Pathogens and pests continue to wreak havoc to some UK tree species

TEN years ago I penned a piece entitled ‘Last tree standing’. The article focussed on the ever-lengthening list of alien insect pests and plant pathogens picking off our native and naturalised trees. Since then, the situation has deteriorated further, with some of the world’s worst pests and pathogens added to the list. I confess to having forgotten about all this until November 2023, following publication of research showing how UK forests could face catastrophic ecosystem collapse in the next five decades unless action is taken now.

Control is within our grasp, but the last 25 years have shown how hopeless this country is at pest and pathogen exclusion and quarantine. 

In May 2023, the Forestry Commission announced a new initiative to protect sweet chestnut (Castanea sativa). The latest threats to sweet chestnut are what CABI describes as the world’s worst disease and the world’s worst pest of the genus Castanea – chestnut blight (Cryphonectria parasitica) and oriental chestnut gall wasp (Dryocosmus kuriphilus), identified here in 2011 and 2014, respectively.

With apocalypse now on the agenda it is time to revisit ‘Last tree standing’. The collapse of our forests was essentially what I was warning about 10 years ago. Some of the pests and pathogens up for discussion today were here in 2014, but have since been topped up with some of the worst the world has to offer.

A TALE OF TWO CHESTNUTS

Oriental chestnut gall wasp causes serious damage including leaf galls and dieback (Image: Patrick Mannix)

In the early 2000s, the main concerns for white-flowering horse chestnut were bacterial bleeding canker (Pseudomonas syringae pv. aesculi) and horse chestnut leaf miner (Cameraria ohridella), but by 2014 there were bigger fish to fry. White-flowering horse chestnut was neither a forest nor a timber tree, its moist white wood being ideal for making milkmaid’s buckets and shelving for Kentish apple stores, but little else. 

Sweet chestnut, with a longer UK domicile and commercial timber credentials, became the focus of attention in 2011 following the identification of sweet chestnut blight (Cryphonectria parasitica) on trees imported from France and planted as nut orchards in Warwickshire and Sussex. UK plant health authorities appeared to take this potential disaster in their stride, but the situation worsened after bulk consignments of planting material from France were imported, split up and distributed to customers around the country. Authorities initiated trace-forward exercises, but not all consignments could be identified because documentation never existed or had been lost. Outbreaks began to pop up across the country and today Forest Research is actively involved with chestnut blight on 65 sites across England.

In 2015, oriental chestnut gall wasp (OCGW) was identified in woodland in North Kent and on mature street trees at St Albans in Hertfordshire. CABI describes OCGW as the world’s worst insect pest of Castanea. Eradication programmes were mounted, but one year later the pest was recorded at multiple sites across London and the home counties. The FC promptly abandoned eradication for containment.

Sweet chestnut was one of the first trees to become infected with Phytophthora ramorum. Things took a nasty turn in 2015 when it was found in woodland in south-west England, with implications of disease spread via spores between trees. The latest figures from Forest Research show 50 infected sites.

HEARTS OF OAK

Acute oak decline, with characteristic, dark-coloured, vertical, weeping fissures known as stem bleeds or cankers, and seeping black fluid through vertical cracks in the bark. This is one of the classic symptoms of AOD, but the causes are complex. (Image: FJ/TM)

In ecology, landscape (park and hedgerow) and timber, English oak is head and shoulders above the rest, which makes it all the more confounding why DEFRA was not quicker off the mark in funding the fight against acute oak decline (AOD). Early advances were almost entirely thanks to funding by Woodland Heritage, topping £1 million by 2014. Research into this highly complex condition has mushroomed into a truly all-encompassing programme. Complex associations involving beetles and plant pathogenic bacteria are under the microscope, but whether we are any nearer to eliminating the problem is another matter.

Oak processionary moth (Thaumetopoea processionea) fared far better from DEFRA funding, probably because it was regarded as a public health pest. The £500,000 budget established in spring 2013 by FC England for three years of surveying was exhausted after one year. In 2014, OPM was still restricted to half a dozen west London boroughs with another infestation in Croydon, south London, and an outlier at Pangbourne in Berkshire.

Lack of forward and lateral thinking held back progress. The FC aerially sprayed the bacterial insecticide Bacillus thuringiensis subsp. kurstaki (BTK) on two small woodlands (Herridge’s and Broom’s Copses) at Pangbourne in Berkshire in May 2013. It took place against the better judgement of Natural England and concerns expressed by conservation organisations, local residents and politicians. 

The FC said it was a one-off attempt to eradicate OPM from Berkshire. No pre-treatment pest surveys were carried out to determine whether there was actually any OPM infestation in the target woodlands. BTK lacks the potency and persistence required to eradicate OPM. Instead, they managed to decimate the non-target green tortrix moth larvae and in doing so hit the breeding blue tit population which relies on these insects to feed its young. The FC tried again in mid-May 2014, spraying nearby Sulham Woods with Btk, but fared no better.

Oak processionary moth (OPM) monitoring and control did well from government funding right from the start because OPM was considered a public health pest. However, that has not stopped it spreading apace since the first finding in 2006. (Image: FJ/TM)

The number of OPM nests rose in 2014 to the highest level yet (12,513). Over 46,500 trees were sprayed across 322 sites. Favourable weather appears to have been the main factor driving pest spread, overwhelming the already huge and costly control programme.

A decade on, the OPM scene has changed out of all proportion, with breeding OPM populations now found in all 32 London boroughs, all Home Counties contiguous with London (Kent, Surrey, Berkshire, Buckinghamshire, Hertfordshire and Essex) and Sussex, Hampshire and Bedfordshire. The number of oak trees sprayed each year now tops 200,000. OPM is clearly here to stay.

CHALARA CONFUSION

This Norfolk woodland is where the very first identification of Chalara ash dieback in the wider environment was made in late summer/early autumn 2012. One year on, the woodland’s common ash, saplings and mature trees, showed advanced dieback. (Image: FJ/TM)

If Phytophthora ramorum was a conundrum, then Chalara ash dieback sowed the seeds of confusion. In 2014, the fourth scientific name (Hymenoscyphus fraxineus) for the pathogen was announced. 

Chalara ash dieback was first identified in the wider environment in late summer/early autumn 2012. The FC claimed its spread had slowed down in 2013, but the disease came back to bite in 2014, moving into new areas and causing an explosive outbreak in Lancashire, subsequently spreading into Yorkshire and Cumbria. The FC started adding red dots to the map early in August 2014, but said nothing and was duly rewarded with no media coverage.

Not until the second week of October 2014, when the Woodland Trust told The Daily Telegraph that ash trees in its Greendale Wood near Grindleton in Lancashire were infected, did this massive new regional outbreak receive any coverage at all. By this time, the initial 10-case cluster around Burnley and Blackburn, first posted in early August, had exploded into over 60 cases stretching west to Blackpool and north and north-east into the Forest of Bowland, the North Yorkshire Moors and West Yorkshire. One week later, outbreaks were posted for the South Lakeland District of Cumbria. 

The very last red dot was added to the map on 3 November 2014 in Pembrokeshire, as far west as you can get in Wales. The FC now started to display an interactive map divided into 10 km x 10 km squares, each with an Ordnance Survey reference. Clicking on a square revealed the numbers and dates of wider environment outbreaks for that 100 km2. 

The FC no longer showed recently planted ash sites infected with Chalara, claiming the distinction had become less important, because “the diseased plants at many such sites have been removed and replaced, thereby eliminating the disease from the area”. But not if the disease had already escaped into the wider environment but had not yet been found. Given the huge number of question marks still surrounding the disease, the last thing the FC should have been doing was removing relevant information from the public domain. Landowners finding Chalara in their woodlands had no way of trying to trace the disease back to newly-planted ash stands in the surrounding area. They would now have to assume that the pathogen had simply dropped out of the sky, which is clearly what the authorities wanted everyone to believe. 

By early November 2014, a total of 902 outbreaks had been recorded in the wider environment, plant nurseries and recent plantings, an increase of 262 since July 2014. Austin Brady, Woodland Trust director of conservation and external affairs, said: “The impacts of the disease in the short term are becoming clearer, though we have yet to see how much natural resistance exists within our native ash population.” 

Sadly, the Woodland Trust was looking for something (genetic resistance) which did not exist in the Fraxinus excelsior/Chalara plant host/pathogen relationship. FC England had already planted 250,000 ash trees for genetic resistance screening, only to realise that in this instance no such thing existed. Differences in the reactions of ash trees to Chalara were almost entirely due to differences in the tolerance of trees to disease. Genetic resistance to Chalara ash dieback had not been documented anywhere in Europe at that time and was unlikely to exist naturally in common ash.

A huge volume of research findings sprang up across Europe, but our plant health authorities only appeared to take notice of that which suited them. Movement of airborne spores from Europe was favoured by the FC, and for obvious reasons, even though it contradicted all their own science at the time. However, there was a clear mismatch between the likelihood of long-distance airborne spore travel and the time lines and distance lines relating to disease movement and patterns across Europe over the previous 20 years, as well as the high susceptibility of Chalara ascospores to UV (ultraviolet) light and desiccation.

Spore-trapping studies carried out by Dr Anne Chandelier in Belgium and presented at the Fraxback Conference in London, threw most light on this matter. She showed how ascospores released from apothecia on the previous year’s fallen leaf stalks concentrate just 0.5 m above ground level. The number of spores recorded 3 m above ground level was 30 times less. Lateral movement of most spores was no further than 50 m from the point of release. 

Anne Chandelier was questioned about the airborne movement of Chalara ascospores and their potential for long-distance travel over hundreds of km. She expressed strong doubts, referring them to her findings and the high susceptibility of Chalara ascopsores to UV light and desiccation. One questioner in the international audience with an upper-quartile English accent persisted and cited the long-distance travel of spores produced by rust fungi such as Hemileia vastatrix (coffee leaf rust). He was cautioned by Dr Chandelier against any direct comparison of Chalara ascospores and spores of rust fungi. Rust fungal spores have thick pigmented walls that prevent desiccation and defy sterilisation by UV light.

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With Chalara ascospores concentrating just 0.5 m off of the ground there is clearly little scope for lift-off into the atmosphere and long-distance dispersal in high-altitude air currents. Moreover, any ‘high flying’ spores would clearly be exposed to correspondingly higher levels of UV light and stronger drying-out forces compared with the same spores close to ground level. In this respect the Chalara pathogen is completely different to Phytophthora ramorum, spores (sporangia) of which are released from infected leaves at the tops of tall Japanese larch trees. 

The UK government’s Chalara Management Plan, announced in March 2013, included laboratory screening of fungicides for activity against the disease, with follow-up field trials starting in spring/summer 2013 of the best ‘in-lab’ performing fungicides for protectant activity (prevention of leaf infection by spores) against the disease. Fourteen fungicides were selected by FERA including five which belong to a chemical grouping that displays little or no protectant action as part of its overall fungicide activity. Two full growing seasons had elapsed (since March 2013) with absolutely nothing worthwhile reported on this work. How long did it take to test 14 fungicides?

In 2014, the FC was still talking about ‘Protected Zone Status’ for areas where Chalara was not in the wider environment. How on earth could they establish and maintain protected zones in a small country like the UK for a pathogen with airborne spores? And with one of the most frequent and widely spread tree species as the primary host, and no means of control? Ten years on there is no corner of the British Isles where common ash has escaped Chalara. The latest predictions are for 80 per cent of ash trees to die from Chalara. Judging by reports of ash tree felling, a high proportion of the 80 per cent has already succumbed. But even that high percentage could be a conservative estimate because reports from an ongoing woodland restoration project in Derbyshire say 99 per cent of ash trees were infected with Chalara by 2020. 

Go back half a century and list the four mainstream hardwoods of prime economic importance – English oak, English ash, sweet chestnut and English elm – and then compare the standing of each tree now. English elm has gone. English ash is on the way out. Sweet chestnut is under siege and English oak is on trial. And all thanks to alien insect pests and microbial pathogens which could and should have been kept out of the country.