To stem the spread, state biologists have posted the Gunpowder from Prettyboy Reservoir downstream to Loch Raven Reservoir with signs that explain what didymo is and what it looks like.
DNR officials are trying to determine whether they have the authority to enact emergency regulations to require anglers to clean their gear. They also are looking into setting up "washing stations" along the Gunpowder's banks.
"We're mounting a full-court press to educate the public," said Susan Rivers, a DNR biologist who has been sampling the Gunpowder.
Anglers, she said, should scrub their gear before leaving a stream. Back home, they should disinfect boots, waders and other gear in a solution of one pound of salt to five gallons of water or scrub them down in hot water and dish detergent and then air-dry. The organism can survive on moist surfaces for two days.
The algae was identified in the 1890s in Great Britain, Scandinavia and China. It was detected on the West Coast a decade ago.
An outbreak in New Zealand in 2004 prompted a "biosecurity lockdown," complete with checkpoints and penalties of five years in prison and $100,000 fines for anglers and boaters who failed to clean their gear. A year later, outbreaks had decreased 90 percent, but officials warned it could be the downside of a natural cycle.
"On the East Coast, didymo hit the radar in the last several years," Rivers said. "Research has not found the connector. That's what is being looked into now."
LeGardeur, who just relocated his fishing shop to larger quarters and employs seven guides, said DNR officials have moved swiftly to assure the public and contain the problem. He has been handing out information and is producing a video for the shop Web site to educate customers.
All of this effort, McKnight said, is to "buy time."
"The longer we can hold it back, the more time we get to find a way to fight this," he said. "We're reaching out to fishermen. They're on the front lines now to protect our streams."
candy.thomson@baltsun.com
YOU CAN HELP
Tips for anglers on preventing the spread of "didymo:"
Do not move from one stream to another until all clothing and gear have been cleaned and scrubbed. Leave dirt behind.
Take gear home, scrub and soak in a 5 percent salt solution - one pound of salt to five gallons of water - or dish detergent, then rinse well.
Unwashed gear should be air-dried for at least 48 hours before visiting another waterway.
Don't move water, fish or any animals from one area to another; drain all live wells and clean thoroughly.
Avoid using felts on wading boots, which carry organisms and are difficult to clean and disinfect.
Source: Department of Natural Resources
