By Maggie Shiels in California

The world's tallest trees, California's breathtaking redwoods, have become infected with a deadly fungus that has killed tens of thousands of oak trees in the past three years.

What the future holds we can't predict

Dr Matteo Garbelotto

The disease known as sudden oak death (SOD) has been found in a number of redwoods in five counties in Northern California.

There is also one case of the pathogen infecting a Douglas fir in Sonoma county.

The revelation comes after months of studies conducted by Matteo Garbelotto, a forest pathologist at UC Berkeley, and David Rizzo, a professor of pathology at UC Davis.

They stress that so far the infections are only in saplings and sprouts and that there is no evidence that the disease can actually kill grown trees.

Early days

Dr Garbelotto told BBC News Online: "We haven't seen any evidence of mature trees being killed or of smaller trees being killed.

"In the case of the Douglas fir, we believe we're looking at something that's just started, so we're looking at a new host. What the future holds we can't predict.

The wilting branch tips of a Douglas fir

"Potentially, it could lead to the death of Douglas firs. In the case of redwoods, I think we're looking at something that's pretty established."

The researchers note that the symptoms have been detected only on the needles and very small branches of redwoods and that more work needs to be done to chart the course of the disease.

"We need to look at it through time and see what the effects are," said Dr Garbelotto. "We have two new tree species that are extremely important ecologically, that are infected in a way we don't know and this complicates our understanding of what the final impact is. At this point it is so early in the game."

Economic impact

Professor Rizzo agrees. "We have a lot of unknowns," he said. "We really don't have a good sense of the progression of the disease over a period of years."

While the study poses more questions that it perhaps answers, the presence of Phytophthora ramorum in both redwoods and Douglas firs has caused alarm among those in the $1bn a year timber industry. Both trees represent two of the state's most valuable timber resources.

Coastal redwoods

Very tall sequoia tree Has fibrous reddish bark Can reach over 100 m (330 ft) Latin name: Sequoia sempervirens Thrive in the fogs that roll in from the sea

Louis Blumberg, of the California Department of Forestry and Fire Protection, said: "Ninety-five per cent of the redwood harvested in California and 45% of the Douglas fir come from areas of infestation. So the implications are quite grave."

California's Governor Gray Davis is in agreement and has called on President Bush to release $10m to combat this highly contagious fungus.

"The announcement about sudden oak death in Douglas fir and coast redwoods significantly raises the stakes," said Governor Davis. "As a state, we will continue to tackle this serious economic and environmental problem but we need federal resources as well."

State lines

To ensure the fungus is not spread around the country, the California Department of Food and Agriculture (CDFA) has also just announced that existing regulations restricting movement of the 15 known species that can harbour SOD will be extended to include redwoods and Douglas fir.

It is not clear yet how the mature trees will be affected

Among the measures enforced by the CDFA is the removal of bark from all lumber moved across county or state lines and an inspection by a county agricultural commissioner.

The presence of P. ramorum in redwoods had been suspected at the turn of the year but these results confirm the rare incidence of the pathogen spreading from one species to another.

Professor Rizzo said the California blight, which is related to the organism that caused the Irish potato famine more than 150 years ago, was most similar to a disease now ravaging trees in Western Australia.

Last ice age

Dr Garbelotto said there was a real urgency about this situation because both trees played an important role in the state's ecosystem. "In forests, size matters.

Dr Garbelotto: Biggest and most important plants set the tone in an ecosystem

"The biggest and most important plants play a bigger role because they set the tone for what the ecosystem is capable of doing and redwoods are the dominant trees in a lot of ecosystems on the coast of Northern California and the Douglas fir is a co-dominant tree as we move up the interior to Oregon, Washington, British Columbia and Canada."

Coastal redwoods are also a major tourist attraction in California and can reach heights of more than 100 metres (330 feet) and live for as long at 2,000 years.

Forestry experts say the ancient trees once grew as far away as Russia, but were killed off by the last ice age in all but the coastal fog belt in California and southern Oregon.