How to Spot a Diseased Tree in Washington State (Signs every homeowner should know)

A shady street tree is a great tree – until it isn’t. Diseased or pest-infested trees can become safety hazards, spread to nearby plants, and be costly to treat or remove. In Washington State, several infections and invasive insects demand attention, so early detection is your best defense. Here’s a practical, research-backed guide to the signs that a tree may be sick, what those signs often mean, and when to call a pro.

Look for these clear, year-round warning signs

1. Sudden canopy thinning or widespread dieback

If large sections of the crown (top of the tree) go bare or show dying branches over one or more years, that’s a major red flag. Dieback can point to root rot, vascular diseases, or borers that disrupt nutrient flow. When dieback accelerates quickly (leaves browning and falling across the canopy within weeks) you may be dealing with serious pathogens like Sudden Oak Death on susceptible hosts.

2. Unusual leaf symptoms

Look for yellowing, early browning, spotting, or distorted leaves at odd times of year. Leaf discoloration concentrated at branch tips often signals vascular or root issues; random spots or powdery coatings can indicate fungal diseases (e.g., powdery mildew, leaf spot). If many different species in your neighborhood show similar leaf problems after wet weather, fungal or waterborne pathogens are likely.

3. Cankers, sunken or dead patches on branches and trunks

Open wounds, oozing sap, sunken areas of bark, or visible cankers are serious signs that wood-decay fungi or bacterial infections have taken hold. Cankers can girdle branches or trunks, cutting off water flow and leading to structural failure. If you see fresh sap, discoloration beneath the bark, or blackened areas, don’t ignore them.  

4. Woodpecker “foraging” or D-shaped exit holes

Heavy woodpecker activity can indicate wood-boring insect larvae (they’re finding dinner inside the tree). For species like emerald ash borer (EAB), an emerging adult leaves a distinctive D-shaped hole in the bark; S-shaped galleries under the bark are another telltale sign of larvae feeding. Washington is actively monitoring and preparing for EAB. Please report suspicious signs to local authorities.  

5. Epicormic shoots (sucker growth) and thin, stunted new growth

When a stressed tree produces clusters of shoots along the trunk or at branch crotches, it’s trying to survive, but the underlying cause is stress: root damage, chronic disease, or severe pruning. Stunted or sparse new growth is another sign the tree isn’t getting enough resources.  

6. Fungal fruiting bodies, mushrooms, or sawdust at the base

Fungi that produce mushrooms, conks, or “brackets” on roots and trunks indicate internal decay. Sawdust or frass (insect droppings) at the base or in bark crevices points to boring insects. Both symptoms can indicate structural weakness. Trees with root rot or internal cavities can fail without much warning.

Common Washington threats to keep in mind

• Sudden Oak Death (Phytophthora ramorum): causes rapid browning in some oak and tanoak species and affects other hosts; WSU runs an active Sudden Oak Death program.  
• Emerald Ash Borer (EAB): a devastating wood-borer of ash trees; Washington agencies urge vigilance and reporting.  
• Root rots (Armillaria, Phytophthora) and wood decay fungi: common in wet soils or stressed trees and often fatal over time.  
• Dutch elm disease and other bark-beetle associated pathogens: especially relevant for elms and other host species.  
  

For up-to-date regional alerts and treatment guidance, check the Washington Department of Natural Resources (DNR) forest health pages and WSU Extension resources.

Prevention tips that actually help

• Keep trees healthy: regular mulching, correct watering, and avoiding root compaction.
• Don’t move firewood: transporting infested wood spreads many pests and diseases.
• Prune properly and remove dead limbs promptly to reduce stress and entry points for pathogens.
• Plant a diversity of species: monocultures mean single pests can cause widespread loss.

When to act immediately

If a large limb is hanging, the tree leans suddenly, or you see heavy internal decay, treat it as an emergency and call a professional tree service to evaluate and, if necessary, remove the hazard.

Spotting disease early can save a tree or at least protect your yard and neighboring trees. If you’re unsure about a tree on your property or want an expert assessment in Washington State, HG Tree Services offers on-site inspections, safe pruning, and removal when needed. Contact us for a professional evaluation and an itemized plan that prioritizes safety and long-term tree health.