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Trees are living organisms and therefore are susceptible to a number of diseases and pests. The most dangerous of these organisms have been spread from abroad leaving our native trees highly susceptible as they have no natural defence mechanisms against them. It is important to look out for these diseases and pests to help mitigate the issues they cause and help stop them spreading further. It is worth noting not all fungi is bad for a tree or a sign of bad health, some can be beneficial. Please read this page for some general information regarding some of the most common and dangerous pests and diseases in the UK.

Ash dieback (Hymenoscyphus fraxineus)

Ash dieback has been affecting ash trees in the UK since 2006, though the fungi causing the disease was only formally identified in 2016. This disease is devastating to ash trees and will most likely kill around 80% of UK ash. For more information of what Monmouthshire County Council please see our Ash Dieback webpage.

Larch disease (Phytophthora ramorum)

Larch disease has been reported in the UK since 2002. This disease is caused by the bacteria phytophthora ramorum which enters via the trees leaves. This bacterium can spread to nearby trees, and it enters by weakening by the leaves or needles. Phytophthora ramorum can also infect other trees such as sweet chestnuts and horse chestnuts. This leads to visible symptoms such as withered and/or blackened leaves/needles leading to dieback of outer branches as well as areas of black “bleeding” on the trunk. The general advice for Larch Disease is to remove the infected tree and any surrounding larches to limit infections. The areas should then be replanted with less susceptible species.

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Acute Oak Decline

Acute Oak Decline (AOD) has been known in the UK for hundreds of years, with the current bout of decline ongoing for at least 2020. AOD is not a traditional form of disease such as a bacteria or fungi, but more a general decline in the vitality of the tree. This is usually to do with environmental factors such as extreme heat or water stress. The symptoms of AOD are general thinning of the crown, extensive stem bleeding (looks like dark weeping patches on the stem), and dark fluid that seeps through vertical cracks in the bark. Whilst AOD isn’t necessarily deadly by itself, the weakened tree fall victim to secondary pests and diseases. The recommended way to fight AOD is to replant oak trees to supplement the numbers.

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Horse Chestnut Leaf Miner (Cameraria ohridella)

The Horse Chestnut Leaf Miner (HCLM) are the caterpillars/larvae of the Cameraria Ohridella moth and are a pest that infests Horse Chestnuts. The HCLM has been confirmed in the UK since 2014. The caterpillars eat their way through the leaves leaving hollow tracks. This causes symptoms of dry and crisp discoloured leaves which can cause a premature leaf drop. Whilst the damage is mostly cosmetic, it can affect the trees’ ability to form seeds (conkers) and subsequent infections over many years can weaken the tree allowing secondary pest and diseases to take hold. There is not much that can be done to prevent or stop this disease, though following biosecurity recommendations can help limit its spread.

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Oak Processionary Moth (Thaumetopoea Processionea)

Oak processionary moth (OPM) is a pest that affects oak trees, the caterpillars of this moth are a hazard to the health of humans and animals. Currently, OPM is contained to the greater London area and there are strict biosecurity practices to ensure it does not spread further. It is still important to know the risks and how to identify this pest to ensure the safety of both trees and people. The caterpillars are a hazard as the small hairs the caterpillars shed are allergens and are especially dangerous for individuals with asthma. Contact with the hairs causes a red itchy rash and can irritate the lungs. The caterpillars also affect the health of the tree as they can devastate the foliage of the trees, leading to the tree weakening and cause secondary pests and diseases.

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Biosecurity best practices

In order to help protect out native species, there are some simple steps that you can take to stop the spread of certain pests and diseases. When visiting forested areas park on hard standing such as tarmac or concrete rather than grass where possible. Remember to wash equipment such as boots and bikes when going between different areas of forest. Ideally, this is best done whilst still on site. If you do frequent different areas of woodland, please regularly clean your car, bike and boots, to make sure there’s no transfer of mud between sites.

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This page provides a general overview of pests and diseases to look out for. If you require more in-depth information please see the following links:

Woodland Trust link page: Key tree pests and diseases – Woodland Trust


Forest Research: