Investigators have determined that the brush fire that tore through five acres of woodland in Lusby on Tuesday was sparked by a dry tree branch touching a power line, Department of Natural Resources spokeswoman Susan O'Brien said Friday.

About 70 firefighters fought the blaze for seven hours before knocking it down.

The fire erupted about 11 a.m. in a right of way off H.G. Trueman Road that is owned by the Southern Maryland Electric Cooperative, said Richard McIntyre, also a spokesman for the state Department of Natural Resources. Firefighters weren't alerted until about 1:30 p.m. and it took them two hours to reach the center of the fire because the area had no road access, McIntyre said.

Firefighters from 17 departments across Southern Maryland responded to the blaze and used five bulldozers and 16 four-wheel-drive vehicles, McIntyre said. Efforts were helped by a late afternoon thunderstorm, he said. The last firefighters left the scene about 9 p.m., he said. There were no injuries.

This season's drought has made small fires an everyday occurrence in Calvert, said Donald Hall, county emergency management director.

In Southern Maryland, there have been 171 brush fires this year, 100 more than the number recorded for the same period last year, McIntyre said.

Ten days ago, a 15-acre St. Mary's County brush fire in the Kitts Point area south of St. Inigoes caused $120,000 in damage to a barn and farm machinery, he said.