How to Spot a Diseased Tree Before Its Too Late

The trees on your property are living organisms, and like any living thing, they can get sick. The difference between a tree that recovers and one that has to come down often comes down to timing. Catching disease early gives you options. Waiting too long can mean losing the tree entirely, along with the shade, beauty, and property value it provides.

Illinois is home to dozens of tree diseases, many of them aggressive enough to kill a mature tree within a single season. Knowing what to look for can save you thousands of dollars in emergency removal costs and help protect the other trees on your property from infection. Here is what every homeowner in the Chicago suburbs should know about spotting diseased trees.

Visual Signs That Something Is Wrong

Trees cannot tell you when they are sick, but they show it in ways that are surprisingly easy to read once you know what to look for. Make a habit of walking your property and looking at your trees a few times each season. Here are the warning signs that should catch your attention:

• Discolored leaves - Leaves that turn yellow, brown, or develop spots outside of normal fall color change are a red flag. Premature leaf drop in summer is another strong indicator. If one section of the canopy looks off-color while the rest appears healthy, disease may be spreading through the vascular system.
• Cankers on the bark - Cankers are sunken, discolored areas on the trunk or branches where the bark has died. They can appear as dark, cracked patches or areas where the bark has peeled away entirely. Cankers often indicate a fungal infection beneath the surface.
• Fungal fruiting bodies - Mushrooms or shelf-like growths (conks) appearing on the trunk, at the base, or on major roots are a serious concern. These are the reproductive structures of wood-decay fungi, and by the time they appear on the outside, the internal rot can be extensive.
• Oozing sap or slime - Sap bleeding from the trunk, sometimes with a foul smell, often signals bacterial wetwood or other infections. Dark, wet streaks running down the bark are not normal and should be evaluated.
• Wilting or drooping - Leaves that wilt even when the soil is moist suggest a problem with the tree's ability to transport water, which is a hallmark of vascular diseases like oak wilt and Dutch elm disease.
• Thinning canopy - A gradual thinning of the crown, where you can see more sky through the branches than in previous years, can indicate root disease, trunk decay, or chronic stress that makes the tree vulnerable to infection.
• Peeling or cracking bark - While some species naturally shed bark, widespread bark loss or deep vertical cracks can indicate disease, sunscald damage, or internal decay.

Common Tree Diseases in Illinois

The Chicago area's climate, with its hot, humid summers and cold winters, creates conditions favorable for several serious tree diseases. Here are the ones we encounter most often in our work across Stickney, Berwyn, Riverside, and Oak Park:

Oak Wilt - This is one of the most destructive tree diseases in the Midwest. Caused by the fungus Ceratocystis fagacearum, oak wilt blocks the water-conducting vessels inside the tree. Red oaks can die within weeks of showing symptoms, which include rapid wilting and browning of leaves starting at the margins. White oaks are more resistant but can still be affected. The disease spreads through root grafts between nearby oaks and via sap-feeding beetles. If you have oaks on your property, never prune them between April and July, when the beetles are most active.

Dutch Elm Disease - Once the most common street tree in America, elms have been devastated by this fungal disease since the mid-20th century. Dutch elm disease causes yellowing and wilting of leaves, typically starting on one branch and spreading through the tree. Infected branches show brown streaking in the sapwood when cut. The disease is spread by elm bark beetles and through connected root systems. Early detection and branch removal can sometimes save the tree if the infection has not reached the trunk.

Apple Scab - Extremely common on crabapple trees throughout the Chicago suburbs, apple scab causes olive-green to dark brown spots on leaves. Severely infected trees drop their leaves by midsummer, weakening the tree over successive years. While rarely fatal on its own, repeated defoliation stresses the tree and makes it vulnerable to other problems. Fungicide treatments applied in spring can be effective when caught early.

Anthracnose - This group of fungal diseases affects sycamores, maples, oaks, and ash trees in Illinois. Symptoms include irregular brown spots on leaves, curling, and premature leaf drop. On sycamores, anthracnose can also cause twig dieback and cankers. Most trees can tolerate mild anthracnose, but severe, repeated infections warrant professional evaluation.

Emerald Ash Borer Damage - While technically an insect pest rather than a disease, the emerald ash borer (EAB) has killed hundreds of millions of ash trees across the Midwest and its effects mimic disease symptoms. Signs include thinning canopy starting at the top of the tree, D-shaped exit holes in the bark, S-shaped tunneling beneath the bark, and increased woodpecker activity. Untreated ash trees typically die within three to five years of infestation. Preventive insecticide treatments can protect healthy ash trees, but once more than 50 percent of the canopy is lost, removal is usually the only option.

What to Do When You Spot Disease

If you notice any of the signs described above, here is what we recommend:

• Do not wait - Tree diseases rarely resolve on their own, and many spread rapidly. A week of delay can mean the difference between saving a tree and losing it.
• Do not prune it yourself - Improper pruning can spread disease to other parts of the tree or to nearby trees. Some diseases, like oak wilt, are actively spread by pruning wounds during certain seasons. Always sterilize tools between cuts.
• Call a professional - A trained arborist can accurately diagnose the problem and recommend the right course of action. At County Tree Service, our team evaluates hundreds of trees every season across the western suburbs. Call us at (708) 484-4808 or explore our full list of services.
• Document what you see - Take photos of the symptoms, noting the date, which parts of the tree are affected, and whether the problem seems to be spreading. This information helps your arborist make a faster, more accurate diagnosis.

Treatment vs. Removal: How We Decide

Not every diseased tree needs to come down. The decision depends on several factors:

• How far the disease has progressed - Early-stage infections in a single branch can often be treated by removing the affected limb. Once disease reaches the trunk or root system, options become limited.
• The type of disease - Some diseases respond well to fungicide or insecticide treatments. Others, like oak wilt in red oaks, move too fast for treatment to be effective once symptoms appear.
• The tree's overall health - A vigorous tree with good structure can often fight off or tolerate a mild infection. A tree that is already stressed from drought, construction damage, or prior disease has fewer resources to recover.
• Safety considerations - If internal decay has compromised the tree's structural integrity, removal may be necessary regardless of whether the disease itself is treatable. A tree that could fall on a house, driveway, or play area cannot be left standing while you wait to see if treatment works.

Our approach is always to save the tree when we can. Mature trees take decades to replace, and their ecological and property value is significant. But when removal is the right call, we are honest about it and explain exactly why.

The Value of Early Detection

The single most important thing you can do for the trees on your property is pay attention to them. Walk your yard in spring when leaves emerge, in midsummer when the canopy is full, and in fall as colors change. Look for anything that seems different from last year. Trust your instincts. If something looks wrong, it probably is.

Early detection turns a potential emergency into a manageable situation. It gives you time to explore treatment options, plan for removal if needed, and budget accordingly. It also protects the other trees on your property by stopping the spread of contagious diseases before they jump from tree to tree.

If you are concerned about a tree on your property, do not wait for it to become a crisis. Book a free estimate with County Tree Service today, or call us at (708) 484-4808. We serve Stickney, Berwyn, Riverside, Oak Park, and communities throughout the Chicago area, and we are always happy to take a look.