Hikers help spread sudden oak death

Researchers have confirmed suspicions that trail users such as hikers and mountain bikers are helping to spread a disease that is devastating Californian forests.

The researchers found the pathogen causing sudden oak death was prevalent along trails through otherwise uninfected forests, but almost absent in soil samples taken two metres away from the trail.

They also found that the disease was more widespread in parks heavily used for hiking, mountain biking and horse riding than in less-visited areas. Previous work has shown that people can carry the pathogen on their shoes, but this is the first study to provide evidence of the consequences.

"Humans are moving the pathogen around, and the result seems to be higher levels of the disease," says J Hall Cushman, a biologist at Sonoma State University in California. He presented the data, collected over the last two years, at the Ecological Society of America conference in Portland, Oregon last week.

Difficult questions

Sudden oak death, caused by the fungus-like pathogen Phytopthora ramorum, is sweeping through forests in coastal California. It has also been detected in the UK and several other European countries.

The pathogen kills some oak species, and causes a non-fatal leaf disease in many other plants such as rhododendrons and California bay. Researchers suspect the disease is also spread by water and by other animals.

Cushman says the results pose difficult questions for land managers in California, where outdoor recreation is hugely popular. If managers do nothing, they may be criticised for not preventing the spread the disease.

Restricting trail access during wet seasons, when the pathogen is most active, would probably be most effective, but would also be unpopular and hard to enforce.

Another possible control method is to ensure visitors clean their shoes and bike tires before and after visits. The National Park Service plans to test this method this winter.

Wild fires

The need for action was highlighted by another study presented at the same conference. This predicted that in heavily infected areas the disease will kill up to up to 69% of a dominant native tree species, coast live oak (Quercus agrifolia), within five years.

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	Related Stories   Genome of Sudden Oak Death bug cracked 11 June 2004   Deadly fungus disease hits UK trees 05 December 2003   Exotic fungus may save India's tea 25 June 2003     For more related stories search the print edition Archive     Weblinks   J Hall Cushman, Sonoma State University   California Oak Death Mortality Task Force   Ecosystem sciences, University of California, Berkeley   Ecological Society of America conference   US National Park Service

A dramatic protective measure against the disease was revealed in a third study presented at the conference – wildfires. "We almost never see infections in areas that burned" since 1950, says Max Moritz of the University of California, Berkeley.

Periodic wildfires are natural occurrences, but managers have historically suppressed them. Researchers are now working to discover why fires should have such a long-lasting effect.

The news that people and fire affect the spread of sudden oak death matches the experience of Patrick Robards, a ranger at China Camp State Park. The park is a notorious hotspot for sudden oak death that gets 300,000 visitors a year.

Robards conducted controlled burns as part of the park management regime, but stopped in 1999 due to lack of funds. He estimates that in burned areas fewer than 5% of oaks show signs of the disease - in areas that did not burn, up to 90% of oaks are dying or already dead.

Mike Faden, Portland