Threats to British trees

Picture of a leaf from an ash tree with a dead and browning tip

Chalara fraxinea fungus

Chalara dieback of ash is a serious disease of ash trees caused by a fungus called Chalara fraxinea

Symptoms include leaf loss and crown dieback and can result in the death of the tree. The Forestry Commission is working with scientists from the Food and Environment Research Agency (FERA), and woodland managers and landowners are urged to check the health of their trees. It is thought that the disease arrived in Britain from imported plants from the Netherlands for the nursery trade.

If you suspect a tree has this fungus then please contact the Forestry Commission.

Other known threats to our trees include:

Phytophthora ramorum


Phytophthora ramorum

A fungal pathogen that infects the commercially important conifer species, Japanese larch, also beech and oak trees (sudden oak death). Symptoms are bleeding lesions on trunk and limbs.

Dendroctonus micans


Great spruce bark beetle

This beetle (which was accidentally introduced from continental Europe) breeds under the bark, weakening the infected tree and in extreme cases, can kill the tree.

Horse chestnut bleeding canker


Horse chestnut bleeding canker

The canker appears as an area of dying bark that oozes liquid. If it spreads around the entire trunk, it cuts off the food supply, killing the tree

Oak processionary moth


Oak processionary moth (Thaumetopoea processionea)

The oak processionary moth is a major defoliator of oak in Europe. The larvae (caterpillars) feed on the foliage of many species of oaks, including English, Sessile and Turkey oaks (Quercus robur, Q.petraea and Q.cerris).

Hornbeam, hazel, beech, sweet chestnut and birch are also reported to be attacked, although mainly when growing next to severely defoliated oaks. Oak processionary moth is also a risk to human health. The larvae (caterpillars) are covered in irritating hairs that contain a toxin and contact with these hairs, or their inhalation, can result in skin irritation and allergic reactions. These problems are significant because oak processionary is often most abundant on urban trees, along forest edges and in amenity woodlands.