How to Identify, Prevent & Treat Tree Decay

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Do you ever look at a tree and wonder if it’s healthy? How can you tell if it’s dying? Or if it’s rotting on the inside, hiding fatal decay that could bring it crashing down? Trees are tough—after all, they live outside all year round and cope with wind, snow, ice, drought, heat, insects, and people with saws who don’t know what they’re doing. But they’re still living organisms with a lifespan, and they get old and damaged, and diseased. And you can’t always tell if a tree is decaying or dying just by looking at it.

In this article you’ll learn:

  • What tree decay is
  • What to look for if you’re worried that your tree is diseased, dying, or rotting (signs and symptoms of tree decay)
  • How trees defend themselves against disease and decay
  • What tree decay looks like on the outside
  • How arborists evaluate a tree for decay and pathogens on the inside
  • What tools arborists use for evaluating tree decay (it’s kind of like CSI for trees!)
  • How to decide if a decaying tree is a hazard and needs to be removed

What is tree decay, and what should I know about it?

Simply put, a tree decays when its internal structure and vascular system (aka its wood) have been attacked and destroyed by pathogens. These can include a wide range of fungi or bacteria (as well as the insects who carry or transmit those pathogens).

Once inside the tree, these microscopic attackers digest or destroy the cell walls of the wood, eliminating their complex structure and the remarkable strength and flexibility that trees are known for. This is what leads to branches or whole trees falling: the diseased wood can’t support its own weight.

But this scenario is the endgame of tree decay. It happens only after a series of events on and in the tree that unfold over time and that are often stopped in their tracks by the tree itself.

Decay starts when

  • a tree is wounded (such as by a pruning cut or broken branch),
  • insects and diseases enter the tree through the wound,
  • the tree can’t successfully close off the wounded area or prevent pathogens from reaching the rest of the tree, and
  • disease starts destroying the tree’s internal structure.

What To Look For If You Think A Tree Is Decaying

There are many diseases and pathogens that can damage a tree. And just as many ways that people can start a tree on a path of decay and decline by unwittingly damaging it. The resulting damage and decay may not be noticed for a long time, compounding its spread.

Some diseases are disfiguring or superficially damaging but not fatal. Other diseases are invisible to the untrained eye but may have advanced to the point where the tree’s vascular system or its structural strength has been irreversibly compromised.

Some decay in trees is noticeable, such as cracks in the tree's trunk, fruiting fungal bodies, or areas of loose or missing bark.

Some decay in trees is noticeable, such as cracks in the tree’s trunk, fruiting fungal bodies, or areas of loose or missing bark.

Visible Signs Of Decay In Trees

There are many outwardly visible signs that a tree may be rotting or dying. They include:

  • Fruiting fungal bodies, called mushrooms or conks, growing on a tree’s bark or at its base
  • Areas of loose or missing bark that can be caused by animals, string trimmers, or car bumpers damaging the exterior bark, or insects and disease that have found their way under the bark
  • Bad pruning cuts that leave open wounds or entryways for the disease. A bad pruning cut can be so severe that the tree can’t effectively isolate it from the rest of the tree or prevent the disease from spreading.
  • Bark wounds or cankers that weep sap. These dark, wet stains or drips that don’t dry out may be the tree’s response to disease. Using their internal pressure, a tree may try to push out disease-causing pathogens, making it look like the tree is “bleeding”. In other cases, the disease has made wounds in sapwood that can’t close.
  • Cracks in the trunk (which may be misdiagnosed as frost cracks) from the tree’s response growth to interior disease decay. This happens when the tree tries to compensate for the loss of strength in decayed wood.
  • Sunken areas of bark on a trunk or around the base (trunk flare) that may be from diseased or rotted internal wood that has collapsed.
  • Changes in soil levels around the tree’s root flare that may be dying roots or decayed wood that have disintegrated.
  • Leaf, twig, and branch die-back in a tree’s crown that might mean the tree doesn’t have the energy or energy stores to send into the crown to support its leaf and twig growth.
  • Leaning trunks may indicate dying roots that no longer anchor a tree in the soil.
  • Splitting and breaking branches that have lost their strong branch or trunk attachment from weakened, diseased wood or because the systems of water and nutrient distribution have been cut off
  • Sawdust around the base of a tree’s trunk that may be insect frass or rotted wood escaping from wounds.

NOTE: Because of the complexity of trees and tree diseases, some of these signs may also indicate a treatable disease or non-fatal damage. That’s why it’s always best to have a professional arborist examine your tree. An untrained “tree guy” may be cheaper but finding out that a misdiagnosis meant you didn’t have to remove a large tree is a hard lesson to learn.

Rayzor's Edge Tree Service removes a tree in New Jersey that is completely hollow on the inside, which is very dangerous

Tree Decay Is Complicated To Diagnose

It can be hard to identify a diseased or decaying tree if you don’t know what to look for. Sometimes, people think a tree is diseased or dying because it’s old and isn’t cosmetically perfect, or because insects or drought have damaged the current season’s leaves. To properly diagnose decay and disease in a tree, you’ll need to hire a trained professional.

Certified arborists and tree care professionals have the education and training—as well as the professional tools—that are needed to thoroughly examine and evaluate your trees for decay. Arborists evaluate a tree visually and manually, using a checklist of decay indicators. Depending on the results, an arborist will do a further internal investigation to verify a tree’s structural health and identify the presence of pathogens that are causing rot.

PRO TIP: Learn what to check for when searching for someone qualified to assess your tree!

Matt from Rayzor's Edge uses a resistograph to assess the health of a tree

Matt from Rayzor’s Edge uses a resistograph to assess the health of a tree

How To Check For Decay Inside A Tree

Visible, outward signs of disease and decay in a tree can only tell us so much, and we’re often left guessing at just how compromised a tree is inside.

While a cavity that you can peer into can give some idea about the internal health of a tree, it may not show enough. This is where professional training and internal testing tools come in handy.

Tools For Evaluating Decay In Trees

Hand Testing For Tree Decay

The first thing an arborist may do is use a mallet to strike a tree’s trunk and listen to the quality of the sound it makes. If you’ve ever rapped on an interior wall while looking for a wall stud to hang a picture, you know the different sound a solid area makes compared to a wallboard with a space behind it.

Arborists are searching for a similar difference: sound, solid wood versus missing or decayed wood that has lost its structural strength and its lignin (the material in wood that makes it rigid and strong).

Looking At Visible Decay

Next, an arborist may probe cavities with a metal rod to test how easily the cavity’s interior gives way when pushed. Sound wood is hard and unyielding when poked while decaying wood will crumble or allow a tool to pierce it. The pattern, appearance, and smell of decay can also help an arborist with a diagnosis.

Internal Tree Decay Investigation

If the arborist suspects internal decay in a tree, he or she may use a resistograph to investigate further. A resistograph is a sensitive instrument that evaluates the internal strength and stability of a tree’s wood. (It’s also used on finished lumber, such as roof beams and wood framing when testing for termite damage.)

A resistograph has a small, very fine-gauge drill that is driven into the bark and interior wood. It measures the condition of the tree’s wood by how much resistance is encountered by the tiny drill as it rotates through the wood. The levels of resistance are recorded and give the arborist a “data picture” of the tree’s interior health.

The resistance drill, which is so thin that it’s more like a needle, leaves a hole through the wood so small that it will close up and will not become itself an entry point for disease.

Identifying The Fingerprint Of Tree Decay

Another cutting-edge testing tool that can diagnose decay and disease inside a tree is a rapid DNA test. While DNA testing is used to identify tree species and to identify illegally traded wood products, it is also used to diagnose tree decay and tree diseases. When used this way, it’s not intended to identify the tree itself, but the genetic identifiers of funguses, bacteria, and insects that are found in the wood. This tool provides a very accurate diagnosis of the tree’s internal condition in a non-invasive way. Like the tiny resistograph drill, there is no damage to the tree from the DNA collection

>> Read more about tree DNA testing 

Trees are showing signs of decay in New Jersey as the canopy dies off

How To Protect Your Trees From Disease And Decay

Prevention is vital, as there are many tree diseases that cannot be treated or eliminated once they gain a foothold in your tree. It’s equally important to avoid diseased or decaying tree failure (falling) that damages property or, worse yet, injures or kills a person.

Tree Decay Prevention

Just like regular checkups at the doctor’s office, we recommend periodic tree inspections, especially for mature or specimen trees. Early discovery of tree problems, combined with professional tree work and treatments, are the best steps you can take to ensure the health of your trees.

Benefits Of Inspecting Trees For Damage Or Disease

Regular tree inspections that find early signs of damage can prevent tree failure that may result in property damage or personal injury. The inspection will assess and document structural defects such as decay and root defects, split crotches, dead and broken limbs, and branch or bark cracks. Whatever conditions exist that can be treated or mitigated should be addressed immediately.

For example, if a tree has a broken or damaged branch that has started to decay, it may be possible to stop the spread of decay by removing the branch. Likewise, if insect damage has affected part of a tree’s crown, sprays, trunk injections, or selective crown pruning may remove the insects and stop the disease from spreading. Sometimes these methods work, but sometimes the diseased areas are too large to ensure the tree will survive.

How To Treat Tree Decay And Disease

The best way to treat tree decay depends on:

  • the source or cause of decay,
  • the extent of decay, and
  • whether or not it can be stopped.

Trees themselves have a sophisticated method of stopping decay called compartmentalization.

This is important to understand because unlike our skin, which heals after an injury, tree wounds don’t heal. Instead, they are sealed off from the rest of the tree. You can compare this to shutting a door to close off a room, except that it’s permanent.

When a tree is wounded and threatened by decay, the tree closes off or plugs up the system of cells that provide water and nutrients to the wounded area. This will kill living tree tissue, but it will also prevent the movement of pathogens from the wounded area into the rest of the tree. The tree effectively sacrifices a portion of itself to protect the health of the rest of its structure. Often this stops the progress of decay, and the tree continues to grow. The damaged internal portion of a tree may not be found until the tree is cut down or its lumber is milled.

Sometimes, however, a wound is too large for the tree to isolate, and decay spreads. At this point, the tree risks being overwhelmed by disease and decay because it can no longer stop it.

This unstoppable progress of decay and disease underscores the importance of early identification and treatment of tree wounds and disease.

When a diseased tree can’t protect itself, arborists will try treating the disease or eliminating its source. Depending on what is occurring, disease control may mitigate or stop the damage, or preventive pruning is done.

Unfortunately, not all trees can be saved.

Rayzors Edge Crew removes a dangerous dead tree in New Jersey

When You Have To Remove A Dying Or Decayed Tree

We’ve discussed how important it is to

  • effectively identify and evaluate tree defects,
  • remove areas of disease if possible, and
  • conduct a thorough assessment of a damaged tree.

If a tree’s decay can’t be stopped or is already too extensive to treat, we’ll perform a tree risk assessment. This will tell you:

  • the magnitude of the hazard,
  • the probability that the tree will fail, and
  • the value of what is at risk if the tree fails.

This will give you the information you need to make an informed decision about removing a tree.

PRO TIP: Trees need an inch of sound, non-decaying wood for each six inches of trunk diameter. When a tree loses that protection, it will inarguably be a hazard, unstable, and unpredictable.

It’s always hard to lose a tree, particularly a large one. Trees are invaluable resources, but when they are dying, creating a hazard, or spreading disease, it’s necessary to remove them.

Sometimes, the course of decay takes a long time so immediate removal isn’t required. But once decay has been identified, you’ll need to closely monitor and frequently evaluate the tree so that it is removed before structural failure is imminent.

REMINDER: if there is a hazard tree on your property, you are responsible for it and liable for any damage it causes.

When Should You Remove a Decaying Tree?

It’s tough (and expensive) to remove a mature tree, and it may be wiser (and less expensive) to remove it earlier instead of later.

>> Learn more about the cost of tree removal

Our trained arborists can give you advice and information, but it will be your decision whether or not to remove the tree. This is when a long view is useful.

Ask yourself if:

  • A known hazard tree is worth leaving in place
  • You want to risk damage to property or human life
  • It’s worth risking the safety of your neighbors, your house, or overhead utility lines
  • You’ll be saving any money by putting off tree removal for later, when it may require more safety equipment and crew members to take down a dangerously brittle or overturning decayed tree

You might end up deciding to remove your dying tree sooner.

We Can Help

We’re an established, professional tree care company with a record of careful work, and we have removed a lot of trees. Our crews are trained and skilled and we have lots of experience with resistance testing and tree decay evaluation. If you have a tree that might be decaying from disease and our evaluations tell us that it can’t be saved, we’ll remove it safely and efficiently. And the upside? We can plant a new tree for you and help ensure its health and vigor as it grows to maturity.

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