The Forestry Commission for the South East and London recently put on a free event at the York Club in Windsor Great Park to look at current and future scenarios. Although aimed at providing the latest information and management advice to assist those at the sharp end in managing the trees and woodlands in their care in London and the Home Counties, there are lessons for all, writes Dr John Jackson.

AS a scene-setter, Paul Beales from the Animal and Plant Health Agency (APHA) talked on ‘Famine, War and early Starbucks’, highlighting cases where epidemics in crops such as coffee, potatoes and grapes had a profound impact on social history.


Five morning presentations were then expertly covered by staff from the Forestry Commission or Forest Research on the life history, symptoms and current panorama for:

• Oak processionary moth

Ips typographicus

• Chalara or Hymenoscyphus fraxinea – ash dieback

• Sweet chestnut blight (Cryphonectria parasitica)

• Canker stain of London plane.

Andrew Hoppit dealt with OPM, Rob Coventry with ash dieback and Helen Carter the other three species.

On biosecurity, Millie Toft from the FC drew attention to their ‘Keep it Clean’ best practice advice and the free learning biosecurity modules online at

Dr Joan Webber of Forest Research, based at Alice Holt Forest near Farnham in Surrey, captivated the audience with an hour-long slot entitled ‘Tree Health – Current and Future Prospects’.

Many of the novel uninvited vectors of pests and diseases that have arrived on these shores first crop up in south-east England. That is especially true for insects and their allies for whom the Channel is no barrier if they are active fliers or disperse when the thermals or surface winds waft them in the right direction – although there is a constant rain of spores too.

The situation in south-east England may be worse than elsewhere in the UK because:

• the ‘heat island’ effect of London and the climate in the region is more akin to parts of continental Europe so amenable to pests from there;

• there is a high density of exotic plant species;

• there is a greater ‘disturbance’ factor as there are lots of trade or entry points.

These all add up to multiple opportunities for the arrival and establishment of new, unwelcome pests and diseases. Hence London and the south-east offer better opportunities to any invasive species lurking in the trees.

Ironically, promoting a greater diversity of tree species to boost resilience may actually give introduced organisms a better chance of getting a foothold here.

The speaker noted too how alien pests and pathogens are more damaging than native ones because they have not co-evolved with their hosts – and climate change may be altering the susceptibility of trees.

Forestry Journal: L–R: Paul Beales (APHA), and Andrew Hoppitt, Helen Carter, Craig Harrison and Rob Coventry (all Forestry Commission).L–R: Paul Beales (APHA), and Andrew Hoppitt, Helen Carter, Craig Harrison and Rob Coventry (all Forestry Commission).


Joan then examined three special case studies:

• Often painted as an alien, Red-band needle blight or Dothistroma septosporum is just possibly native here. It was first reported in Dorset in the mid 1950s, long before it decimated Corsican pine plantations in East Anglia. It is likely that after the great storm of 1987 wreaked havoc, infected stock was inadvertently used to replant the Brecklands, resulting in massive problems.

• The Asian long-horned beetle (Anoplophora glabripennis) hit the headlines in 2012 when it reared its antennae in poplars in Paddock Wood in Kent as a breeding population. A major campaign eradicated it – but it had probably been around there for 10 years before – on the northern limit of its climatic distribution.

• The gypsy moth (Lymantria dispar) has been around at a low level for maybe 20–30 years but is only now radiating out – climate change is implicated.

So, some pathogens may already be native and one day swap to a new introduced host – or they may be alien yet latent until some factor changes and they become troublesome challenges. New conditions may include climate change, novel silvicultural practices or tree crops, the plant trade or the organism itself evolving to become more virulent.

The bacterium Xylella fastidiosa is a potentially massive problem, affecting over 550 plant species and causing sleepless nights for plant people. There are at least four subspecies described, all spread by sap suckers (spittle bugs).

X. f. subsp. multiplex is endemic to the USA and has turned up quite recently in Europe, infecting olive groves in the Mediterranean area, starting in southern Italy where it is politely translated as olive quick-decline syndrome. More concerning for foresters is that this subspecies of the bacterium can also infect and affect species of oaks, maples and sycamores. However, infected trees may not show symptoms unless stressed, so it may go undetected for some time yet still be spreading.

Meanwhile, it is better to be safe than sorry so the BRIGIT project is underway, coordinated by the John Innes Institute near Cambridge, to stop Xylella reaching these shores and to understand the challenges to confront should it arrive. Work involves encouraging volunteer networks and modelling scenarios if it should turn up uninvited.

Forestry Journal: Joan Webber explains Xylella.Joan Webber explains Xylella.


A reminder too from Joan was that disease and decline are not one and the same – many factors are implicated, for example, in sudden oak decline.

She floated a series of thoughts on the future; how long may it take before we humans witness and realise the impact of an introduced pathogen?

Legislation is usually only set up after an organism is already present and causing problems; and tree diseases are far more difficult to spot compared with those in agriculture. In natural environments such as woodlands, things stay below the radar for longer than in agricultural systems – and tend to be worse as they have built up to a point of no return when they are eventually detected and outlawed.

Examples are:

• Dutch elm disease that changed the face of the English landscape in the 1960s was first recorded in the UK back in the 1920s in a mild form.

• The Asian long-horned beetle was first confirmed here in Kent in 2002 – likely arriving in dunnage – but the breeding outbreak wasn’t until 2012, a decade later.

• Although ash dieback was first officially confirmed in 2012, ring dating infected trees showed it was already on the loose in the UK by 2004 and possibly even back to the mid 1990s, so there may be a ‘latent’ period of some 20 years.

• Is it premature to predict where we will be with ADB 10 years from now – what percentage might prove ‘tolerant’ and will the epidemic peak and some ash make a comeback over time? Who knows!

• And does each and every alien pathogen introduction end in disaster? The straight answer is ‘no’, as most fail to become established.

On a brighter note, Joan cautioned about over-dramatising and half-truth news – an example is with Dutch elm where some current headlines declare there are none left – but that is not quite the full picture. And with bleeding canker in horse chestnut, not all conkers have died and some are showing signs of recovery and a degree of resistance.

So, although the pests and diseases scene is grim – with no room for complacency – it is not always as gloomy as it is sometimes painted.

The question of whether we can help boost tree resistance is sometimes posed, but Joan suggested there was no single solution or magic bullet to the tree disease challenge in the UK.

Summarising, she urged tree people not to jump to conclusions, beware fake news, to minimise stress on trees, to diversify and accept that some pests and diseases are here to stay but can be managed to avoid epidemic levels.