A fungus that caused widespread loss of bee colonies in Europe and Asia may be playing a crucial role in the mysterious phenomenon known as Colony Collapse Disorder that is now wiping out bees across the U.S., University of California, San Francisco researchers said Wednesday.

Researchers have been struggling for months without success to explain the disorder, and the new findings represent the first solid evidence pointing to a potential cause. But the results are “highly preliminary” and are from only a few hives from Le Grand in California’s Merced County, UCSF biochemist Joe DeRisi said. “We don’t want to give anybody the impression that this thing has been solved.”

Other researchers said Wednesday that they too had found the fungus, a single-celled parasite called Nosema ceranae, in affected hives from around the country – as well as in some hives that have continued to survive and live. Those researchers also have found

two other fungi and a half-dozen viruses in the dead bees.

“N. ceranae” is “one of many pathogens” in the bees, said entomologist Diana Cox-Foster of Pennsylvania State University. “By itself, it is probably not the culprit . . . but it may be one of the key players.”

“We still haven’t ruled out other factors, such as pesticides or inadequate food resources following a drought,” she said.

Historically, bee losses are not unusual. Weather, pesticide exposures and infestations by pests, such as the Varroa mite, have wiped out significant numbers of colonies in the past, particularly in the 1960s and 1970s.

But the current loss is unprecedented. Beekeepers in 28 states, Canada and England have reported large losses.