Mysterious willow die-off in northern San Diego County

A mysterious pest has damaged willows along the Escondido Creek watershed of northern San Diego County, leaving conservation officials scrambling for answers to the die-off.

Officials with the Escondido Creek Conservancy originally suspected the damage was caused by the shot hole borer beetle, which attacks 137 tree species including willows, oaks and sycamores.

The insect has decimated the Tijuana River Valley, where denuded tree trunks stand in the place of formerly lush habitat. Tests at UC Riverside came back negative for a fungus associated with the beetle, but didn’t resolve the threat to the watershed.

The problem began over the summer, when the conservancy learned that trees in Elfin Forest were rapidly wilting and dying, Executive Director Ann Van Leer said.

The collapse of willow groves ripples through the watershed’s ecosystem, Van Leer said.

“It’s one of the foundation plants, she said. “A lot of birds and insects rely on the willows. It’s an indicator species of the watershed.”

Worried that Escondido Creek could suffer similar damage, the conservancy closed its trails to the public on Aug. 3 and submitted samples from the trees to plant pathologist Akif Eskalen at UC Riverside.

“We didn’t want people and bicycles spreading the infection by moving through the area,” Van Leer said.

The samples showed no signs of Fusarium fungus, which transports and spreads the beetle infestation. Instead, the lab found a different fungus, Phaeoacremonium.

The results only compounded the puzzle, since Phaeoacremonium is a slow-growing pest that isn’t associated with sudden die-offs.

Eskalen said he plans to visit the site Friday and hopes to get additional clues to the die-off.

“This is a new disease that we haven’t figured out yet,” he said. “We still don’t know what caused the decline of those willows.”

deborah.brennan@sduniontribune.com

Brennan writes for the San Diego Union-Tribune