Tree diseases are easy to misdiagnose, and incorrect treatment is worse than no treatment. The first step in disease management is always accurate identification — understanding whether you're dealing with a fungal pathogen, a bacterial infection, an abiotic (non-living) disorder, or something else entirely. Here's a guide to the most common diseases affecting Illinois trees.
Oak Wilt (Bretziella fagacearum)
One of the most serious tree diseases in Illinois. Causes rapid wilting and dieback in red oaks (often killing within weeks) and more gradual decline in white oaks. Spreads both through root grafts between neighboring oaks and by sap beetles carrying spores to fresh wounds. Avoid pruning oaks between April 1 and July 15. Treatment with propiconazole injections is effective for early-stage infections and for preventive applications in high-risk situations.
Dutch Elm Disease (Ophiostoma novo-ulmi)
A fungal pathogen spread by elm bark beetles and through root grafts. Causes rapid wilting and yellowing of leaves on individual branches that spreads throughout the crown. American elms are highly susceptible; Siberian and Asian elms are resistant. Propiconazole trunk injections are effective for prevention and treatment of mildly infected trees. Prompt removal of dead elms and their root systems prevents spread through root grafts. Elm bark beetle breeding sites (stressed or dead elm wood) should be removed to reduce beetle populations.
Apple Scab (Venturia inaequalis)
An extremely common fungal disease of apple, crabapple, and hawthorn trees. Causes dark, scabby lesions on leaves and fruit. In severe years, affected trees may defoliate entirely by midsummer. Repeated defoliation weakens trees over time. Fungicide programs starting at bud break and continuing through late spring are effective for managing scab on valued specimens. Scab-resistant crabapple varieties are the best long-term solution for new plantings.
If your crabapple or hawthorn drops all its leaves by August and then re-leafs in September, that's almost certainly apple scab. One defoliation typically doesn't kill a tree, but repeated annual defoliation causes progressive decline. A preventive fungicide program can break this cycle.
Fire Blight (Erwinia amylovora)
A bacterial disease affecting apple, crabapple, hawthorn, serviceberry, and other members of the rose family. New shoots and blossoms turn brown and curl in a characteristic "shepherd's crook" shape. The bacteria can move into main branches and even the trunk. During active growth, pruning infected wood out well below the visible margin (12 to 18 inches) and sterilizing tools between cuts slows spread. Copper-based bactericides applied at bloom can reduce infection from bee transmission.
Anthracnose
A group of fungal diseases causing irregular brown or tan spots on leaves, often along the veins and leaf margins. Common on sycamore, white oak, dogwood, ash, and maple. Anthracnose is most severe in cool, wet springs. Most established trees tolerate repeated infections without serious harm. For trees that defoliate annually or where early leaf drop is causing concern, fungicide applications at bud break can reduce severity.
Cytospora Canker
A fungal disease that attacks stressed conifers — particularly spruce, pine, and Colorado blue spruce. Causes branch dieback from the lower crown upward. Lower branches on blue spruce dying back progressively over several years is a classic presentation. The disease enters through wounds and stress-weakened tissue. There is no curative treatment — management focuses on pruning affected branches and improving tree vigor through proper care.
Chlorosis: Not a Disease, But a Major Problem
Iron and manganese chlorosis — yellowing leaves with green veins — is not caused by a pathogen but by alkaline soil chemistry that makes micronutrients unavailable to the tree. It is extremely common in Chicagoland's naturally high-pH soils and affects pin oaks, silver maples, river birch, and many ornamentals. Treatment includes trunk injections or soil applications of chelated iron or manganese, combined with soil acidification where appropriate.
Accurate diagnosis is the foundation of effective disease management. If your trees are showing unusual symptoms, don't guess — a trained eye can usually identify the problem quickly and recommend the most targeted intervention. Contact Emerald Tree Care for a disease diagnosis and management consultation.
