FORT PIERCE — Citrus greening is spreading like wildfire through Florida's commercial citrus groves, and it's not known how long it will take to find a solution to the devastating bacterial disease, leading scientists from around the nation were told Tuesday.

"Citrus greening is an unknown unknown," said Bob Norberg, director of economic and market research for the Florida Department of Citrus. "We don't know how long it will take us to get rid of it."

Norberg also offered some chilling statistics: Greening spreads through the average Florida citrus grove at an infection rate that doubles every year. Within seven years, a grove will be 100 percent infected, he said.

Researchers from The National Academies and a U.S. Department of Agriculture special greening task force gathered Tuesday at the USDA's Horticultural Research Laboratory off Rock Road in Fort Pierce.

Today, they will huddle in closed-door sessions, and on Thursday they plan to present reports.

"We are at a crossroads. We need everyone's help now. Get to work," said Mike Sparks, chief executive officer of the 8,000-member grower group Florida Citrus Mutual.

Citrus greening, also known by its Chinese name of huanglongbing, is a bacterial disease spread by the Asian citrus psyllid. First detected in September 2005 in Homestead, it since has spread to 30 citrus-producing counties. The disease turns fruit inedible and often is fatal to trees.

One scenario projects that within nine to 10 years, all the citrus trees currently in the ground will have succumbed to greening, Norberg said. The industry is replanting, using trees produced in secure nurseries.

"We hope that before then, (scientists) will have come up with a new greening-resistant variety or some way to manage the disease," he said.

Peter McClure, chairman of the Florida Citrus Production Research Advisory Council, said the industry needs to maintain enough productive acres to support the capital investment represented by the processing and packing plants.

Some experts have placed the minimum tree acreage necessary for the industry to remain viable at 500,000. The industry was at 554,000 productive acres in 2006-07, according to a National Agricultural Statistics Service report issued in February.

"The number one goal is that the citrus industry of Florida survives," McClure said.

Bill Dawson, a University of Florida eminent scholar in plant virology, said that so far researchers have not found any plants that are resistant to the citrus greening bacterium. Dawsonsaid inserting greening-resistant genes into citrus is the most promising possible solution.

Christopher Mundt, a National Academy of Sciences member and an Oregon State University grain scientist, said he's been to many meetings about agricultural diseases where it was predicted the sky was falling, but it did not.

"This is one time that it looks like the sky is falling," Mundt said, though he added: "But I've heard about a lot of good research. I'm impressed."