"It is a risk either way," he said. "Not grow them and face a lot of demand. Grow them and people stay away."

A spokesman for Home Depot said that the company will carry impatiens in the same numbers as in the past and that sales associates have been trained to answer customers' questions.

George Ball of Burpee, which offers seeds and live plants via mail order, reports his greenhouses are disease-free.

Downy mildew, like so many plant diseases, is a puzzle. It has existed in the wild in the United States since the late 1800s and has never been seen in the production market. It first appeared in England in 2003, but there was no history of the disease in that country.

Did it cross over from the wild, or catch a ride from England?

"Nobody knows the answer to that," said Daughtrey of Cornell.

Downy mildew is limited to the Impatiens walleriana, the common garden impatiens, and will not spread to other plants.

The spores, which are closely related to a water-born algae, act as parasites, sapping the energy reserves of the plant. The first signs are slightly yellowing, curling leaves. The disease progresses quickly until the plant is nothing but bare, browning, leafless stems.

It began to show up in Florida in the fall of 2011. By the end of the last season, growers knew they were facing the tough choice of taking this plant — which represents more than $100 million in wholesale sales — off the market until a disease resistant cultivar can be bred or risking the ire of the unsuspecting gardener.

"In all my years as a gardener, a grower, seed salesman and retail garden center owner," said Ray Greenstreet of Greenstreet Gardens in Lothian, "I have never experienced such a devastating disease hit our industry so rapidly in basically one season."

Daughtrey said so little is known about the habits of this disease that it is possible weather plays a primary role in its spread, meaning that there can be good seasons and bad, depending on temperature and rainfall.

"It is sad," said Daughtrey, "because the crisis is now and we don't have anything to offer the gardener that makes it better. We don't have a Band-aid."

She works with a number of growers in New York, and some of them were on the fence this winter. But ultimately, she said, "they decided they couldn't produce a plant under false pretenses."

Landscapers have the chemicals at their disposal to perhaps keep their installations disease-free for the season. But those fungicides are not available to the home gardener and, in any case, it would cost far more to save a $1.25 plant than it is worth.

For the homeowner determined to purchase impatiens, it makes more sense to use it as a specimen in a planter or a hanging basket instead of as a carpet of color in a shady spot.

"We are talking to our customers about a new kind of drama in the garden," said Engel of Valley View, who admits that almost all the substitutes are more expensive than this colorful little bloomer, and some are not as carefree to grow.

"Maybe we were in a rut and this will chase us out of it," said Daughtrey. "But you have to respect how much effect this one little plant has on us."

susan.reimer@baltsun

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Impatiens downy mildew