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Pests and diseases

Oak Processionary Moths are an invasive species from Europe with no natural predator in the UK. They lay large, webbed nests of caterpillars in only oak trees. First spotted in Richmond, they have spread throughout London and the South East. Although the arboriculture industry is trying to eradicate them, they are fighting a losing battle.

If spotted, it is advised to get them removed ASAP and not go near them, as they are harmful to both humans and animals – they can cause respiratory problems, eye irritation and large rashes or boils. There is an NHS health warning page here


They can be found on the underside of branches, most commonly joining the trunk of a tree, inside large webs with grey/black caterpillars. They have been known to cause damage to oaks, albeit only on very old ones with a small canopy.

Disposal of this species is done in full contamination suits, carefully removed and disposed of by fire as they are such a serious health risk


Leaf Miners (cameraria ohridella) are found on horse chestnut. It is a moth which lays larvae in the leaves, and as they eat their way out, they defoliate the canopy. They have been known to eat large portions of tree canopies by late summer. They are not a large epidemic, but have led to many horse chestnuts being in very poor condition; aesthetically it is not nice to see a tree covered in brown leaves in summer.


Wooly Aphids live most commonly in apple trees and sometimes elders and other fruit trees. They are often mistaken for fungi or mould, as they encase themselves in white fluid for protection. They can sometimes be known to defoliate tree from tip to branches, from mid to late summer, then go into hiding in the bark and sometimes the roots in winter. Later they develop into a small fly, which if becomes a problem, there are various methods of treatment. Badly affected branches can be pruned off, and a hose with soapy water used for those less affected. People have been known to use insecticides but this is not necessary and may damage the surrounding environment


Massaria is a fungus that attacks the London plane tree. It is becoming more widespread throughout the country , especially in summer when there is less water in the trees. It is rarely visible from the ground – you can only see the start of the rot from the upper side of branches, so usually requires aerial inspection. Massaria can cause whole stems or limbs to fall from the tree canopy, causing potential health and safety problems.


Ganoderma is a perennial fungus that attacks a wide range of species, leaving a large woody bracket at the base of trees. It has a dark leathery surface on the upper side, with a pale, milky underbelly.

It is always a good idea to leave the fungal brackets on trees for identification purposes, and removing them will do no good to the tree. Unfortunately the only treatment is to cut down the affected tree. A heavy reduction can be performed to make the tree safe, where possible.

Ganoderma is an especially dangerous fungus, as it rots trees’ primary anchor roots while leaving the fibrous ones behind – this means the trees look perfectly healthy from the outside when in fact they are ready to fall over. Many council trees have been felled for this reason as it is a major safety concern.