Swedish take on Ash Tree killer fungus

From the Telegraph reporter Tom Rowley. Nov 4th 2012

ash-treeI am standing in the remote Swedish countryside on a freezing but cloudless day studying two trees. One is blackened, its bark clinging to it in shrivelled clumps. The other is in good health, standing 30ft tall, its sturdy bark covered with a generous layer of frost. Yet the two stand barely an arm’s-length apart, and both are infected by the same disease. They are among a small sample in a hectare of land that scientists hope will help them save Europe’s ash trees — up to 80 million of which are threatened in Britain.

Until last week, few had heard of ash dieback. Soon it may be too widespread to ignore. First spotted in Buckinghamshire in February, the fungus, Chalara fraxinea, has been identified at more than 40 sites from East Anglia — where it appears well established in mature trees — to Knockmountain in Renfrewshire. The Forestry Commission plans to contain it by burning up to 100,000 trees and possibly banning the public from forests to stop them transporting fungal spores on their boots. But the Swedish scientists, who have been studying the fungus for more than a decade, say the destruction might be counterproductive. If we want native ash to continue to thrive, we might have to “learn to live with” the fungus.

Prof Jan Stenlid, of the University of Agricultural Sciences in Uppsala, has been monitoring the 1,300 trees on the plot, about an hour’s drive north of Stockholm, and the healthy tree is not a one-off. In fact, only a third of ash trees have died over the period of the study. It seems that some are able to find an “equilibrium” with the fungus. Prof Stenlid says this natural resistance could be bred into saplings, to ensure that the ash does not die out entirely in Europe. “You could take branches from this tree and graft it to root stock, like with apples and pears, to produce seeds in controlled environments. This would preserve good genotypes to breed good trees,” he said. The research provides a glimpse of hope in a crisis that has drawn parallels with Dutch elm disease, which killed 28 million trees in Britain after arriving in the 1960s.

Since the domestic threat of ash dieback emerged last week, the Government has been criticised for inaction. Andrew Motion, the former poet laureate who is now head of the Campaign to Protect Rural England, said the dithering showed a “lack of care” for our landscape. Prof Stenlid shares these concerns, pointing out that the disease was first observed in Poland 20 years ago, where it probably arrived from Asia through the nursery trade in saplings. It spread rapidly to 20 European countries. He also criticised the European Union for failing to categorise the fungus as a “registered” disease, which meant that inspections of ash saplings were not required before their export to Britain. But it might not be too late to save Britain’s native ash, he says.

The Swedish study suggests that attempts to contain the disease by burning trees could be counterproductive. If resistant trees can grow alongside dying ones, burning all trees will “throw the baby out with the bath water”, leading to the eradication of native ash. Prof Stenlid suggests a more cautious approach. “You should at least wait to find which ones are best off and then try to build a future from the ones that are not suffering so badly,” he says. “Otherwise you will remove the possibility of preserving the tree species because you will cut down the resistant ones.” Back in the university’s laboratory, experiments by Stina Bengtsson, a PhD student, show that the fungus thrives in both cold and warm climates, suggesting that there are “no limitations” to its survival in Britain. “From Stina’s work, it is clear that the fungus can develop lesions [and so release spores] even at low temperatures,” says Prof Stenlid. “Although it seems to like warmer temperatures.” Ms Bengtsson says the fungus adapts to its environment. “There is an arms race between the fungus and the tree,” she explains. “They both have the ability to develop new genotypes [showing different and possibly advantageous characteristics] but the fungus is quicker.” In the short-term this is depressing news: clearly the fungus is well equipped to survive in temperate Britain. But the Swedish team believes that tree and fungus will eventually form a “symbiotic” relationship, with healthy but infected trees. If the fungus does not adapt, it will kill its hosts and so die out itself. “At the moment the fungus is entering a paradise [in Britain], with a naive host population so it does not have to do anything more than just grow and there will be lots of susceptible trees,” says Prof Stenlid. “But the restriction will come when a significant part of the host population has died. Then there will be [evolutionary] pressure on the fungus to change or die.”

The theory is supported by research in Denmark. Iben Margarete Thomson, a forest pathologist at Copenhagen University, hopes to accelerate this process by breeding from the most resistant trees. “The fungus kills off the host by mistake at the moment,” she explains. “There is an imbalance and that will right itself when there are more resistant ashes. For ash to do that itself might take 50 years, but we are trying to shorten that by finding those trees ourselves.” This process could take dozens of years, during which ash could all but disappear from such cherished landscapes as the Peak District and the Yorkshire Dales, removing the wood favoured by Thomas Gainsborough and David Hockney and used to make longbows in the Hundred Years War. Joan Webber, the Forestry Commission’s principal pathologist, says she and her team have studied European research but supports the policy of burning trees because the risk of destroying resistant ash is outweighed by the need to contain the disease. Prof Stenlid disagrees. “The only smart way to face the future,” he says, “is knowing it is impossible to eradicate the fungus.” He says it is likely to be here for decades, but, if we take heed of his research, so could the native ash.



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