How to Tell If a Tree Is Sick: A Homeowner's Guide to Spotting Disease and Decay

Trees don't get sick overnight, and they rarely show it the way a person would. There's no fever, no obvious symptom that jumps out at you from the kitchen window. Instead, a tree in trouble sends up a slow, quiet string of signals — a few dead twigs here, some discolored bark there, leaves that come in a little thinner each spring. By the time the problem is impossible to miss, it's often already serious.

The good news: most of these signals are visible from the ground if you know what you're looking at. You don't need a degree in arboriculture to notice that something's off — you just need to know where to look and what's normal versus what isn't. Here's a practical walkthrough of what disease and decay actually look like on a living tree, broken down by where you'll spot them.

Start at the Top: What the Canopy Is Telling You

The crown of the tree — the leaves and outermost branches — is usually the first place a problem shows up, because it's the part of the tree furthest from the roots and most dependent on everything underneath working correctly.

Dieback at the top or edges. "Crown dieback" is when the outermost branches, especially at the top of the tree, lose their leaves and go bare while the rest of the tree still looks fine. This is one of the most reliable early indicators of stress — root damage, disease, soil compaction, drought, or a vascular problem that's restricting how water and nutrients move through the tree. A few dead twigs at the tips after a hard winter is normal. A pattern of dead branches creeping inward toward the trunk, especially at the crown, is not.

Thin, undersized, or off-color leaves. Compare this year's leaf-out to what you remember from a few years back. Leaves that are noticeably smaller, paler, yellowing between the veins, or coming in patchy and sparse can point to root problems, nutrient deficiencies, fungal infections, or insect damage to the vascular system. One off year during a drought isn't necessarily alarming. The same pattern showing up two or three years running is worth a second look.

Leaves dropping at the wrong time. Trees dropping leaves in October is normal. Trees dropping green, healthy-looking leaves in June or July is not — that's often a stress response, where the tree is shedding canopy to conserve resources because something underneath isn't working right.

Read the Bark and Trunk

The trunk is the tree's central support structure and its main plumbing system rolled into one. Damage here tends to be serious because there's no redundancy — everything the tree needs travels through this one column.

Cracks, splits, and seams. A vertical crack running up the trunk, especially one that's open rather than just a healed-over seam, is a sign of internal stress — sometimes from frost, sometimes from included bark where two major limbs grow too close together, sometimes from internal decay pushing outward. Any crack that's growing, oozing, or running deep into the wood is a structural concern, not a cosmetic one.

Peeling, missing, or discolored bark. Healthy bark is intact and consistent for the species. Patches of bark that are peeling away, sloughing off in sheets, or missing entirely — especially if you can see discolored or soft wood underneath — usually mean the tissue underneath is already dead or actively decaying. Look for staining, oozing sap in unusual patterns, or a sour smell, all of which can indicate active infection.

Cankers. A canker is a localized dead spot on the bark or trunk — often sunken, discolored, cracked, or oozing. These form where a fungus or bacteria has killed an area of the tree's living tissue. A small canker on a branch might be manageable. A canker on the main trunk, especially one that's expanding or wrapping around the trunk's circumference, is a serious problem because it can cut off the tree's ability to move water and nutrients past that point entirely.

Mushrooms, conks, and fungal growth at the base or on the trunk. This is one of the clearest signs of internal decay there is. Fungi don't grow on healthy wood — they grow on wood that's already breaking down. Shelf-like growths (called conks or brackets) on the trunk, or mushrooms sprouting from the base or root flare, mean there's active decay happening inside the tree, often well beyond what you can see from outside. This doesn't always mean the tree has to come down immediately, but it does mean a professional needs to assess how much structural wood has been compromised.

Check the Base and Root Zone

What's happening below ground is harder to see directly, but the tree usually telegraphs it through what's visible at the base.

A leaning trunk, especially a new or worsening lean. Some trees grow with a natural lean and have for years without issue — that's just how they're shaped. A new lean, or a lean that's progressing, is a different story. It can mean root damage, soil erosion, root rot, or that the tree's anchoring system is failing. If you notice soil lifting or cracking on the side opposite the lean, that's an active warning sign that shouldn't wait.

Mushrooms or fungal growth in the grass near the base. Just like fungal growth on the trunk, mushrooms sprouting from the root zone in the lawn around a tree often indicate root decay underneath. This is especially worth noting if it's a new development and concentrated near one particular tree rather than scattered generally across the yard.

Exposed, damaged, or girdling roots. Roots that have been cut during construction, compacted by heavy equipment, or wrapped around the trunk (girdling roots, which slowly strangle the tree's own vascular system) all create long-term problems that show up as canopy decline years later. If you've had recent excavation, grading, or driveway work near a mature tree, keep an eye on it for the next few seasons — root damage often takes time to show above ground.

Watch for These Insect Signs

Insects and disease often go hand in hand — pests create entry points for pathogens, and weakened trees attract more pests. A few things worth watching for:

What to Do If You Spot Something

Finding one or two of these signs doesn't automatically mean your tree is dying or dangerous — trees are resilient, and many problems are manageable if they're caught early. What matters is not ignoring a pattern. A single dead branch is normal tree maintenance. A trunk with a canker, fungal brackets at the base, AND a noticeable lean is three different systems telling you the same thing, and that's not something to wait on.

If you're not sure whether what you're looking at is a minor issue or a real problem, that's exactly the kind of question a second pair of trained eyes can answer quickly. We'd rather come take a look and tell you it's nothing to worry about than have you find out the hard way during the next storm.