Curse of the Californian giant-killer

They're a wonder of the natural world. But America's famous giant redwoods are threatened by a lethal disease

Two years ago, a Californian tree surgeon called Ken Bovero got a disturbing call from one of his clients. Something seemed to be very wrong with two redwood trees on the client's property in Mill Valley just outside San Francisco.

Mr Bovero noticed that the trees seemed to be dying from the top down, and from the branches inwards. He submitted samples to a lab, and initial results suggested they were infected with a potentially lethal new tree disease that has been ravaging some of California's rarer oak varieties for the past six years.

And that was just the beginning. More recent testing has shown up traces of the same disease, called Phytophthora ramorum, on samples of redwood trees taken from the coastal wilderness at Big Sur and from the campus of the University of California at Berkeley – both places that have lost numerous oaks to the disease.

All of a sudden, a grim sense of foreboding has overtaken the scientific community. Could the glorious, world-famous redwood forests of the American West be facing the onset of a devastating epidemic? After decades of abuse by logging companies that have depleted the region's redwood population by more than 90 per cent, could the rest be felled with the same devastating efficiency as Dutch elm disease wiped out elms from much of Europe?

These are early days. So far, only Mr Bovero's trees have shown signs of disease, and it is not yet certain that the strain of Phytophthora he found is related to the organism that has been killing oaks on the California and Oregon coasts. As for the samples at Big Sur and Berkeley, they were taken from dying shoots at the base of redwoods. For now, it has only been established that the disease was present on the surface of the trees; further tests are under way to see whether there was any penetration into the deeper wood tissue. Scientists at UC Berkeley have also injected the disease into healthy redwood shoots in the laboratory to see how susceptible they might be.

The Berkeley plant scientist Matteo Garbelotto, a lead researcher on the disease, said this week: "I can tell you we have reason to believe they are a potential host and we are doing experiments to see if they are." However, he urged caution, however, saying it was not yet certain whether redwoods were at risk.

But on the negative side, no tree species has yet escaped infection after DNA samples of Phytophthora ramorum have been confirmed. Mr Bovero says he has seen seven infected redwoods in the past two years, and although the lab testing isn't complete, he is convinced something serious is afoot. "You watch; they'll confirm it, and this is just the tip of the iceberg," he said.

When Phytophthora ramorum was first detected, it was nicknamed "sudden oak death" because it only appeared to affect oak trees. The fungus-like pathogen is believed to be related to the blight that caused the great Irish potato famine in the 19th century. There is no known cure. Several rare species were struck down, including tanoaks and coastal valley oaks – both prized by environmentalists and the subject of much legal wrangling when loggers or housing developers threaten to move in on hillside or forest.

By now, the disease has jumped to at least 14 species, including rhododendrons, California bay laurels, huckleberry, manzanita, madrone, big leaf maple, California honeysuckle, buckeye and coffeeberry. It has turned up in 10 Californian counties, from Monterey, two hours' drive south of San Francisco, to Mendocino, four hours to the north. Traces have been found in southern Oregon, and in arrow-wood trees in Germany and the Netherlands. Dr Garbelotto, among others, believes the disease may have originated in Europe because of the greater biodiversity there. How it reached the US West Coast is a mystery for the moment.

What happens next is a matter of some speculation. Everything depends on how lethal the disease turns out to be. Unlike valley oaks, redwoods have a high economic value, since they are widely used in construction. That, in turn, means that plenty of money would be available – from both state and private industrial sources – to try to isolate infected trees and minimise damage.

From the naturalist's point of view, the biggest disaster would be an epidemic in the few remaining large redwood forests under state or federal protection. For now, three state parks in the Big Sur area have tested positive for Phytophthora ramorum, but they are not nearly as significant as the old-growth redwood groves in Humboldt County, several hundred miles to the north.

The scientists are saying it is far too soon to panic, but forestry officials are already taking precautions. California and Oregon have imposed restrictions on the movement of infected wood, and ramblers have been told to wash off their shoes after visiting infected areas.