A destructive Oregon wildfire that ranks as the largest among dozens raging across the drought-parched Western United States in recent weeks was ignited by lightning but smoldered for days before it was detected, forest officials said on Wednesday. The origin of the so-called Bootleg fire, which was first reported July 6 in the Fremont-Winema National Forest some 250 miles south of Portland, came to light as ground crews made increasing headway in curtailing the blaze. Strike teams were taking advantage of calmer winds, cooler temperatures, and slightly higher humidity over the past two days to extend and bolster containment lines around the fire’s periphery, incident commanders reported in their latest summary. Although the blaze continued to expand, its growth was stunted as flames advanced into an old fire-scarred area with less vegetation available to burn, officials said. As of Wednesday, the Bootleg had charred 395,463 acres of tinder-dry brush and …