The Six Most Devastating Tree Diseases of the 21st Century

Tree diseases cause huge damage to the tree stock in the UK, with the last twenty years or so seeing some of the worst blight brought upon UK woodlands and forests in living memory. For tree conservation to be successful diseases need to be correctly identified and remedial measures taken. This can be achieved if the signs and symptoms of the most devastating diseases are known and action is taken quickly to halt the spread.

Education is a powerful weapon in the drive to protect trees, not just within the arborist and conservation community, but amongst the wider public as well. With more people able to spot specific tree diseases the more likely professionals will be alerted to a new outbreak and a tree health survey conducted in order to take action to halt the spread.

Let’s take a look at the six most devastating tree diseases of the 21st century.

1. Acute Oak Decline

Acute oak decline (AOD) emerged in Britain around 30 to 40 years ago. Thousands of oaks are affected but it is recorded most often in the Midlands and South East England. Weakened trees are most likely to be affected. Infected oaks have dark patches that weep (bleed) on the stem. It will be possible to spot dark fluid seeping through the cracks between bark plates. It may also run down the trunk but at certain times of the year it may dry up and cake. The disease can develop rapidly over a year or so and death can occur within six to eight years. As the tree nears death, the canopy will appear thin.


2. Chalara dieback of Ash

Chalara dieback of ash (sometimes referred to just as ‘Chalara’) was first recorded in 2012 and is caused by a fungus called Hymenoscyphus fraxineus. It was first discovered in a consignment of infected trees at a nursery in Buckinghamshire. Soon, a small number of cases were recorded in Norfolk and Suffolk and now most regions of the UK are affected. Infected Ash trees lose their leaves, suffer crown dieback and develop lesions on the bark. It is usually fatal as the weakened tree succumbs to attack by other pests. Some ash trees appear resistant to infection and may hold the key to controlling the spread.

3. Phoney Peach Disease

Phone Peach Disease is a huge potential future threat to the UK. This disease is caused by the bacteria Xylella fastidiosa and affects woody commercial trees including grapevines and olive plants although it can also affect herbaceous plants. Until 2013, it was confined to the Americas and Taiwan but then established a foothold in olive trees in Italy and is threatening the whole of mainland Europe. Symptoms are different in each species. It could be anything from a mild leaf scorch to foliage wilt, branch withering, dieback, stunting and death. When oak and elm trees are infected, they exhibit leaf scorch and dieback.

4. Dutch Elm Disease

As the single most serious tree disease on the planet, Dutch Elm Disease has already killed over 60 million elms in two waves of epidemics and is still a major threat. It is caused by a fungus of the Ophiostoma genus. The first epidemic was in the 1920s and the second started in the 1970s and is on-going. The fungus is spread from one tree to another by the Scolytus bark beetle and infects all species of elm in the UK. You will notice it first in early summer when clusters of leaves turn yellow and start to wilt. Soon they turn brown and fall off. The affected shoots will die, starting at the tip, and twigs tend to bend into a characteristic ‘shepherd’s crook’ shape with dark streaks or spots or rings in cross section.

5. Bleeding Canker of Horse Chestnut

First recognised in the USA in the 1930s, Bleeding Canker of Horse Chestnut trees was first recorded in Britain in the 1970s and was caused by a pathogen called Phytophthora. In recent years, its incidence has increased and now at least half the horse chestnuts in Britain show signs of infection. These new cases are attributed to a bacteria called Pseudomonas syringae pv aesculi. Trees of all ages can be affected and whilst some die, others recover. The predominant symptoms are cankers and bark cracks which both occur on the stem. There may also be bleeding on the trunk and branches. The progression of the disease can be very slow and the trees can be managed by simply removing branches that show signs of infection.

6. Phytophthora Disease of Alder

The disease was first recorded in the UK in 1993 and all alder species can be infected. It is caused by Phytophthora alni, which only affects alder. It is a major threat to UK trees where the common alder dominates areas along streams and rivers. It is estimated that 20% of trees are now affected all over the UK although it is most prevalent in south east England. In infected trees, the summer leaves are small and yellow and the crown is thin and sparse. After a few years, there are dead twigs and branches and heavy cone production.



Other diseases that are of concern in the UK include:

  • Sweet chestnut blight
  • Dothistroma needle blight of pine
  • Massaria disease of plane trees
  • Phytophthora diseases including:
  • Phytophthora alni
  • Phytophthora austrocedri
  • Phytophthora kernoviae
  • Phytophthora lateralis
  • Phytophthora ramorum
  • Sirococcus blight


There is a wealth of information on all the listed tree diseases on the website if you want to find out more. The Woodland Trust, Royal Forestry Society and website also have a lot of information and resources.

If you think you have spotted a tree infected with a disease then you can report it to Forest Research, which are the UK’s principal organisation for forestry and tree related research. They have a form you can submit a report using their Tree Alert tool, so if you do see something that doesn’t look right whilst enjoying the British countryside, make sure to report it.

About the Author: Paul George is the managing director of Landmark Trading, and has worked in the arb industry for 14 years. Landmark Trading are one the UK’s leading suppliers of arborist equipment. You can connect with Paul on Twitter, Facebook or call Landmark Trading on 01780 482 231.