What are the most common tree diseases in the US?
The most common US tree diseases are anthracnose, apple scab, and powdery mildew (cosmetic); fire blight and cytospora canker (prune out infected wood); oak wilt, Dutch elm disease, and verticillium wilt (often fatal); and Armillaria root rot (no cure). Emerald ash borer is an insect, not a disease, but kills more trees than any of them.
Most tree "diseases" a homeowner notices are leaf spots after a wet spring, and most do no lasting harm. A handful get into the wood or roots, spread tree to tree, and kill. The table sorts the ones you are most likely to meet in a US yard. One entry is not a disease: the emerald ash borer is a beetle, but it has killed more yard trees in twenty years than any fungus.
| Disease | Main hosts | What you see | Serious? | Treatable? |
|---|---|---|---|---|
| Anthracnose | Sycamore, ash, oak, maple, dogwood | Brown blotches along leaf veins in a wet spring, early leaf drop | Rarely | Not needed; rake leaves |
| Apple scab | Apple, crabapple | Olive-black spots on leaves and fruit, leaf drop by midsummer | Cosmetic | Spring fungicide, or resistant varieties |
| Powdery mildew | Lilac, dogwood, oak, crabapple | White powder on leaves in late summer | Cosmetic | Rarely worth it |
| Fire blight | Pear, apple, crabapple, hawthorn | Branch tips wilt, blacken, and curl like a shepherd's crook | Yes on pears; kills young trees | Prune 12 inches below damage in dry weather |
| Cytospora canker | Spruce, esp. Colorado blue; peach, poplar | Lower branches die one at a time, white resin on bark | Slowly disfiguring | Remove dead branches; no cure |
| Diplodia tip blight | Austrian, Scots, mugo pine | New shoots stunted and brown, black specks on cones | Kills over years | Fungicide at bud break |
| Oak wilt | Red oaks mainly; white oaks slower | Leaves brown from the edges and drop in summer; whole tree in weeks | Fatal to red oaks | Injection for nearby trees; trench to cut root grafts |
| Dutch elm disease | American and other native elms | Wilting leaves on one branch, then spreading; brown streaks in sapwood | Fatal | Injections every 2–3 years on high-value trees |
| Verticillium wilt | Maple, ash, redbud, catalpa, smoketree | Sudden wilt on one side, green streaks in sapwood | Often fatal over years | No cure; water, mulch, replace with resistant species |
| Armillaria root rot | Almost anything | Thinning canopy, honey-colored mushrooms at base, white fans under bark | Fatal | No cure; do not replant the same spot |
| Thousand cankers disease | Black walnut | Thinning crown, small cankers under bark | Fatal | No cure |
| Emerald ash borer (insect) | All North American ash | Thinning from top, D-shaped exit holes, S-shaped tunnels under bark, woodpecker damage | Fatal without treatment | Trunk injection every 2–3 years, $100–$500 per tree per visit |
Which tree diseases are actually serious?
The ones that get into the water-carrying wood or the roots. Oak wilt can kill a red oak in a single summer. Dutch elm disease took most of the country's American elms. Verticillium lives in the soil and shuts down maples a limb at a time. Armillaria works through the roots and is why a tree with a full crown and mushrooms at the base is a hazard; see mushrooms or fungus on my tree. Emerald ash borer kills nearly every untreated ash it reaches. Anthracnose, apple scab, and powdery mildew are leaf problems: ugly, but not fatal to an established tree.
Can tree diseases be treated?
Some, depending on the disease and the value of the tree. Fire blight and cytospora canker are managed with a saw: cut well below the infected wood in dry weather and clean the blade between cuts. Apple scab and Diplodia respond to fungicide applied in spring before symptoms show. Oak wilt, Dutch elm disease, and emerald ash borer are treated by trunk injection, typically every 2–3 years, at $100–$500 per tree per visit, worth it for a large, healthy, well-placed tree and not for a small or declining one. Verticillium, Armillaria, and thousand cankers have no practical cure. What an arborist can do for a sick tree is on can an arborist save my tree?, and whether pruning helps is on can pruning save a dying tree?
How do tree diseases spread between trees?
Four ways. Leaf and canker diseases spread by spores on wind and rain, so they explode in a wet spring and raking fallen leaves helps. Soil diseases like verticillium and Armillaria spread through roots and persist for years, so you do not replant the same species in the same spot. Oak wilt and Dutch elm disease move through root grafts between neighbors and on beetles drawn to fresh wounds, which is why oaks in affected regions are not pruned April–July and infected elms are removed fast. Wipe pruning blades with alcohol between trees. Timing by species is on when should I prune, by species?
Should I spray my trees to prevent disease?
Not as a routine. A large tree cannot be sprayed well from the ground, most yard trees have nothing a spray would prevent, and the money is better spent on deep water in a drought and a mulch ring. Spraying earns its cost for a specific, identified disease on a valuable tree: a crabapple that loses its leaves to scab every July, a pine with Diplodia. Be wary of door-to-door "tree health programs" that sell a schedule without naming a disease. If nobody can say what the tree has, there is nothing to treat.
How do I get a tree disease identified?
Start with your county or state extension office. Most identify common problems from photos for free or a small lab fee, and their advice is not tied to selling a treatment. Photograph the whole tree, leaves front and back, bark symptoms, and a fresh cut through a small dying branch, since sapwood streaking identifies wilts. For a valuable tree, an arborist site visit at $75–$150 gets a diagnosis and a plan in one trip. If more than half the canopy is already dead, treatment is money spent on a removal you will need anyway; the checklist on how do I know if my tree is dead or dying? helps you make that call.
No independently confirmed professional review is on file yet.
Not yet independently reviewed. Figures are typical US ranges for planning purposes, not quotes.