher: Margery Daughtrey, Senior Extension Associate Department of Plant Pathology, Long Island Horticultural Research and Extension Center, Cornell University
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