Dealing with Ash dieback - Disease strategy

As cases of ash dieback hit our shores, is there still time to protect the UK's trees against the infection spreading from mainland Europe? Jack Shamash reports.

Ash dieback - image: PA
Ash dieback - image: PA

Throughout mainland Europe, the effects of ash dieback disease are becoming increasingly apparent. The sight of wilted, brown leaves and dying crowns is becoming common in forests. In many cases, particularly among younger trees, the condition has killed off entire stands of trees.

In Denmark, where vast numbers of ash trees are affected, research from the University of Copenhagen’s forest and landscape department shows that growers have simply stopped planting ash trees. The North American Plant Protection Organisation suggests that up to 60 per cent of ash stands in Lithuania have been killed by the disease, while 80 per cent of ash in Poland is affected.

Ray Jenkins is production director at Wyevale Transplants, which sells up to 200,000 ash trees a year, mainly for hedging. "It’s a major concern for UK growers," he says. "I first saw it in Denmark a couple of years ago and it was causing serious problems. I’ve just come back from Germany, where there’s no market for ash due to the disease."

Pathogen identified

Ash dieback first started to appear about nine years ago. Two years ago, the fungus Chalara fraxinea was identified as the pathogen responsible for the disease. In 2009, researchers found that the teleomorph — the sexual stage of the fungus — was Hymenoscyphus pseudoalbidus, which, worryingly, seems to be native and widespread in Europe.

Researchers are not sure how the fungus spreads, but the most probable vectors are rain splash or insects. Symptoms include brittle and drying twigs, wilted leaves and black spots that can turn into cankers on the tree’s bark.

Until recently, Britain was thought to be free of the fungus. However, in recent months, the situation has changed. Inspectors discovered that a consignment of ash trees imported from Holland had been distributed to mail-order customers by a British nursery. In June, the disease was discovered in young ash trees that had been planted in a Leicestershire car park, apparently linked to a nursery in Lincolnshire.

But these two isolated incidents may just be the tip of the iceberg. HTA business development director Tim Briercliffe says:

"We don’t know how far it has spread but because of trading with Europe it could be widely present across Britain." The disease is notifiable — anyone finding it has a duty to inform the authorities and any trees infected or likely to be infected will have to be destroyed. However, Briercliffe believes the disease may have spread too far for this sort of selective destruction to have any real effect. "

If the disease is endemic, control measures may be a waste of time," he warns. The HTA wants a voluntary moratorium on all ash imports and compensation for any destruction.

According to experts, there are around 80-million ash trees in Britain and it is the fourth most common tree in the country. However, owners of these trees might be reluctant to investigate any possible outbreaks of ash dieback. Unlike massaria — a recently introduced disease that affects plane trees — the effects of ash dieback are not catastrophic. The trees are unlikely to shed boughs or fall over so there are no pressing health and safety concerns for the public.
If the disease is found, whole stands of trees will have to be cut down, double-bagged and taken to a landfill site for disposal. All of this is extremely expensive, so the temptation for many local authorities and landowners is simply to do nothing and hope that the problem never arises.

As yet, the outbreak of ash dieback has had little impact on the day-to-day running of the industry. Mark Townsend, area manager at tree contractor Gristwood & Toms, says: "We’ve never come across it. Because of massaria, many local authorities are less willing to specify plane trees, but we’ve seen no change in demand for ash."

Similarly, Barcham Trees sales director Keith Sacre points out that his firm has yet to feel any major impact. "Customers are not expressing concern but we are considering reducing the amount of ash that we hold on the nursery," he says.

Serious criticism

There has been serious criticism of the official response to the crisis. Even after the disease was identified in Britain, the Government refused to limit ash imports, claiming that any restriction would breach EU law on the free passage of goods. "An overhaul of the regime is vital," says Briercliffe. Others go further. Jenkins describes the present situation as "absolutely ridiculous".

The most difficult question is what to do next. Most bodies believe it is vital to get more information on the extent of the problem. A consultation letter has been put out by the Food & Environment Research Agency (FERA) asking for views on the subject. It points out that inspectors are currently assessing possible outbreaks.

The consultation paper raises the possibility that it may already be too late to prevent widespread infection:

"If [the disease] is found to be widespread, then further statutory action would not be appropriate, although FERA, the Forestry Commission and the devolved administrations would continue to work with those in the industry and with other relevant stakeholders to minimise any impacts." In this case, a policy of suppression would be used. Infected trees would be cut down and there would be some quarantine areas, but ash would still be allowed to enter the country freely.

If the outbreak is confined to relatively few sites, then the Government would continue with the policy of eradication. The consultation suggests the UK would have to be declared a protected zone. Ash being moved into the country would have to be "plant passported" to confirm that it derives from areas free of C. fraxinea. The consultation procedure is detailed on FERA’s website and replies have to be received by 26 October.

To combat the disease, many firms are looking at products that could delay or prevent the spread of the fungus. Tree consultant Jonathan Cocking has licensed Dutch product Allicin, based on extract of garlic. Now being tested at the University of Padua, it was initially devised as a treatment for animals but is proving effective on bleeding canker. Cocking believes it will be also be applicable for ash dieback.

The liquid is pushed into the stem of the tree at pressure through a special steel applicator. Treatments cost around £300 per tree. "We hope that we can market this in about a year’s time," says Cocking. He believes the testing and treatment of ash dieback could be a potentially lucrative area of work for tree firms.

Dr Glynn Percival at the University of Reading is developing a treatment based on salicylic acid — a chemical related to aspirin. The chemical switches on the plant’s own defence system in a process known as "systemic induced resistance". In experiments done on apple scab, the severity of attacks is reduced by up to 60 per cent. Percival believes that the chemical could have a similar effect against ash dieback.

He explains: "If the disease becomes more prevalent, we will be looking for trial sites where we can test the effect." The chemical can be sprayed on the trees or used to drench the soil around them.

Natural resistance

In the longer term, foresters and land managers may have to look for trees with more natural resistance. Research done by Professor Erik Dahl Kjaer at the University of Copenhagen shows that around five per cent of trees have a very high level of natural resistance against ash dieback.

The studies, published in the journals Heredity and Forest Pathology, seem to indicate that some trees have an "active defence" and will attack the fungus itself. The researchers believe it will be relatively easy to pick out the most resistant trees and use them to clone new strains of ash that would be able to survive the effects of the fungus.

However, all these measures lie in the future. For most nurseries, the only current option is constant vigilance. Last month, Defra inspectors visited Wyevale Transplants looking for ash dieback. The nursery was given a clean bill of health. "It’s very worrying because if anything suspicious is found it will be tested. If the disease is confirmed, our entire stock of ash will have to be destroyed," says Jenkins.

Because of this, Wyevale Transplants has been particularly careful. "I walk and check our crop regularly. We generally buy British seeds and grow it ourselves." He is particularly wary of imported trees. "In two weeks, I will be going to Belgium to buy ash and make sure that it is OK. I won’t take stock that I haven’t inspected personally."

At present, most people remain blissfully unaware of ash dieback. But unfortunately, this situation is unlikely to continue. Within months, rather than years, ash dieback could be casting its shadow over Britain’s woodlands, parks and street scene — and within the industry, everyone will have to take appropriate steps.

See further information on Chalara dieback of ash from the Forestry Commission.

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