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DE LUZ, Calif. (CNS) – A wildfire erupted amid a scorching summer heat wave Thursday in a sparsely developed area at the San Diego-Riverside county line, blackening dozens of open acres but causing no immediate structural threats.

The blaze broke out for unknown reasons shortly before 11:30 a.m. in the area of Los Gatos Road and Via Volcano in the unincorporated De Luz community, moving through heavy brush where homes are spread out on large lots along hillsides, according to Cal Fire.

By 1:45 p.m., the burn area had grown to about 50 acres, the state agency reported.

Personnel aboard five air tankers and two water-dropping helicopters were the first to reach the site of the blaze, dubbed the Volcano Fire. Crews from the Riverside area, San Diego County and Cleveland National Forest also were dispatched but had difficulty immediately reaching the remote location, officials said.

Under a mutual-aid agreement signed earlier this summer, fire crews from both counties generally conduct a joint response whenever a blaze is within five miles of the county line.