25 Sep The 5 Deadliest Tree Diseases
Table of Contents
We live in a lucky world with endless travel. So much so ‘Before the COVID-19 pandemic, the number of international visits had more than doubled since 2000.’ Our world in data recorded. We are getting out and about there!
With exports and imports so accessible from all corners of the earth, it does mean everything can travel.
The types of tree diseases are constantly evolving and spread far and wide, the busier our world gets.
So, what are the most deadly tree diseases lurking in the UK currently? Where did they come from? Is it game over? Can it be treated? Why is it so serious?
Fret not, we’ll give you all the information you need below.
1. Ash Die Back (Hymenoscyphus fraxineus)
what is it?
Luckily this disease has a pretty self explanatory name (the same can’t be said for its Latin counterpart). It only affects ash trees and it causes them to dieback until they die completely .
This is probably the most deadly tree disease in the UK. The first case confirmed in the UK was in 2012. Since then it has spread across the whole of the UK including the Republic of Ireland. The very first time it was detected was in Poland in 1992.
How it works and the symptoms
This savage disease is caused by ash dieback fungus (Latin: Hymenoscyphus fraxineus). It can also be known as the Latin name Chalara fraxinea and normal name of Chalara Ash dieback. Although it’s two separate names it is the same disease. Chalara fraxinea is simply the name for the sexual stage (the first stage) and Hymenoscyphus fraxineus is the name for the asexual/second stage but is recognised under the latter name.
Essentially in the fungus’ asexual stage (the first stage being the sexual where the fungus spreads) it halts the transport of water through the tree, causing branches to dieback. Once this happens, the damage really starts to happen. It’s symptoms include:
- Dark lesions or cankers on the tree
- Wilting leaves that are affected
- Dead branches
- Blotches/blackening of leaves and shoots in mid-late summer (July-September). If observed in autumn, normal changing of leaves can be mistake for ash die back.
Treatment
Unfortunately there isn’t a cure for ash die back yet. There has been a small percentage of ash trees that are tolerant or resistant however. If this is the case these trees are important for carrying on new ash trees with increased tolerance.
A majority of the time to prevent further spread of the disease the tree is cut down. There are other management strategies though, such as ensuring dead branches are removed frequently for safety purposes. It all depends on the unique set of circumstances the tree has.
Why is it so deadly?
Having no cure for this disease bumps it up in severity, however coupled with the fact that it spreads rapidly means it’s no laughing matter.
According to gov’s Forest Research it’s ‘predicted to kill 70-90% of native ash (>100 million trees) over the next 20 years.’
Ash trees are one of the most common trees in the UK and are used for a variety of products. Being such a common tree, losing them, especially at such a speed can have a big knock on effect for the ecology and on biodiversity. It spreads through wind dispersal and can come through imports.
2. Sudden Oak death/Phytophthora ramorum
What is it?
Phytophthora Ramorum, commonly known as Sudden Oak death in the USA or Ramorum Dieback in the UK is a disease caused by an algae-like water mould called Phytophthora Ramorum (hence the name of course!). It affects a wide variety of trees and shrubs, particularly rhododendrons, causing damage and death.
It’s also known to affect larch trees over other species. Although Americans call it sudden oak death, UK oaks are safe as they are a more resilient species compared to the USA species.
How it works and what are the symptoms
This pathogen travels through mist, rain, waterways etc. If water’s involved it’ll be joining the party. Once it’s transferred onto its host it reproduces quickly. Now the tree is infected the symptoms or death will soon follow.
Things to look out for are:
- Lesions on bark that ooze out dark liquid
- Branch dieback
- Resinous cankers (on branches and upper trunk)
- Wilted/withered shoot tips/blackened needles
Treatment
There’s no cure available for this disease. Generally to maintain and get rid of the disease the tree will need to be felled. If possible this is best to be done before spring or autumn as that’s when sporulation begins. If you do notice it however or find a plant with the same symptoms you can get your local tree surgeon to double check it.
This is a notifiable tree disease due to the importance of containing the spread and destruction. Currently it’s mostly plant nursaries that are affected rather then the general wild. Once notified if the tree is on your property they might give you a statutory management plan.
You can report a suspected diseased tree on the Tree Alert Website.
Why is it so deadly?
A disease for plants/trees is classed as deadly when it can cause death, damage and destruction to our ecology and industries. Ramorum disease definitely sits in high ranks and ticks all of these.
3. Acute Oak Decline
What is it?
Unlike most of the common deadly diseases so far, acute oak decline is thought to be a bacterial disease. This by no means makes it less serious however…
It’s got a few other name variations kicking around such as Chronic Oak Decline and Oak decline. These are used interchangeably however Chronic Oak Decline has some different tendencies. As the name suggests this disease only affects full or hybrid oak trees. It seems to target more mature trees however it can still reach the young.
Death can follow in a brisk 4-6 years after infection. The first case was in the 1920s but only recently has it become a rising problem.
How it works and the symptoms
Acute Oak Decline is still being investigated and there’s uncertainty on what actually causes this and how it works. It’s generally suspected to be a complex disease meaning it’s not just one single strain of bacteria but a few or a combination or bacteria and other activating factors combined together.
It’s believed by some that jewel beetles have something to do with the disease. Although it’s still very mysterious, it does have some clear consistent symptoms. An oak will often display one or more of the following:
- Dark weeping fluid from trunk cracks in bark (5-10cm long)
- Multiple bleeding patches
- D shaped exit holes from jewel beetles
Treatment
As there’s not much knowledge of insight into this disease there’s no cure to date. The next best thing is management. It’s generally advised to leave the tree as it is if it’s solo. This means no lopping, chopping, chucking infected branches in your compost etc. If it’s surrounded by other healthy oak trees however, to prevent spread it is safest to have it felled.
Why is it so deadly?
For a casual sounding name, it’s actually far from it. Causing death puts it high on the deadly list for the trees. The UK largely relies on Oak wood for production and out in the general wild, off plantations it plays a key part in biodiversity and the ecosystem.
4. Dutch Elm Disease
What is it?
Dutch elm is yet another rampant fungal disease that’s nearly wiped out the elm species a few times. If you’re a Latin fanatic, the fungus is called Ophiostoma novo-ulmi, it’s spread by the elm bark beetle.
How it works and the symptoms
Similarly to ash dieback the fungus blocks the water transport in the tree (the vascular system). This slowly kills it as the rest of the tree can’t be nourished. This is when the elm starts to experience dieback and a variety of other symptoms, before dying. Young elms may only have months to survive whereas mature elms will be able to fight it for a couple of years before giving in.
As well as spreading through elm bark beetles, the fungus can spread through root grafts to healthy trees up to 15m away.
Key symptoms to watch out for are:
- Leaves turn yellow, then wilt, then die in the summer months
- Young shoots affected turn into a shepherd’s crook shape.
- Brown/black discolouration under bark of wilting branches
- Branches eventually die and break off
Treatment
Again like with many tree diseases, the only thing to do is focus on management. This is dependent on the stage of the disease. If it has already spread to the root system and badly affected the tree it is too much of a safety hazard, the tree will quickly die. Either way it will need to be disposed of in a controlled way to prevent future spread. There is also options of using fungicides and insecticides if it’s caught early enough along with pruning affected crowns. Otherwise it’ll most likely need to be removed.
Why is it so deadly?
More than 20 million elm trees died in the second Dutch elm disease epidemic in the 1960s. Dutch elm disease seems to come in waves of coming back.
5. Bleeding Canker - Horse Chesnut
What is it?
Bleeding Canker disease causes an infection of the Bark in predominantly Horse Chestnut, Lime and Alder trees. The disease is down to a fungus-like organism with the latin name of ‘phytophthora’. It was first recognised in the UK in the 1970s but wasn’t much cause for concern.
Things flipped however when cases began to rise in the 21st century. It’s believed from 2000 onwards generally it’s caused by a previously unknown pathogen from the Himalayas called (Pseudomonas syringae pv. Aesculi, quite a mouth full).
How it works and the symptoms
Phytophora is a genus, we won’t get too technical but essentially it’s a group of pathogens that like to travel together and cause havoc together. Imagine them as a gang.
These fungal-like organisms have zoospores, they are attracted to the chemicals that the root emits. Zoospores then travel through water or saturated soil to the roots and infect the plant from there. This is why it thrives under moist and warm conditions. Once it’s infected its host it proceeds to damage the plant’s root and vascular systems (water transport system). In turn this prevents nutrients and hydration nourishing the plant. Over time this can kill the plant however some do survive it.
It’s important to note – The symptoms of Bleeding Cankers do cross over with Ramorum Disease/Sudden Oak Death which are notifiable pathogens.
General Symptoms include:
- Cankers that bleed out reddish/brown fluid or bleed from splits in the bark
- Branches may start to dieback
- Foliage may be pale and sparse
Symptoms vary from species of tree so there might be additional symptoms.
Treatment
Unfortunately this pathogen can’t be removed from the plant once it’s infected its host. It can however be managed and controlled for the best outcome.
Some things you can try:
- Scrape back cankers on tree to dry them out and get rid of them.
- Remove smaller infected branches and dispose of them in a way so that the infection can’t spread elsewhere.
Why is it so deadly?
Although this wasn’t taken too seriously in previous centuries, due to the change in spread and damage caused we’ve bumped this up to our deadly list. It causes trees to die and there’s no cure for it. It may not be as widespread as Ash dieback or Ramorum Disease but it’s next on the rankings.
Prevention Methods
Now you’re in the know, hopefully you can keep an eye on your trees to prevent any of these diseases.
There are a few things in your control to keep diseases at bay.
- Keep new plants/trees away from the others for a few weeks to see if there’s sign of any disease.
- Prune regularly to ensure your tree is healthy and that any infection on the tree’s crown is being removed.
- Keep an eye on your trees for changes in colour, bleeds or signs of abnormal activity/decline in health.
- Choose disease resistant trees if you’re looking to plant new ones.
- Contaminated soil can be a quick way to spread diseases. If your trees were infected make sure to manage it properly through treatment or new planting.
As we all know, knowledge is power. Hopefully this has given you some confidence and know how to keep our trees right.
If you’re ever unsure about anything you can always phone your local arborist.
Did you know about these 5 deadly tree diseases?
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