Defending tree health

Research and legislation are working to stop the spread of potentially devastating threats to trees. Jez Abbott reports.

When will Royal Botanic Gardens, Kew call up the British army, just as the Belgians mobilised theirs to fend off an invasion of the dreaded oak processionary moth?

Sadly not even the military might of Belgium and the spectre of troops with flame throwers could repel this invasive enemy. France, Germany and the Netherlands had already fallen and England was the next stop on the pest's European campaign.

Oak processionary moth was found on several London sites in 2006 and Kew was one of them. Immediate controls failed to eradicate the moth and this July saw staff at the arboretum remove caterpillar nests from affected trees.

"Our knowledge of the moth has moved on in leaps and bounds since it first appeared in the gardens," says the head of the arboretum, Tony Kirkham. "Our scientists have been researching this new pest by studying nests, caterpillars and moths.

"This work is not only vital to prevent oak processionary moth spreading from west London to other parts of the UK, but for the future management of other exotic pests and diseases that can thrive in the UK thanks to our warmer weather."

Under attack

Kirkham has good reason to fear further attacks at Kew and elsewhere. Although there are no records of oak processionary moth killing trees, the pest feeds off leaves causing massive defoliation.

And like all wars, there is a human cost. The silky white hair shrouding the larvae can cause severe skin irritation and allergic reactions, and this nasty combination makes oak processionary moths so worrying to tree and health experts.

Once established, the moths tend to flourish on urban trees, along forest edges and in amenity woods, where the probability of it coming into contact with people is high. But forming a battle plan is proving as controversial as the moth itself.

And the moth is not alone. Researchers at the RHS have found the number of plant disease outbreaks is up by 60 per cent on last year. Burgeoning imports and continuing climate change "suggest the problem will only get worse", they forecast.

The collective worries of the RHS were enshrined in a report this summer called Non-native diseases and the future of UK gardens. In it, the society calls for a worldwide risk-management system similar to supermarket food-assurance schemes.

RHS director of science and learning Dr Simon Thornton-Wood says: "Current inspection works well for diseases we already know about. But it's the diseases on plants that would not normally be considered problematic that are the real cause for concern."

Previously unknown Phytophthora kernoviae and P. ramorum, for example, caused havoc with rhododendrons, he says, calling for a quality-assurance system on the £870m of plant imports to Britain.

"An industry code of conduct could provide answers at every stage of the production and transport process."

This code of conduct could include: supplier certification and an inspection report; an auditing process for pest and disease control; post-entry quarantine until inspection has been completed; and continued monitoring of imported plants until sold.

However, not everyone is convinced by the RHS's statistics. The HTA doubts disease outbreaks are up by 60 per cent and is calling for "objectivity", "realism" and "factual" debate. Any code of practice should be agreed across Europe to ensure UK growers aren't disadvantaged.

And it needs to be voluntary, says HTA director general David Gwyther. Producers and retailers are highly sensitive to risks, and destroying stock deemed by inspectors to be infected costs the trader dear, unlike food traders, who are bailed out by government.

Gwyther says: "We have a robust plant passport mechanism on imports and the Plant Health & Seeds Inspectorate checks stock at ports of entry, nurseries and garden centres. If there were legitimate concerns, then these are the mechanisms that should be strengthened."

Taking action

The finance and economic implications of pests and diseases, however, are becoming more tangible. That said, HTA director of business development Tim Briercliffe is keen to avoid "spreading panic". Both the RHS and HTA belong to a working group that was set up two years ago to look at the issues.

Forest Research, the science wing of the Forestry Commission, also belongs to the group and says the need for coordinated action has become "acute".

Head of plant health Roddie Burgess says balancing good protection with unfettered trade is hard. "Regulate too tightly, and you could end up in a Geneva court for creating artificial trade barriers. Plant controls must be justified and the minimum necessary to give an acceptable level of protection. But the system is not perfect: no system is."

Forest Research principal pathologist Dr Joan Webber echoes the RHS fears that numbers of pests and pathogens are on the up. Either that, or advances in science such as molecular screening have improved methods of detection, she adds.

Degree of control

Dr Webber will not be drawn on the need for a code or tighter controls because "scientists should not trespass into areas of policy". But the damage caused by new threats including oak processionary moth pale beside the king of all pests and diseases.

"Dutch elm disease was the worst tree disease by far, and is still having an impact. For some of the other diseases, such as Phytophthora, there is a very strong climatic association so changing weather patterns may have an impact on the kinds of pests and diseases that reach our shores and our methods of eradication in future."

Defra is holding a consultation on future disease management and it will not comment on the possibility of more rules until the completion of talks this month.

Hillier Nurseries director Hossein Arshadi wants a code of practice because disease control is an evolving science, which is new to many landscapers and councils. He says: "I'm not against nurseries importing young trees and liners to grow them on as they have the expertise to inspect them. But I think the problem lies with imports by traders who don't detect problems."

Although commercial nurseries are checked twice a year by the Plant Health & Seeds Inspectorate, it is hard for local authorities or landscape firms to know about new pests and diseases, Arshadi says, citing canker, leaf miner and moths on chestnuts.

In west London, Kew is like a test case on counter-insurgency, and Kirkham talks tough: "We have worked hard by monitoring and introducing measures to destroy nests and caterpillars, and manage outbreaks of oak processionary moth."


- Oak processionary moth, Thaumetopoea processionea, was found across London in 2006, the caterpillars feeding on leaves of English, Sessile and Turkey oaks. Hornbeam, hazel, beech, sweet chestnut and birch near badly affected oaks have also been hit. The pest is native to central and southern Europe, but climate change is thought to have prompted its spread north.

- Bleeding canker on horse chestnut was first reported in the 1970s, caused by the fungal pathogen Phytophthora. Until recently the bleeding cankers were considered rare and were seen only in the south of England, but in the past four or five years infection rates have shot up. In 2006 more than 110 cases were reported, as far north as Glasgow and Fife, against four cases in 2000.

- Horse chestnut leaf miner, Cameraria ohridella, was first seen in Greece in the late 1970s and was dubbed a new species of the genus Cameraria. By 2002 it had gained a foothold in the UK, in Wimbledon, from where it has spread rapidly. It is found across central southern England, East Anglia and the Midlands. The rate of spread, up to 60km per year, is akin to that on the Continent.

- Red band needle blight causes premature needle defoliation, resulting in loss of yield and, in severe cases, tree death. Until recently the disease was mainly found in the southern hemisphere, usually on radiata pine. Since the 1990s there has been an increase in Europe. It is in Britain and threatens one of the most productive commercial pine species, Corsican pine.

- Phytophthora, Greek for "plant destroyer", is one of the world's most destructive pathogens. P. infestans blights potatoes and contributed to the Irish potato famine in the 1880s. Two species, P. ramorum and P. kernoviae, were discovered in the UK in 2003. The former is the cause of sudden oak death in the US, while both cause lethal stem cankers on beech and red oak in the UK.

- Dutch elm disease first appeared in north Europe around 1910 and in the UK by 1918. The first disease pandemic, spread by native and European bark beetles, swept across Europe in the 1930s and 1940s, and a more destructive outbreak struck in the 1960s. More than 20 million elms in Britain were killed, and by the 1970s it had colonised woods in north and western Britain.

Have you registered with us yet?

Register now to enjoy more articles and free email bulletins

Sign up now
Already registered?
Sign in

Read These Next

Follow us on:
  • Facebook
  • LinkedIn
  • Twitter
  • Google +
Horticulture Jobs
More Horticulture Jobs

HW Top UK Arboriculture Businesses

See our exclusive RANKING of arboriculture businesses by annual turnover. 


Build your business with the latest public sector tenders covering landscape, arboriculture, grounds care, production and kit supplies. To receive the latest tenders weekly to your inbox sign up for our Tenders Tracker bulletin here.


Find out more about the outstanding parks, gardens and arboricultural projects and teams that became our Custodian Award 2019 winners.

Jeremy Barrell On...

Jeremy Barrell

Tree consultant Jeremy Barrell reflects on the big issues in arboriculture.

Products & Kit Resources