A quick guide to the pests, diseases and fungi that trouble your trees

4th March 2020

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Most of the year it’s important to look out for the signs of pests, diseases and fungi that will weaken the trees in your garden, so we’ve put together a quick guide on what to watch out for and when to call in your trusted local tree surgeon or arborist.

This is by no means an exhaustive list, but it contains most of the main ones you might find.


Bugs, beetles and borers are the main problem pests in the UK. They cause damage to trees through their diets and reproductive activities and while some cause purely aesthetic issues, others can be far more damaging.

  • Eight-toothed spruce bark beetle

So far this pest has only been found in Kent, but it could pop up anywhere, so watch out for them. If you notice 2cm to 2.5cm holes in the bark and sawdust under branches or trunks, this is the sign of an insect infestation. These particular bugs are around 5mm long, shaped like a cylinder, dark brown/black, and shiny with little brown hairs. They are partial to spruce or pine trees. They bore into the tree and create chambers into which the larvae are placed – these chambers become wider as the larvae grow and can damage the tree’s water transport system, causing dieback.

  • Elm zig-zag sawfly

This aptly named pest lays its eggs on the edge of the elm leaves and when the larvae emerges it munches its away along leaves in a zigzag pattern. Populations of the elm zig-zag sawfly can explode rapidly, causing a significant number of leaves to be consumed and weakening the tree as this happens year after year.

  • Great spruce bark beetle

If you see 2cm to 3.5cm holes and sawdust-like material at the base of the tree and white, purple or brown resin oozing from these holes as the tree protects itself from the infestation, then you may have a beetle problem. These amber/brown bugs are 6–8mm long and around half as wide. Much like other bugs, it’s the damage they do with their burrowing that can kill a tree, because as their chambers and tunnels criss-cross inside the trunk they can cut off circulation, causing dieback to anything above.

  • Horse chestnut leaf miner

These little guys leave tracks inside the leaves of their horse chestnut hosts as their larvae munch away for sustenance. When they grow up they become tiny brown and white moths, but in their larval stage they range between 0.5mm and 3.5mm in length. If you notice that the leaves on your horse chestnut tree are going brown and falling before all others around it, then check the leaves for the tracks of the miner.

  • Oak processionary moth

As the name suggests, in April and May, the caterpillars of this moth march nose to tail around and up the trunk of the trees in a rippling furry train. Their nests are located on trunks and made from white silken webbing that goes brown with age. The danger to trees is the stripping of leaves as the hungry procession makes it way up and down the tree.

  • Oriental chestnut gall wasp

A gall looks little like an odd fruit growth at the base of a leaf or a leaf stalk, a bulbous inflation. They start off green or pink and go browner throughout the summertime. The galls are caused by the wasp larvae and can cause leaves to die and drop prematurely.

Diseases and fungi

Fungal infections are especially difficult to irradiate as the fungus often survives the winter in leaf litter and lets fly its offspring as thousands of spores on the wind throughout the summer months. Spores can travel many miles on the breeze before landing on leaves and starting the damage they will cause.

  • Ash dieback

This is a national problem and really dangerous for ash trees. Caused by fungal infection, it can be recognised by dark patches on leaves in summer, wilted and blackened leaves shed in early autumn, and diamond-shaped dark brown lesions where branches meet trunk. Ash dieback is often fatal for the tree as the fungal growth blocks the flow of water around the tree.

  • Dothistroma needle blight

This fungal infection affects pine trees and can cause needles to turn reddish-brown or brown with yellow spots and fall from the tree, leaving a telltale ‘lion’s tail’ effect with just the needles on the end of branches left. All this discolouration and needle shedding will weaken the tree’s ability to photosynthesize.

  • Dutch elm disease

The fungus that causes this disease is called Ophiostoma novo-ulmi and like other fungal infections will cause leaves to yellow and fall early, but this one also causes the twigs of the tree to bend down in what’s known as a ‘shepherd’s crook’. This fungus is often introduced to the tree by feeding elm bark beetles and once established will release toxins that impede the tree’s ability to transport water around its system, causing extensive dieback.

  • Horse chestnut bleeding canker

If you notice cracks in the bark which ooze a reddish-brown liquid in the summertime then your tree could have been infected by a bacterial pathogen which multiplies and blocks the tree’s water transport system. Often this is nothing to worry about, but if the cankers grow and spread across the tree then this can cause serious problems.

If you have noticed any of these signs on your garden trees, then call in the experts. The longer you leave things, the more your trees – and those of your neighbours – could be impacted and the problem will become much more serious.

To book a survey and consultation with one of our ARB Approved arborists for advice on your trees or to book our tree surgeons to prune/maintain or fell your tree(s), call us on 0208 292 8992.

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