It is a fact that once introduced, a tree pest has never been eradicated from the UK. This means that our only real defence is to keep pests and diseases out of the country. Once present, stopping them spreading is critical to tree health.
Climate change and rapid globalisation has led to an unprecedented level of threat to our trees, woods and forests. Legislation around plant imports is so complex that it has become almost unenforceable. For decades, the main threat was to non-natives and part of the problem was that they were grown in monocultures but there are now new threats to our native trees.
Common pests and diseases
Ash Dieback (Chalara fraxinea)
Only identified in the UK in 2012, now thought to have spread naturally into the UK from the Continent, as well as being imported on nursery ash trees. There is no cure for this disease; it will kill young trees in one season though mature trees can survive for many years with on-going infection. Planting of ash trees is currently not allowed. There is research taking place into genetic resistance on use of fungicides to control it.
This is a new bacterial type disease affecting the larch species of conifer, which has spread from rhododendron as its main host. There is no cure and trees are usually killed in one year. The only control measures we have are strict biosecurity and sanitation felling in infected stands. The west of the country is the higher risk zone and Cumbria currently has on-going sites of infection.
Acute Oak Decline
Currently most prevalent in the Midlands and South-east; it is not known what causes this but it's thought to be one or combination of a beetle, bacterial causes and climate change or other tree stresses.
Sweet Chestnut Blight (Cryphonectria parasitica)
A fungal disease discovered in the UK in 2011; it is often fatal to the tree and is thought to have killed over 3 billion trees in America. The disease was discovered on newly planted stock and the trace forward has currently resulted in only a few other sites; the disease presents a huge threat to the tree in the UK.
Infects juniper and is currently affecting trees in Cumbria; it is not known how this disease arrived here and there is no cure.
Red Band Needle Blight (Dothistroma septosporum)
A fungus-like pathogen that affects pine species, particularly Corsican pine. It is thought that Scots pine is less vulnerable to attack; some Cumbrian stands currently have this infection. It reduces timber growth and defoliates trees, rarely resulting in tree death.
Bleeding Canker (Pseudomonas syringae)
This pathogen has spread rapidly across the whole of the UK. It affects the tree by causing bleeding wounds and early defoliation, rarely resulting in quick tree death but over a period of years trees become weakened and eventually succumb to this or another pathogen such as honey fungus.
Oak Processionary moth (Thaumetopoea processionea)
The caterpillar defoliates oak trees; endemic in Europe but currently affecting only a few sites around London. The main concern around this pest is not the damage to trees but to human health. The hairs of the caterpillar cause irritation and an intense skin rash - in extreme cases, there can be an anaphylactic reaction and hospitalisation of the patient.
Emerald Ash Borer (Agrilus planipennis)
This beetle has devastated ash tree (Fraxinus) populations in North America, after having been introduced there from Asia, but is not yet known in the UK. Apparently, a biological control has been developed.
Asian Long-horn Beetle (Anoplophora glabripennis)
Not yet in the wild in the UK but intercepted several times on imported wood. Attacks many tree species and extensive damage to forests and street trees is likely if it becomes established here.
Citrus Long-horn Beetle (Anoplophora chinensis)
This SE Asian species has also been intercepted on numerous occasions. It has EU quarantine status and if it becomes established will be extremely damaging to a wide range of tree species.
Horse Chestnut Leaf Miner (Cameraria ohridella)
This is a leaf-mining moth caterpillar that causes severe damage to leaves, though rarely results in tree death; causes loss of growth and makes the tree look rather sick and unsightly; recently discovered in South Cumbria.