A devastating fungus that has hit California has now reached Britain and is putting Ireland's oldest trees at risk, writes Karlin Lillington.
In 1994, the people of Mill Valley noticed something was very, very wrong with one of the most beautiful and distinguishing features of their hilly California landscape: their ancient oaks were dying. The leaves had begun to turn brown, and the rough bark of the trees, many of them hundreds of years old, bled sticky dark juice from strange sores. First, it was the tan oaks. Then, the graceful, twisted coast live oaks and the mighty black oaks.
Teams of entomologists came from the universities to see if this was an insect infestation, similar to the Dutch Elm Disease which had felled so many of those once-common trees.
Then the situation got worse: the dying oaks were no longer confined just to Marin, across the Golden Gate Bridge from San Francisco. Whole swathes of trees in neighbouring counties began to get the mysterious disease, now called Sudden Oak Death, and then slowly rot away.
It took several years and the arrival of a plant pathologist from the agricultural stronghold of the University of California at Davis to nail the culprit. To the surprise of many researchers, the killer turned out not to be beetles or other insects, but a newly-discovered species of the phytophthora fungus, given the name P. ramorum.
That may not ring any chimes, unless you're a botanist - or a student devoted to the minutiae of Irish history. For phytophthora is none other than the fungus that caused the rot and death of the Famine.
As elusive to present-day researchers as an understanding of the Famine blight was to the people of the 19th century, P. ramorum is a puzzle. No one knows where it has come from, or how long it has been in California, or exactly how it spreads and infects.
And it has come to Ireland, where it could threaten some of the most loved trees in the Irish countryside: the beech and the chestnut, stripping these ancients out of the last remaining acres of our old growth forest.
"I was fairly new to California when oak trees started dying in the mid-1990s," says University of California (UC) Davis Professor David Rizzo, the man to identify the new phytophthora. "At first, beetles were thought to be associated with it, but even the entomologists weren't sure. By the summer of 2000, there were a lot of dying oak trees." Rizzo and a fellow researcher based at UC in Berkeley, Matteo Garbelotto, eventually guessed it might be a fungus. They took a culture, and Garbelotto did a DNA sequence of the blight.
"It didn't match anything else we'd seen. On the one hand, that was exciting. On the other hand, it was scary, because it meant we weren't dealing with something we already knew about." Phytophthora is not uncommon - there are about 60 varieties, says Rizzo; the potato blight was caused by P. infestans. As a species, they tend to like damp climates - hence the devastation caused in Ireland - but most are fairly innocuous.
However, occasional species can turn into a grim reaper for plant populations: one species that popped up in Australia in the 1920s turned thousands of acres of eucalyptus trees into treeless grassland.
Californians are deeply worried that their landscape, so defined by the oaks that cover the valleys and hillsides, could change utterly in the space of decades. Even worse, it is now known the term Sudden Oak Death is a misnomer, as phytophthora infects other species, including the towering California redwood, the state's official tree.
P. ramorum has reached epidemic proportions in several California counties, says Rizzo, and may lead to "a cascade of changes" in the make-up of forest ecosystems, causing long-term, landscape-level changes.
Rizzo heads a group of 15 researchers investigating the virus at UC Davis. He doesn't hold out much immediate hope, mainly because treating millions of acres of wild forest is not the same as spraying an agricultural crop or treating your garden plants.
However, researchers have found that the blight seems to spread to the trees by infecting low-growth plants like rhododendron and viburnum. In California, a particular culprit is the Bay Laurel, which can appear as a shrub or a tree. "It's the Typhoid Mary of the forests," says Rizzo.
Another breakthrough was finding a way to inject a chemical to protect an uninfected tree, a new measure many homeowners with oaks on their property are likely to avail of in California.
And to the relief of Californians, it was also found that the disease doesn't kill every oak - a few seem to weather its assault, says Rizzo. And the redwoods, like some other trees, don't seem to die from phytophthora, though Rizzo cautions that it is too early to be sure.
Researchers now realise it isn't just Californians that need worry. Phytophthora has been found in Europe, including Ireland - indeed, says Rizzo, a fellow researcher in Britain realized it had been identified in Europe in the 1990s, its origins unknown, and had seemed to be confined to nursery plants in the Netherlands and Germany.
Such is the level of worry that the disease has been declared a special emergency across the EU, $3.8 million has been earmarked in the US to decode P. ramorum's genome, and the California state government has promised $2 million to tackle the disease.
Last week, the Department of the Environment in Britain announced P. ramorum had been found on nine trees, including beech, sweet chestnut and horse chestnut (it doesn't seem to infect European oaks), and its spread is being closely monitored. A spokeswoman for the British Forestry Commission says a second species of phytophthora has also been found. The affected trees and plants are all in the west of the country, she says. In the lab it also infects, but doesn't seem to kill, Douglas fir and Sitka spruce.
In Ireland, P. ramorum is being monitored, and has been seen on rhododendron and viburnum - plants commonly found heavily carpeting the forest floor in the State's national parks. Its attack on beech and chestnut could pose a serious threat to Irish forests in some of the most beautiful parts of the countryside.
For now, US research is concentrating on the affects not on oak, but on redwoods and firs. It's just plain old economics, says Rizzo - those species are at the centre of the logging industry in the US. Meanwhile, Irish forestry experts can do nothing but watch developments in the UK and hope the blight doesn't jump to Irish trees.