The leaves of an impatiens plant affected by IDM will turn yellow and fall off before the plant dies.
Photo courtesy of Jill Calabro

In May 2018, the Horticultural Research Institute (HRI) announced that it was going to invest more time and resources into Plasmopara obducens — the causal agent for impatiens downy mildew (IDM). According to HRI, IDM’s spread has caused a decline in sales of impatiens; the USDA’s National Agricultural Statistics Service estimates that impatiens sales dropped from more than $266 million in 2009 to just under $215 million in 2014.

As a result, a team of researchers is looking further into IDM and how growers can best treat it. Some, like Margery Daughtrey from Cornell University and Dr. Mary Hausbeck from Michigan State University, are researching which products best treat IDM. Others, such as Dr. Cristi Palmer from Rutgers University and Dr. Catalina Salgado-Salazar from the USDA, are looking into the disease’s history to determine what makes it such a problem. And others, such as Dr. Nina Shishkoff from the USDA, are working to better understand IDM’s life cycle.

The history of IDM

Plasmopara obducens was first reported in the United States in the 1880s when it was spotted on a native impatiens called orange jewelweed. Impatiens walleriana, the most common host of IDM and considered to be the most susceptible of the roughly 1,300 impatiens species, was introduced to the U.S. in the 1960s, according to HRI.

But IDM was not a significant issue for the green industry until 2004 when widespread infections were reported in multiple states for the first time, says Dr. Jill Calabro, the research and science programs director at HRI. It’s also unclear exactly why it took more than a century for the disease to wreak havoc on impatiens in production.

“It quickly grew from a minor problem,” Calabro says. “There are reports of businesses going out of business because of this one disease. And that all happened within a decade. That’s an extremely quick shift.”

While Calabro says the answer is still unclear, there is one cause researchers are exploring.

“One theory is that there was a population shift among the pathogen, Plasmopara obducens,” Calabro says. “There is one team of researchers right now [working on this project] that is able to go in and compare the genetic population of modern times — genetic isolates from now — to isolates from the late 1800s. It’s a powerful molecular technique that can provide a lot of information. They are still working through this, but early indicators seem to indicate that the populations are very distinct.”

Current IDM research aims to establish how the disease infects plants and the best ways to combat it.
Photo courtesy of Jill Calabro

How to spot IDM and how it spreads

According Calabro, IDM has defining characteristics that are telltale signs a plant has been infected. Growers, Calabro says, should also understand that once a plant is infected, it’s going to die.

“Impatiens downy mildew starts as really slightly chlorotic leaves,” she says. “There might be a bit of stippling associated with it, but it quickly advances and leaves quickly become yellow and then they’ll eventually fall off. There will be a dramatic leaf drop. IDM will kill the whole plant, but first you’ll start seeing these yellow leaves, the leaves will drop, it will kill the flowers and basically it will leave nothing but these stems — sad, little sticks of impatiens — sticking out of the ground devoid of flowers and stems.”

Calabro adds that, in some cases, a white powdery substance will appear on the undersides of the leaves.

“That doesn’t always happen with [IDM], unfortunately, but if you see the white downy on the underside of leaves, that’s a pretty good indication that you’ve got impatiens downy mildew,” she says. “It’s very much dependent on the environmental conditions.”

IDM spreads across the greenhouse in a number of ways, including wind, rain splash and irrigation splash. Cool, wet conditions are also the most common characteristics that allow downy mildew to appear.

“Shaded areas tend to be hot spots, just because they stay wet for long periods of times,” Calabro says.

Growers should also be aware of two types of IDM spores. On the underside of leaves are the short-lived spores that will not survive the winter. But there are long-term resistant spores that are formed in infected plant material — meaning they cannot be spotted. Once they infect a plant, and kill it, the spores are released into the soil.

“Those spores can last years,” Calabro says. “One of the unknowns right now is how long these spores can survive, but it’s thought that it could be as long as five to 10 years.”

What the research hopes to accomplish

Since 2004, impatiens has gone from being the No. 1 bedding crop in annual sales to No. 3 because of IDM, moving it behind petunias and geraniums. According to Calabro, IDM research matters because impatiens are still a major crop in the green industry, with growers, landscapers and homeowners looking for a solution.

“Impatiens is a very important crop in our world,” Calabro says. “But it has kind of fallen since the rise of impatiens downy mildew.”

Researchers face a few different difficulties. With the estimated 1,300 varieties, there are species like Impatiens walleriana that are highly susceptible to IDM and others (like New Guinea impatiens) that are not entirely resistant, but more resistant. The hope is also that new products can be found to replace some of the fungicides that IDM is becoming resistant to. Calabro also hopes that the inoculate point of the disease can be found, as no one is currently sure how exactly IDM gets started each year.

“That’s a long list [of topics] and there are probably more,” Calabro says.