Most tree problems homeowners notice are caused by one of a handful of common diseases. Knowing what you're looking at is the first step to knowing whether to treat, prune, or remove. Here are the ones we see most often in the Fox Valley.
Oak Wilt
What it is. A vascular fungal disease that plugs the tree's water-transport vessels. The tree wilts and dies because it can't move water to the canopy.
What you'll see. Sudden, complete browning of leaves at the top of the canopy in mid-to-late summer. Red oaks (red, pin, black, scarlet) die rapidly — often within weeks. White oaks (white, bur, swamp white) decline more slowly over multiple seasons.
What to do. Stop oak pruning April–July. Confirm diagnosis with a lab test. Infected red oaks should be removed promptly to limit underground spread to neighboring oaks. Trenching to sever root grafts is sometimes warranted.
Anthracnose
What it is. A broad family of fungal diseases affecting many species — sycamores, maples, ash, oaks. Cool, wet springs make it worse.
What you'll see. Irregular brown or tan blotches on leaves, often along veins. In bad years, significant defoliation in spring. Sometimes twig dieback.
What to do. For most mature, healthy trees: nothing. The tree pushes new leaves and outgrows it. For repeated severe outbreaks on important trees, fungicide treatment can help — but it's preventive, not curative.
Apple Scab
What it is. A fungal disease specific to apples and crabapples.
What you'll see. Olive-green to black spots on leaves and fruit, leading to defoliation, sometimes by August. The tree often loses all leaves and re-leafs again before fall — a major energy drain.
What to do. Cultivar selection matters most. Resistant crabapple varieties exist; susceptible old varieties are best replaced. For trees you're keeping, fungicide programs starting at bud break are effective but require multiple applications.
Dutch Elm Disease (DED)
What it is. A vascular fungal disease, spread by bark beetles and through root grafts.
What you'll see. “Flagging” — sudden yellowing and wilting of leaves on one upper branch first, then spreading. Brown streaks visible under the bark of affected branches.
What to do. Most native American elms were lost to DED decades ago. Modern resistant cultivars exist. For mature surviving elms, professional injection treatments every 2–3 years are effective preventive — but the moment the tree shows symptoms, the prognosis is poor.
Verticillium Wilt
What it is. A soil-borne fungal disease affecting many species — maples, ash, redbuds, smoke bush, magnolia.
What you'll see. Wilting and browning of leaves on one branch or one side of the tree. Often greenish-brown streaks visible in cross-section of affected branches.
What to do. No reliable cure. Maintain tree vigor with proper watering and mulching; remove and dispose of dead branches; some trees compartmentalize the infection and live for years with it.
Tar Spot on Maples
What it is. A cosmetic fungal disease that hits Norway maples particularly hard.
What you'll see. Large black raised spots — like tar splatters — on the leaves in late summer.
What to do. Cosmetic only — doesn't hurt the tree. Rake and dispose of fallen leaves to reduce next year's spore load. No treatment usually needed.
A general principle. Many tree diseases look alarming and aren't fatal; a few look subtle and are. The diagnosis matters more than the symptoms — and a misdiagnosis often leads to either unnecessary removal or unnecessary expense on treatment that doesn't address the actual problem. If you're not sure, a free on-site evaluation will tell you what you're actually looking at.
