Written by Josh Goode, ISA Board Certified Master Arborist (MA-5752B), Tree Risk Assessment Qualified (TRAQ), Plant Healthcare Manager at Monkeyman’s Tree Service Reviewed by Chris Ruvalcaba, ISA Certified Arborist (PN-9560AT), Tree Risk Assessment Qualified (TRAQ), Owner/President of Monkeyman’s Last updated: 05/24/2026
Ash tree disease refers to the pests, fungal infections, and disorders that damage and kill ash trees — the most serious being the Emerald Ash Borer (EAB), an invasive beetle now confirmed in the Portland, Oregon area. Common signs include early leaf drop, canopy thinning from the top down, and strange holes or marks in the bark. The right treatment depends entirely on the cause: some conditions are easily managed, while an advanced EAB infestation usually means the tree can’t be saved.
If your ash is showing any of these symptoms, it’s right to be concerned. This guide explains what’s actually wrong with most sick ash trees, how to tell the problems apart, what can and can’t be treated, and when a struggling tree becomes a safety hazard — written from what we diagnose in the field across the Portland metro, not a generic checklist..
First, the One That Changes Everything: Emerald Ash Borer
If you only read one section, make it this one. The Emerald Ash Borer (EAB) is a small metallic-green beetle whose larvae tunnel under the bark and cut off the tree’s ability to move water and nutrients. It kills the vast majority of untreated ash trees it infests, and there’s no bringing a tree back once the damage is advanced.
It matters locally because EAB is no longer a distant problem. It was first found on the West Coast in Forest Grove, Oregon in 2022, and has since been confirmed in several Portland-area counties, including Multnomah. If you own an ash tree in our region, this is the threat to take seriously first.
Signs of EAB:
- D-shaped exit holes in the bark, about ⅛ inch wide — the single most distinctive sign
- Canopy thinning and dieback starting at the top of the tree
- S-shaped tunnels (galleries) winding under the bark when you peel it back
- Heavy woodpecker activity — birds stripping bark to get at the larvae
- New shoots (suckers) sprouting from the trunk or base as the tree tries to survive
- Vertical splits in the bark
A key point homeowners miss: by the time the canopy is visibly thinning, EAB has usually been inside the tree for two or three years. Early detection is everything, which is why an annual inspection of any ash tree in our area is worth it.
Can it be treated? Yes — but only proactively or very early. Healthy ash trees can be protected with a systemic insecticide (the most proven is emamectin benzoate), injected directly into the trunk by a licensed professional. It’s effective and safe for people, pets, and pollinators, but it’s a preventive measure repeated every couple of years — not a cure for a tree already in steep decline. Once a tree has lost the upper third of its canopy, removal is usually the safer and more economical path.
The Other Common Ash Problems
Not every sick ash has EAB. These are the other conditions we regularly diagnose, and several are far more treatable.
Ash Yellows
A slow-moving but ultimately fatal disease caused by a phytoplasma (a bacteria-like organism). It disrupts how the tree moves nutrients, so growth stalls over several years.
What to look for: pale or yellowing leaves, foliage that’s smaller than normal, slow or stunted growth, a thinning canopy, and dieback of branches. A telltale sign is tufts of weak, bunched shoots called “witches’ brooms.”
There is no cure for Ash Yellows. Management focuses on keeping the tree healthy and safe for as long as possible, and planning for removal once it becomes a hazard.
Verticillium Wilt
A soil-borne fungus that invades the tree’s vascular system and chokes off water flow to the upper branches.
What to look for: sudden wilting of leaves (often on just one side or section of the tree), brown or olive streaking in the wood just under the bark, dead branches, and patchy canopy thinning.
Mild cases can sometimes be managed with proper watering, pruning of affected limbs, and soil care to help the tree wall off the infection. Severe cases are usually fatal.
Anthracnose
A leaf-and-twig fungus that thrives in Portland’s cool, wet springs — which makes it one of the more common things we see locally.
What to look for: dark brown or black blotches on leaves, distorted or curled foliage, and early leaf drop, often heaviest on the lower and inner canopy.
The good news: anthracnose looks alarming but rarely kills an established tree. It’s largely cosmetic and stress-related, and it’s managed with good sanitation (raking and removing fallen leaves), pruning for airflow, and supporting the tree’s overall health.
Ash Rust
A fungal infection spread by airborne spores. Like anthracnose, it’s stressful but seldom fatal on its own.
What to look for: yellow-orange spots or raised pustules on leaves and sometimes on leaf stems, leaf distortion, early leaf drop, and a powdery residue.
It usually clears with seasonal changes and good tree care. Repeated heavy infections are a sign the tree is under stress and worth a professional look.
How to Tell What You’re Dealing With
A quick way to narrow it down before you call anyone:
- Problem starts at the TOP of the tree and works down, with D-shaped holes → think EAB. Act fast.
- Leaves are spotted, blotchy, or distorted but the tree’s structure seems fine → likely a leaf fungus (anthracnose or rust). Usually not an emergency.
- Whole sections wilt suddenly with streaking under the bark → possibly Verticillium Wilt.
- Slow, multi-year decline with small pale leaves and witches’ brooms → possibly Ash Yellows.
This is a starting point, not a diagnosis — several of these overlap, and EAB in particular is often confirmed only by peeling bark or trapping. When an ash is involved in our region, it’s worth having it looked at properly rather than guessing.
When a Diseased Ash Becomes a Safety Issue
This is the part that turns a tree problem into a property problem. Diseased ash trees — especially EAB-killed ones — become brittle remarkably fast. Dead ash wood is notoriously prone to dropping limbs and failing without warning, which makes them dangerous to be around and dangerous to remove.
Get a tree assessed promptly if you see:
- Large dead limbs over a house, driveway, walkway, or power line
- A significant lean that’s new or worsening
- Mushrooms or conks at the base (a sign of root or trunk decay)
- More than about a third of the canopy dead or bare
A tree in this condition shouldn’t be climbed or cut by a homeowner — by this stage even professional crews treat it as a hazardous removal.
How Monkeyman’s Helps With Ash Trees
We approach every ash the same way: figure out what’s actually wrong before recommending anything. Our ash tree services include:
- Professional diagnosis and tree health assessments by certified arborists
- Targeted treatment for treatable conditions, including trunk-injection for EAB protection
- Tree risk assessment by TRAQ-qualified arborists when safety is a question
- Safe removal of trees that are too far gone, including hazardous EAB-killed ash
- Stump grinding after removal
- Emergency storm response for failures and dangerous limbs
Our certified arborists know how these problems present specifically on Portland-area trees, and we’ll always tell you honestly when a tree is worth saving and when it isn’t.
How to Protect Your Ash Trees
Whether or not your tree is currently sick, a few habits make a real difference:
- Have ash trees inspected annually. With EAB in the region, this is the most important one — early detection is the difference between treatment and removal.
- Don’t move ash firewood. This is how EAB spreads fastest. Buy and burn firewood locally.
- Prune properly and at the right time. Good airflow reduces fungal disease — but note that ash pruning and removal is restricted in our area during the warmer months when beetles are active, so timing matters. We can advise on the current rules.
- Avoid overwatering. Excess moisture feeds the fungal diseases above.
- Keep the soil and roots healthy. A vigorous tree resists infection far better than a stressed one.
- Consider replacing young ash. Officials now recommend planting other species rather than new ash trees, given the EAB outlook.
FAQs About Ash Tree Disease
What’s the most serious ash tree disease? The Emerald Ash Borer (technically an insect, not a disease) is by far the most destructive threat to ash trees, and it’s now present in the Portland area. It kills most untreated ash trees it infests.
Can a diseased ash tree be saved? It depends on the problem and how early it’s caught. Leaf fungi like anthracnose and rust are rarely fatal and very manageable. Healthy trees can be protected from EAB with preventive injections. But a tree already in steep decline — especially from EAB or advanced Ash Yellows — usually needs removal.
How do I know if my ash tree has Emerald Ash Borer? Look for D-shaped exit holes about ⅛ inch wide, dieback starting at the top of the canopy, S-shaped tunnels under the bark, and unusual woodpecker activity. Because the early stages are hidden, a professional inspection is the only way to be sure.
Should I remove my diseased ash tree? If it’s a safety risk — large dead limbs over a structure, a worsening lean, or more than about a third of the canopy dead — yes, and promptly, because dead ash wood becomes brittle and unpredictable. We can assess it and recommend the safest option.
How can I prevent ash tree disease? Annual inspections, proper and well-timed pruning, healthy soil, not overwatering, and not moving ash firewood. For valued ash trees in our region, preventive EAB treatment is also worth discussing with an arborist.
Is the Emerald Ash Borer in Portland? Yes. It was first detected in Oregon in Forest Grove in 2022 and has since been confirmed in several Portland-metro counties. If you own an ash tree here, it’s worth having it inspected.
Get a Free Ash Tree Inspection
If your ash tree is showing any of the signs above — or you simply want to know whether it’s at risk — the safest move is to have a certified arborist take a look before the problem advances.
Request a free estimate and we’ll assess your tree’s health honestly. You can also learn more about our arborist services, plant problem diagnosis, tree removal, and emergency tree service.
Josh Goode is an ISA Board Certified Master Arborist (Credential No. MA-5752B) and Tree Risk Assessment Qualified (TRAQ), serving as Plant Healthcare Manager at Monkeyman’s Tree Service — a family-owned company caring for trees across Portland, Lake Oswego, Beaverton, Tigard, Hillsboro, and the surrounding Portland metro since 2008.