Still, annual bedding plants are a $2 billion industry in the United States, and the colorful Impatiens walleriana has long been one of the best sellers. For years, it was sold by the millions by growers that buy seed from Ball Horticultural Company, Syngenta Seeds and others. And as recently as 2009, it reaped $174 million wholesale annually, according to the Agriculture Department. But now, the endangered pinup girl is having a rocky time in the marketplace. Some growers are still pumping it out, but many have stopped cold.

Walter Gravagna, an owner of Van de Wetering Greenhouses in Jamesport, N.Y., which produces 300 million plugs, or seedlings, of annuals a year, said his production is down by 20 percent. “We supply wholesalers from Michigan to Maryland and up to Maine, and 40 percent of that used to be impatiens,” he said. But “with downy mildew really hitting the East Coast, from the mid-Atlantic to Maine, that supply has gone down considerably.”

Home Depot is still selling the flats, but is training its staff to spot the disease and is educating customers about the risk involved if they insist on the one and only, said Stephen Holmes, a spokesman at the company’s headquarters in Atlanta. And “we’ll have less supply east of the Mississippi,” he said, where the flow of plants has been reduced by 25 to 40 percent.

The disease is a mold (Plasmopara obducens) that thrives in cool, damp conditions and first appears as a white, downy coating of spores on the undersides of leaves, so it’s easy to miss. By the time gardeners notice the flowers drooping, it’s too late to do anything.

So far, it has swept through 33 states and the District of Columbia, and has been confirmed in Ontario, with possible outbreaks in British Columbia and Quebec. It showed up in England in 2003, and spread so quickly through greenhouses and gardens that Britain’s Department for the Environment, Food and Rural Affairs placed a quarantine on the movement of diseased plants, Ms. Daughtrey said, “banning their sale, requiring disposal, that kind of thing.”