How to recognize a diseased tree

Spotted leaves, cracked trunks, fungus invasions… Two arborists explain how to recognize and care for a sick tree.

Why call an arborist

If you are having problems with your trees or woody shrubs, you should first call an arborist.

A certified arborist has specialized training in the field and knows the latest methods for planting, caring for, diagnosing, treating and cutting trees. He or she has access to essential safety equipment and powerful tools, and is also licensed to apply certain chemicals, such as fungicides, that will help save a tree. If you don’t have a green thumb, these tips will help you resurrect a dead plant.

Trees may look tough, but when signs of distress show up, many times there is a problem that has been brewing for a while. Of course, you can follow best practices for planting and keeping your trees healthy, but major work – or at least specialized knowledge – is needed to extend a tree’s life or prevent the spread of a tree disease. Here are some signs to watch for.

Terminal dieback

An arborist’s examination of a tree is done from top to bottom, something you can do too.

Start at the top of the tree. Are there any leafless branches at the top? That’s called terminal dieback, says Lou Meyer, an arborist with The Davey Tree Expert Company. “It’s an almost obvious sign that there’s a root problem in the soil.”

These problems vary. An experienced arborist will be able to tell you if it’s because of root compaction, a fungus or even poor soil conditions. In this case, there are few ways to save the tree. It’s a common problem with silver maples, he says, but it can affect many species.

Sap or sawdust

If you see sap coming out of the trunk through holes that look like knife wounds, there’s a good chance it’s a stinging insect problem. (Note: Some trees, including elms, exude sap naturally, so it’s not a worrisome sign in this case.)

You’ve probably heard most about the emerald ash borer, which lays its eggs in the tree, lives in the cambium (plant tissue), tunnels into it, and then literally starves the tree. Most untreated ash trees that are infested with EAB will eventually die.

Because EAB can damage the structure of the tree, a professional should handle the treatment or cutting. And while the EAB is probably the most well-known insect, others can infest a tree by leaving sap, sawdust or sawdust tubes in their wake.

“The presence of sawdust is a bad omen,” says Lou Meyer. It warrants calling an arborist.


If you see mushrooms on the trunk of a tree, there is a good chance that there is a rotting problem. Fungi are most often light reddish-brown in color, but they can also be orange or red. “Saprophytes (a plant, fungus or microorganism that lives on dead or decaying organic matter) feed on rot; if you see any, then you’ll know there’s rot inside the branch,” Lou Meyer says.

Arborists can sometimes successfully remove large branches in hopes that the rot hasn’t spread or use fungicides to slow its spread. Some species almost always have fungus protruding from their trunks, he adds, such as carob trees. So don’t panic right away and call your tree specialist instead.

Other fungi don’t look like a conventional mushroom and vary by species. Randy Nelson, a certified arborist with Monster Tree Service, says a fungus often appears in Colorado blue spruce planted outside their USDA (U.S. Department of Agriculture Plant Hardiness Zone). It takes the form of black spots on the needles or even terminal dieback.

Loose bark

Bark peeling off the trunk is often a sign of rot caused by a fungus, according to Lou Meyer. Some species shed their bark naturally, such as sycamore, largerstroemia or river birch.

If a white or silvery plaque is visible under the bark – a kind of scale that looks like a dead lesion – it’s a sign of a disease called hypoxylon canker.

In addition to hypoxylon canker, you may also see sap coming out of the trunk. This is not something you can treat yourself, and it often doesn’t get better. With professional assessment and treatment, however, you may be able to save the healthy parts of the tree and extend its life.

Black lesions

If spring is particularly rainy, black spots or lesions can appear on a tree’s leaves. “It’s more of a fungus related to high humidity,” according to Randy Nelson.

Depending on the tree species and season, routine pruning to thin the foliage and create amplified air movement can help. Using chemicals can sometimes help, too. “It’s usually more of an aesthetic problem than anything else, but it can stress the tree if it happens repeatedly.”

Your tree may not be doomed to certain death in this case, but it will require good care to keep it healthy and extend its life.

Leaf scorch, browning and dryness

People think lawn sprinkler systems are enough to water trees. That’s often not the case, according to Randy Nelson. That water doesn’t reach the tree’s roots, and if you have peat moss, it will benefit instead.

He suggests using perforated black garden hoses. If you don’t have one, let a trickle of water flow from your hose and let it slowly soak into the soil for a full day. Set up the hose a few feet from the trunk but within the plumb line (from the trunk to the outer edges of the tree’s branches) as you circle the tree.

“Trees need an average of one inch of water per week,” says Randy Nelson. They may not show immediate signs of leaf scorch, browning or dryness, but rather the next year or the year after, especially in larger trees.”

Smaller or younger trees may show signs of stress in the same season. Since trees and leaves can also dry out and fall off if overwatered, continue to monitor to understand what is causing this problem.

Trunk like a pole

If, like a telephone pole or mast, the trunk does not flare out at ground level, your tree may not have been planted properly. This can cause problems, including roots that bulge and grow in a circle or spiral around the trunk or above or just below the ground, gradually strangling the trunk. This condition will significantly shorten the life of your tree.

“You want the roots to flare out at the base of the trunk just above ground level so you can see that bulge,” says Randy Nelson. If you spot one, an arborist can excavate the soil around it and prune the roots.”

Injuries or cracks in the trunk

Trees often take a beating. Think, for example, of that tree on the boulevard that someone backs into while trying to park, or the UPS truck that repeatedly snags a branch while making deliveries. “These injuries become a prime gateway for disease,” according to Lou Meyer.

Deliveries during the pandemic are a particularly hot topic: large branches that used to get hit once a week may now get hit four to five times. This can cause structural damage to the tree, deteriorating it and potentially posing a danger to people and property if the branches break off.

Similarly, storms can inflict injury on an otherwise healthy tree. Large cracks in the trunk or bark are signs of structural damage that require attention. Finally, the appearance of horizontal cracks is also a warning. They indicate that the wood fibers are cracking and splitting, and that the tree may fall.

Exposed roots

Exposed roots can be a problem. While the root system of some species is near the soil surface, Lou Meyer says the roots of most trees are not as visible. Exposed roots can be a sign that the tree doesn’t have enough room to grow.

Soil heaving – something Randy Nelson says often happens after heavy rains or severe weather – is another good reason to call an arborist. While small trees can sometimes be straightened or staked, an arborist will be able to assess whether the tree is stable or needs to be cut down.