Ash dieback disease is ravaging trees across Europe. Credit: B. Clarke/Getty Images

The UK government today announced an action plan to control the spread of 'ash dieback', a disease caused by the fungus Chalara fraxinea, but this will not stop the pathogen from killing up to 99% of the ash trees in the country, say scientists.

Diseased trees in nurseries — and those that have been newly planted — will be identified and destroyed. Mature trees will be left standing, as they take longer to die and are valuable to wildlife, and can help in the search for naturally resistant trees. The import ban on ash trees that was implemented at the end of October will remain in place.

These measures, however, will not eradicate the disease from the United Kingdom. “There is absolutely no magic wand we can wave to make this disappear,” UK environment secretary Owen Paterson said at a press briefing in London this morning. Ash is the third most common tree in the United Kingdom, and with as many as 90 million ash trees at risk, the disease threatens to irreversibly change the shape of the British countryside.

This all comes on the heels of the largest tree survey ever undertaken in the United Kingdom, in which 500 staff and volunteers combed through 2,500 square kilometres of British countryside looking for sites of infection, which as of today number 129. It is possible that the disease reached the United Kingdom via infected ash timber or imported plants, but Ian Boyd, chief scientific adviser for the Department for Environment, Food and Rural Affairs (Defra) says that it is more likely that the spores arrived naturally. Because the sites of infection are scattered across the country, the spores were probably blown on the wind from continental Europe, where the fungus has ravaged ash trees from Poland to France for more than a decade.

On the upside, ash trees reproduce and grow quickly, with a high capacity for self seeding, so reforestation may not be too arduous. The main task now is to identify resistant strains of ash — a challenge that European scientists are already trying to tackle — so that they can be bred, cultivated and used to repopulate ash trees across the continent.

“If a small number of trees have survived the very intense epidemic in Demark, then there is hope for us here,” says Boyd. “By next season, we could have resistant ash growing as saplings in this country.”

Zig-zag science

Part of the reason it has taken so long to tackle the disease was confusion in Europe over what exactly was causing the dieback, says Joan Webber, a pathologist at Forest Research in Surrey, UK. Was it a new species of fungus, or a variant of an old, endemic species of fungus?

Mycologists first attributed ash dieback to Hymenoscyphus albidus, a species endemic to Europe that they thought had developed into a new, more virulent strain. But in 2011 a group of mycologists determined that the disease was caused by a different species altogether, which they named Hymenoscyphus pseudoalbidus1 (H. pseudoalbidus is the sexually reproducing form of C. fraxinea).

“We had this zig-zagging in the science — is it new or is it old? — which led to the European Union being unable to regulate this as a quarantine organism,” says Webber. “Mycologists argued back and forth, and meanwhile the pathogen moved west across Europe.”

The latest research indicates that H. pseudoalbidus is native to Japan, says Stephen Woodward, a plant pathologist at the University of Aberdeen, UK, who was part of the group that advised the UK government on the action plan. When the spores reached European ashes in Poland, the trees had little ability to cope with the pathogen, and Woodward says that there is little that can be done now. “Estimates of 90% fatality are over-optimistic. It will be far more than that,” he says.

Global problem

Even if biologists can halt the spread of C. fraxinea in the United Kingdom, the worldwide spread of plant pathogens shows little sign of abating in a globalized economy.

“We are going to have to re-prioritize the Department for Environment, Food and Rural Affairs. We need to treat plant diseases as seriously as we do animal diseases,” Paterson said this morning. “We need a radical rethink in how we deal with plant diseases, and the word is ‘radical’.”