Mysterious Flower-Killing Mildew Makes Gardeners Lose Impatiens

Judi Whalen suspected something was wrong with her backyard garden in suburban Maryland when some of her normally colorful impatiens plants turned black.

“For no reason, it seemed over the last two years at least one or two of the plants would suddenly wither and just die,” she said in a phone interview.

The cause probably was a mildew that has devastated the flower in Europe and has immigrated to the U.S. Across the country, gardeners and greenhouse operators planning for the spring season are scrambling for alternatives for the normally ubiquitous plant.

The disease was first detected in the U.S. in 2004 and has since been found in 33 states, making it harder to find the popular flower used to provide a jolt of color to shady nooks of yards and gardens. Standard varieties are almost non-existent in Europe, where the disease wrought floral devastation. Ball Horticultural Co., a major domestic impatiens supplier, is instructing consumers on how to guard against it and suggesting alternatives. Sakata Seed Corp. is selling a new variety not affected by the malady.

“It will be dog-eat-dog at the nurseries this spring, when we try to find other cheap flowers that love shade,” said Lucy Dalglish, a professor and dean at the University of Maryland’s journalism school, who said the mildew wiped out $400 of flowers in her yard last year. “It was a nightmare.”

$11.6 Billion Industry

U.S. horticulture is an $11.6 billion industry, according to the Department of Agriculture, with nursery plants and annual flowers such as impatiens, violets and geraniums accounting for more than half that amount. About 80 million Americans, more than a quarter of the population, have planted gardens, according to the USDA.

The fungal disease affects all varieties of the genus Impatiens walleriana, a common perennial native to Tanzania and Mozambique and known as “bizzy Lizzie” in the U.K. It first appears as a white, downy-like growth of spores on the underside of leaves, thus it’s difficult to spot. By then the infection may be as much as two weeks old, contributing to the unwitting transportation of affected plants.

As the disease progresses, leaves and flowers become yellow and die -- turning the normally hardy plant, which needs little sunlight, into a shadow of its former self. Downy mildew tends to thrive during prolonged periods of wetness, with moderate temperatures hovering around 70 degrees Fahrenheit (21 Celsius), Scott Aker, head of horticulture at the U.S. National Arboretum in Washington, said in a telephone interview.

‘Sneaky Disease’

“It basically defoliates the plant entirely, and that can happen pretty quickly,” said Aker, who said he had downy mildew in his own garden about seven years ago. “It’s kind of a sneaky disease.”

Still rare in the U.S. two years ago, the disease has since been found in all states bordering the Atlantic and Pacific oceans and the Gulf of Mexico and Great Lakes. It has yet to be reported in the Plains and Mountain states, and in West Virginia.

Drier areas are less likely to be affected, Aker said. While the current downy mildew outbreak “ranks pretty serious” among infestations, it’s difficult to tell how bad it will be this year because most flower gardens in the northern part of the U.S. haven’t been planted yet, Aker said.

Many home gardeners probably weren’t aware of the disease in the impatiens crop last year, Ben Radebaugh, president of John H. Radebaugh Greenhouses Inc. of Freeland, Maryland, said in a phone interview.

Deer Blamed

“A lot of people thought the deer ate them or the plants just died,” he said.

The wholesale grower this year has cut back production of impatiens to about 25 percent of his normal crop. To fill the gap, he’s selling a variety of the plant not affected by mildew, and recommending other flowers such as begonias and caladiums. Radebaugh also sprays the impatiens with fungicide before they leave the greenhouse, which protects them from the airborne disease for about six weeks.

West Chicago-based Ball Horticultural, a wholesaler of seeds and plants, is taking steps to ease concerns among its global customers. Katie Rotella, a spokeswoman, said the company has distributed recommendations to greenhouses and landscape contractors in English and Spanish on how to prevent the disease.

Meanwhile, alternatives exist for gardeners who have lost their impatiens.

Impatiens Hawkeri

The related Impatiens hawkeri, or New Guinea impatiens, is tolerant of downy mildew disease. Sakata Seed, a Tokyo-based seed-seller, used it to develop a hybrid that’s had double-digit annual sales growth in recent years, according to Jeanine Standard, a company spokeswoman.

That product, called SunPatiens, has been successfully planted in Europe and the U.S. It blossoms best in sunlight, though it will still bloom in the shade settings where gardeners typically plant impatiens, she said.

“We’re trying to get the word out because the traditional gardener is going to be surprised when she goes to get her traditional impatiens,” Standard said.

Treatment options for afflicted plants are less than ideal, Aker said. “If you want to spray your impatiens on a regular basis with a fungicide, you could protect them from infection,” he said, adding that most people probably won’t choose such a time-consuming, expensive route.

Why downy mildew, which has existed for centuries and targets many plants, has struck impatiens so suddenly and lethally remains unclear, said Gary Moorman, a plant pathologist at the Pennsylvania State University in University Park. Until more is known, gardeners may need to adjust their habits, or take the risk of early, shady plant deaths.

“It’s difficult, because impatiens is a really good shade plant, and there aren’t many of those,” he said.

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