South London is home to a wide variety of trees both in parks, streets, residential gardens, nature reserves and on common land. You only have to look at the names of many streets to get an idea of what can be found (or was once found on that road). Elm Grove Road, Cedar Close, Walnut Tree Close, Holm Oak Close, Oakhill Road, Chestnut Avenue, Larches Avenue, these are just a few examples.
Unfortunately, many of these trees are now under serious threat from new, foreign pests and diseases that are arriving here, giving local councils and tree surgeons many new problems to tackle on a regular basis. Good tree surgeons will be up-to-date about all the new pests and diseases and should you need to employ one for general tree surgery work, most responsible tree surgeons will be vigilant when working on a tree and look out for any signs of these pests and diseases.
Local councils now have to be far more vigilant also, checking council owned trees much more regularly for signs of pests and diseases along with tackling any they find as quickly as possible to prevent their spread, with tree surgeons often having to cut down diseased trees to try and halt the infections from spreading.
Although we have always had to deal with the threat of pests and diseases it has never been so serious until now (see timeline below) and this, along with the high costs of maintaining our large trees has caused councils to rethink their planting strategy so that they now plant, height wise, smaller trees, with a lot more diversity in species.
Planting a variety of species is certainly important due to our large trees being under threat and with the possibility of a whole species of tree being wiped out as with the elms, which would result in London losing thousands of trees in one go causing devastating consequences, it may still be short-sighted of councils in the long term to discontinue with the planting of the much taller growing trees and in particular the London Plane tree.
The London plane was once very much the street tree of choice, its benefits to London are immense and cannot be underestimated and although under threat itself from Massaria, this fast growing tree is still a very hardy specimen which is able to resist most pests and diseases.
As well as being such a hardy tree, one London plane is able to do far more than many small trees put together of shade it offers, how much water it can absorb and thus helping to reduce flooding and for the high amount of pollutants it can remove from the air.
Therefore long term benefits surely outweigh the short term savings in having to maintain them. Perhaps it is time for councils to think again and return to planting them, with the smaller, diverse varieties as under planting.
TIMELINE OF PESTS & DISEASES FROM THE 1970s to 2015
Dutch Elm Disease (which wiped out most of our elms and is considered to be one of the most serious tree diseases in the world)
Great Spruce Bark Beetle,
Phytophthora Disease in Alder Trees
Gypsy Moth (feeds on broadleaved trees)
Dothistroma Needle Blight in pine trees
2002 – 2015
Phytophthora Ramorum in beech trees
Horse Chestnut Tree Leaf Miner
Phytophthora Kernoviae in beech trees
Bleeding Canker of Horse Chestnut trees
Phytophthora Pseudosyringae in hornbeam and beech trees
Oak Processionary Moth
Pine Tree Lappet Moth
Phytophthora Ramorum in larch trees
Acute Oak Decline
Phytophthora Lateralis in Lawson cypress trees
Phytophthora Austrocedrae in our native juniper trees
Chestnut Blight in sweet chestnut trees
Asian Longhorn Beetle in broadleaved trees
Chalara Ash Dieback
Oriental Chestnut Gall Wasp
This shows a very significant increase since 2002 on previous decades.
WHY WE ARE UNDER ATTACK FROM NEW PESTS AND DISEASES
Many pests and diseases arrive here on the exotic trees and plants we are so fond of importing to adorn our parks and gardens and whilst stricter inspections and controls have been put in place, many arrive undetected in the soil of potted shrubs, trees and plants for example.
It is thought that some can even arrive here through windblown spores, whilst others arrive on the wooden packing materials used to create the various goods we import.
Most of these pests and diseases do not cause much in the way of devastation in their native habitats but when they arrive here many can be virulent and fast spreading and without the natural predators of their homelands to keep them in check it is easy to understand why their spread can be so rapid.
It is thought that climate change also has its part to play. With much milder winters than we were once use to, the warmer conditions allow emerging caterpillars to survive, whereas a good long cold spell, with snow and frost would most certainly help to kill most, it not all of them off!
HOW YOU CAN HELP TACKLE THESE PESTS AND DISEASES
The general public have a vital role to play in helping combat these pests and diseases before they have a chance to wipe out many of our much loved trees by regularly checking their own trees for signs of anything untoward and also by regularly clearing up fallen leaves in the autumn and winter months, pests and diseases love to overwinter in them, regular removal will deprive these pests and diseases of a nice warm habitat in which to survive the winter.
Most people have camera phones nowadays so instead of calling out a tree surgeon unnecessarily, if you spot something that looks strange or causes you concern, take a few photos and then contact some local tree surgeons by email attaching the photos, this should enable them to see if there is anything to worry about or not.
Likewise if you notice anything strange on street trees or on the trees at your local park or common a few photos emailed to your local councils parks department, with as much detail as you can of where the tree is located will save them a lot of time if there is actually nothing to worry about or if they are already aware of the problem.
Finally, and although it may seem a bit extreme it does make perfect common sense, when visiting a woodland ensure you go with clean boots and clothing (including equipment, for example bikes) if going for sporting reasons.
When leaving, brush plant debris and mud (as far as is practicable) from boots, clothing, pushchairs, dogs paws and bikes and then give everything a more thorough cleaning when you arrive home.
The idea is – don’t take any unwanted pests and diseases with you when visiting wooded areas and don’t bring any back with you either!
SOME OF LONDON’S MOST COMMON TREES
Some of the more common trees that you are likely to see in London are: Oak, Plane, Sycamore, Beech, Larch, Birch, Ash, Maple, Elm, Hazel, Hornbeam, Sweet Chestnut, Horse Chestnut, Alder, Lime, Buckthorn, Spindle, Willow, Poplar, Walnut, Spruce, Bay, Pine and Fir trees and many of these, including Horse Chestnut, Ash, Oak, Spruce, Pine and Plane are all either under threat or suffering from various different types of pests and diseases.
Below we will take a look at some of the pests and diseases that are either affecting our south London trees or that pose a serious threat to them along with the damage they cause, which trees are affected and the signs and symptoms to look out for.
HORSE CHESTNUT LEAF MINER (Cameraria ohridella)
The horse chestnut leaf miner was first seen in Wimbledon in 2002 and it is now very noticeable on many of our horse chestnut trees in London. Unfortunately this pest has managed to spread in most directions and can now be found in most of England, parts of Wales and has even been spotted in Scotland.
The horse chestnut leaf miner was identified in 1985 in Macedonia, northern Greece, was classified as a new species in 1986 and has now become widespread throughout both eastern and central Europe, being very easily spread. Adult moths in flight are blown to other areas by the wind or through transportation on vehicles, including on infested leaves attached to various forms of transport.
The damage to leaves caused by the larvae of this mining moth is very noticeable with most of the leafs tissue often destroyed, leaves become shrivelled and brown, dropping very early and certainly before they should in autumn.
This destruction happens on a yearly basis but so far does not seem to be affecting the health of horse chestnut trees, it just spoils the look of them but there are concerns that climate and interaction with other pests and diseases, including bleeding canker of horse chestnut trees may lead to this pest having a greater impact on these trees.
Therefore long term monitoring and studies of over three hundred horse chestnut trees at various sites across southern England are being carried out, with all of the trees being inspected twice a year for signs of die back, impaired growth, crown disease and infestation.
The moths are tiny, brown and can be spotted swarming around horse chestnut trees, most unpleasant when you have to walk through them.
Reporting sightings to the Forestry Commission is not required but you can help monitor its spread by reporting any new sightings of trees that have become infested by them.
The only way at the moment to help prevent the horse chestnut leaf miner from multiplying and spreading is to clear fallen leaves during the autumn and winter months and then thoroughly composting them, as this will destroy any overwintering pupae. Alternatively, small heaps of leaves can be covered with a layer of soil as this will help stop the adult moths from emerging in the spring.
OAK PROCESSIONARY MOTH (Thaumetopoea processionea)
A native of southern Europe where its impact on oak trees is minimised by natural predators along with various environmental factors, the oak processionary moth was thought to have been accidentally introduced into Britain on young oak trees imported from Europe as overwintering eggs.
The oak processionary moth was first spotted in Richmond in 2006 with further large outbreaks now to be found in west and south west London and parts of Surrey, including Ealing, Wandsworth, Hounslow and Brent. Unfortunately the Forestry Commission believe that it will be impossible to eradicate these large outbreaks, with continued work being carried out as the only way in which to try and contain and minimise its impact, size and spread.
To highlight just how bad things are, in 2012 Richmond Park removed 4,188 nests with over 1,500 trees having to be destroyed, heartbreaking.
The caterpillars are now starting to emerge from the affected areas in London, with the first ones being reported on 6th April 2016 and controlled treatment with an approved insecticide to kill these caterpillars is being carried out, although some will survive and start descending down the trees where they will be noticeable to the human eye by early May.
Other, smaller outbreaks have been found in various places but quick action to remove nests and regular monitoring has helped to eradicate this pest in those areas.
IDENTIFYING THE OAK PROCESSIONARY MOTH
The brown moths are indistinctive but may be spotted when they become active during mid to late summer busily laying eggs on the branches and twigs of oak trees.
On the other hand the caterpillars are easy to spot, are covered in masses of hairs and move around from nose to tail in late spring to early summer on oak trees (they do not inhabit fences, walls etc).
If you spot any when out walking under no circumstances should you touch or go near the caterpillars or their nests and keep children and pets away from the area as the hairs are highly toxic. Contact can lead to itchy skin rashes, irritation to the eyes and in some instances sore throats along with breathing problems.
Should you see any on an oak tree that you own then you can either contact your local authority for advice or a local tree surgeon. Many London tree surgeons now offer an oak processionary moth nest removal service. Please bear in mind that public-sector and residential oak tree owners are responsible for controlling the oak processionary moth in their trees if they are in the oak processionary moth core zone.
TREE DAMAGE CAUSED BY THE OAK PROCESSIONARY MOTH
The caterpillars of the oak processionary moth feed on the leaves of several of the oak tree species, with large populations easily defoliating large parts of oak trees.
This defoliation makes the oak tree extremely vulnerable to attack from other diseases and pests and less able to deal with extreme environmental conditions that we can suffer from these days such as floods and droughts.
Should large populations of oak processionary moth exhaust their supply of oak leaves they will start feeding on the leaves of other trees and they have now been seen on, birch, hazel, sweet chestnut and beech trees.
New regulations and restrictions have been introduced to control the spread, with this pest now appearing to be here to stay, making treatment and control likely to be something we will now have to live with, certainly for the foreseeable future.
ASIAN LONGHORN BEETLE (Anoplophora glabripennis)
A native of the Korean peninsula and China, it is thought that our much milder climate could lead to the Asian longhorn beetle being able to survive and adapt in the south and south east of England and it was first spotted near Maidstone, Kent (Paddock Wood to be exact) back in March 2012.
Having been accidentally introduced into Italy and the United States, it has gone on to cause extensive damage to the tree population of those countries and there have now been reported outbreaks in several other countries that belong to the European Union too.
Not only is this beetle a very serious threat, it is able to cause extensive damage to many different species of broadleaved trees, with the following species of trees particularly susceptible and known to be hosts: the Acers (maples and sycamores), horse chestnut, alder, birch, hornbeam, hazel, beech, ash, plane, poplar, Prunus (cherry, plum), elm, North American red oak, American pin oak, Robinia (false acacia / black locust), Pagoda tree, Salix (willow, sallow), Sorbus (including whitebeam, service tree, and mountain ash) along with apple and pear trees, with apple trees being the most common tree found in the residential gardens of London.
Luckily the breeding population spotted in Kent in 2012 was dealt with before the adult beetles could emerge and then go forth a multiply, with a total of two thousand one hundred and sixty six host trees removed, including sixty six infested trees.
It is thought that these particular beetles arrived here on untreated wood packaging material from China. Any wood packaging used to import goods into the European Union should be marked in order to prove that it has been treated and it is illegal to import any wood into the United Kingdom should it display any signs of this beetle.
Since the 2012 outbreak (which appears to have been contained), surveillance at all the United Kingdoms ports has been increased. As this beetle would be able to inflict so much damage on many of our deciduous trees, vigilance will certainly need to be maintained, particularly in light of what has already happened in Italy and the United States.
IDENTIFYING THE ASIAN LONGHORN BEETLE
On occasion mistaken for our own common native beetles, the Asian longhorn beetle is quite distinctive, adults are a shiny black with varied, white markings. They are also large in size, ranging from 20mm to 40mm in length and have very noticeable antennae which are black in colour with either light blue or white bands and these antennae are about twice the length of the beetle’s body!
They are also very similar in looks to the citrus longhorn beetle, another threat to our British trees and a non-indigenous species.
Signs of their presence are:
Circular exit holes normally found on the trees main trunk and above on branches. The holes are made by the emerging adult beetle and are around 10mm in diameter.
Droppings at the base of infested trees which look like piles of sawdust.
Areas on the tree where eggs have been laid may be subject to sap bleeding.
Shoots, small branches and bark may show signs of feeding damage.
If you see or suspect you have seen an Asian longhorn beetle then you are legally obliged to report it to the Forestry Commission. If you can capture it and secure it in, for instance a sealed glass jar so that a Forestry Commission inspector can collect it, that would be perfect.
These beetles are not harmful to humans but can nip so handle with care should you see one. Apparently this nip is no more uncomfortable than a nip by one of our own, larger native beetles and is unlikely to draw blood or penetrate the skin. A little nip would surely be worth it if your actions save hundreds of trees!
ACUTE OAK DECLINE
Acute oak decline mainly affects our own two native oaks, the pedunculate oak and the sessile oak but other non-native species found in the United Kingdom can become infected as well.
First noticed in the 1980s it has now been affecting our oak trees for over thirty years with several thousand oak trees now suffering from this disease and it seems to be spreading wide and fast.
Large numbers of oaks in both Richmond Park and Bushy Park are showing acute oak decline symptoms, with about sixty percent of the oaks in areas such as Lawn Plantation and Sheen Gate Wood being symptomatic.
It is still not known what causes this disease with studies into this condition continuing but some ideas put forward have been, bacterial infection from the oak jewel beetle found on many of the affected trees and drought stress.
Although many trees seem able to recover from this acute oak decline, those that become severely affected often die within four to five years of developing the disease and it has also been found that this disease is able to transfer from oak tree to oak tree.
SYMPTOMS OF ACUTE OAK DECLINE
Long fissures or splits of about 5cm to 10cm in length which form in the cracks between bark plates.
Bleeding patches on trunks which can go on to extend high up into the trees canopy. This bleeding or weeping starts in the spring and stains the bark as it flows downwards.
When bleeding stops, which it often does at certain times of the year, dry, black streaks are left on the stems which often then cakes or forms a crust around the split.
Signs of the oak jewel beetle may be present in the form of D shaped exit holes.
The oak trees canopy may show signs of deterioration. Unfortunately this only normally happens when the tree is close to death.
ADVICE FOR TREATING AND MANAGING ACUTE OAK DECLINE
Unless posing immediate health and safety issues it is recommended that an infected tree is left in place and regularly monitored. Where possible infected trees should be cordoned off to help lessen contact with any bleeds and in order to reduce the chances of the disease being transferred from tree to tree.
When visiting or working in areas that have infected oak trees everyone can help reduce the spread of infection by following a few simple rules.
Do not touch infected trees unless you have to carry out any works to them.
All footwear should be cleaned between visits to the infected area.
Those working on trees in infected areas should disinfect tools after working in those areas.
Do not remove plant material from any infected areas for example sticks or leaves, this probably applies particularly to those taking children on a woodland walk.
ASH DIEBACK (Hymenoscyphus fraxineus)
Ash dieback may also be referred to as Chalara or Chalara dieback of ash and is a very serious disease which particularly affects common ash including the ornamental variety Pendula, along with narrow-leaved ash. Both of these species of ash tree are affected in the United Kingdom.
Having first been reported in Poland in 1992 when a high incidence of ash trees were found to be dying it has since been found in large areas across Europe and was confirmed to be present in the United Kingdom in 2012, arriving on imported trees from the Netherlands, although it is entirely possible that the disease has been here for much longer. It was first reported in London in 2014 at Orpington in south east London and near Upminster in east London.
Before the ban on the movement of ash trees in October 2012 this would have been the most likely way that the disease has been able to spread itself over wide distances. However spread will most likely continue by way of spores on vehicles, clothing, footwear and due to the fact that the spores can be blown for up to tens of miles by the wind, whilst continued monitoring for ash dieback is constantly bringing new instances of the disease to light.
ASH DIEBACK SYMPTOMS & SIGNS
Ash dieback actually blocks the way ash trees transport water resulting in lesions on the bark, leaf loss and crown dieback. Young trees seem to be the most susceptible to the disease, with older trees resisting the disease for longer but generally, once a tree has been infected with ash dieback the outcome is usually fatal, with sick trees left vulnerable to being attacked by other pests and diseases and in particular honey fungus.
Signs of the disease include blackened, dead leaves, saplings will be seen with dead side shoots and tops, whilst mature trees will have dieback of branches and twigs to their crowns and dark lesions can be spotted on the trunk of the tree to the base of dead side shoots which are often a diamond shape. The leaves stalks and veins become brown or blackened and between July to October white fruiting bodies can be observed on these blackened stalks. All healthy ash trees should be in full leaf my the middle of June.
For London the news may not be as bad as elsewhere, London does not have a high number of ash trees and it has been reported that city trees fare far better than their country cousins, partly due to the fact that leaf litter is regularly removed, whilst drier conditions lead to less fruiting bodies, although there are over one thousand ash trees across the Royal Parks, making these ash trees possibly more susceptible to the condition.
Anyone who has an ash tree in their garden can help towards keeping the spread of the disease to a minimum by removing all ash litter in the autumn and then disposing of it by either burning, composting or burying it as this break the life cycle of the fungus.
HORSE CHESTNUT BLEEDING CANKER
Yet another serious threat that the much loved horse chestnut tree is having to cope with and found in horse chestnut trees over the whole of the United Kingdom, with park officers in London continuing to report the loss of horse chestnut trees to this disease.
Although not new in itself, with horse chestnut bleeding canker having been around since the 1970s and caused by two fungal organisms which have not given cause for concern, this new strain, originating from the Himalayas certainly is.
Thousands of trees are affected with a higher proportion being found in towns and cities, many of which are very valuable and highly visible amenity trees in public gardens and parks or important landmarks and features of avenues and historic gardens. Approximately seventy percent of horse chestnuts and ninety five percent of red horse chestnuts in the Royal Parks are infected with many mature chestnuts trees now becoming hazardous.
Germany and the Netherlands also have this strain of the disease and although it is not known how the disease arrived here it is thought to have been through the importation of infected trees intended for planting here.
Unfortunately there are no chemical treatments available to either cure or arrest this disease, some trees do manage to recover, whilst other either die or have to be chopped down for safety reasons.
The long term outcome of this disease is still not known and much research is still needed.
THE SYMPTOMS OF BLEEDING CANKER OF HORSE CHESTNUT TREES
This disease affects horse chestnut trees of all ages and shows both internal and external symptoms. Usually leading to the death of the tree, some trees actually go into remission and even recover.
The most noticeable symptoms are the bleeding lesions which are areas of dying bark on the stem or branches of the tree which seep a rusty coloured liquid. In dry summers the bleeding can stop with the liquid drying into a dark and brittle crust near the exit points, with bleeding often starting again in the autumn when conditions become moist and mild.
After a few months the bleeding bark area may crack apart from the centre, whilst fungi may be seen on the surface of the dead bark.
After several years cankers can eventually spread around the trees entire trunk resulting in visible crown symptoms such as early leaf drop, yellowing leaves and eventually to the death of the crown.
If you have a horse chestnut tree that appears to be infected in your garden then my best advice would be to contact a local, fully qualified, professional tree surgeon. They will inspect your tree to confirm if the disease is present and will be able to suggest the best way forward. In some instances it may be safe to leave and monitor the tree as it may go on to recover, or it may require a few branches to be removed that could easily and suddenly snap off, whilst it may be advisable to cut down a smaller, younger tree as infection is much more rapid to trees with small girths.
When a horse chestnut tree has to be removed due to bleeding canker it is not recommended that you replace it with another of the same species as this has often resulted in the new tree developing the same disease.
MASSARIA (Splanchnonema platani)
A fungal disease of plane trees, Massaria is affecting our London plane trees causing branches to die back thus making them prone to breaking and falling off. Whilst the disease does not appear to be affecting the trees health in a severe way it does cause an eventual loss of canopy shape which has always been of great landscape value.
The real concern at the moment is the dropping of large branches which poses a threat to public safety, with the London plane tree now requiring continual re-inspection and dead wood having to be removed by tree surgeons before it gets the chance to become a hazard.
It is not sure how the disease arrived here, it may have been imported on infected trees but it is also known that the spores are easily spread by the wind for very long distances so it could have be blown here from the Mediterranean.
Massaria can be identified as a strip of dead bark which begins at the base of branches and then continues along the top, tapering to a point. Due to being on the top of branches makes Massaria hard to detect from the ground and aeroplanes are now frequently used to enable inspection from above. However, should branches be seen to decline during summer months, this too can be an indication that the tree has been infected with Massaria.
Once branches become infected with Massaria the bark then dies, followed by branch death and failure, which can be extremely rapid in some instances and may occur as early as three months after the first noticeable symptoms have appeared, whilst other branches may take a year or more to fail.
Spores produced by the fungus create lesions which turn black at the edges, causing the wood to decay and this can also result in branch death and fracture.
Unfortunately, as with so many other tree diseases, there is no treatment available to combat Massaria and research into this disease continues.
Article was written by Karen Arnold