Managing Tree Pests & Disease in 2026: Key Takeaways from the PNW-ISA Conference

Tree problems in the Pacific Northwest rarely begin with dramatic failure. More often, tree decline starts subtly: thinning canopies, reduced growth, or minor dieback that can easily be mistaken for seasonal stress. But as pest pressure and root disease increase across Oregon and Washington, those early symptoms require closer attention.

On February 13, 2026, arborists, plant pathologists, and urban forest health professionals gathered for the PNW-ISA Regional Tree Health Conference to discuss emerging threats in Pacific Northwest tree health. The focus included root rot pathogens, invasive tree pests in Oregon, and advances in tree diagnostics.

One theme stood out: tree disease management in the Pacific Northwest is becoming more complex. Urban landscapes now face layered stress from soil compaction, redevelopment, climate shifts, and global pest movement. Early detection through certified arborist inspection and structured tree health assessment is no longer optional — it is essential for protecting long-term landscape investment.

Several arborists from Priority Tree Service attended the conference. The following overview highlights the most important insights for homeowners, HOAs, and commercial property managers across Oregon and Washington.

A tree disorder refers to any abnormal condition that disrupts normal tree growth, structural stability, or long-term health. Accurate tree diagnostics are the foundation of effective tree disease management in Oregon landscapes.

The conference emphasized the importance of distinguishing between two primary causes: 

 Abiotic factors: soil issues, water stress, environmental conditions, weather events, and mechanical injury 

Biotic factors: fungi, insects, and other pathogens 

 Confusing the two can lead to wasted treatments, unnecessary removals, or missed pathogen spread. Accurate diagnosis starts with a systematic approach: 

• Identify the species 
• Examine the crown, stem, roots, and collar 
• Evaluate site conditions 
• Ask about history and recent changes 
• Research and confirm before making recommendations 

Professionals also categorize stress factors as predisposing, inciting, or contributing, helping us understand not just what is happening to a tree, but why. For example, compacted soil may predispose a tree to stress, drought may incite decline, and opportunistic fungi may contribute to eventual failure. Understanding that sequence changes how we respond.

While advanced diagnostics are critical, speakers repeatedly emphasized that foundational care prevents the majority of long-term issues.

• Proper mulching 
• Correct watering 
• Structural and health pruning 
• Fertilizing only when indicated 

These remain some of the most effective tools for long-term tree vitality. These may sound basic, but improper mulch depth and inconsistent irrigation remain two of the most common contributors to stress in urban trees. When trees are chronically stressed, they become far more susceptible to secondary pests and pathogens.

When visual assessment leaves uncertainty, laboratory testing becomes essential. Arborists were encouraged to: 

• Collect clean, labeled samples 
• Maintain a diagnostic kit 
• Document findings with photos and notes 

Accurate diagnosis supports defensible recommendations. For commercial properties managing dozens or hundreds of trees, documentation is also critical for liability protection and long-term maintenance planning. 

Root rot in the Pacific Northwest remains one of the most destructive and misunderstood causes of tree decline. Root disease often develops quietly and can go unnoticed until canopy decline becomes visible, and it remains one of the most significant threats to conifers and hardwoods in the Pacific Northwest. Key indicators include: 

• Resinosis at the root collar 
• Thinning or reduced canopy 
• Distress cone crops (excessive cone production as a stress response)
• Secondary beetle activity 

    Laminated Root Rot: (Phellinus sulphurascens) delamination, white rot, setal hyphae, brown fruiting bodies. Remains a primary conifer killer in Oregon forests and landscaped properties.

    Armillaria: white rot, mycelial fans, black rhizomorphs. Affects both hardwoods and conifers and can persist in soil for decades.

    Heterobasidion: stringy white rot, conks. Continues to spread in areas with previous stump removal and soil disturbance.

• New trees can become infected from old stumps 
• Some pathogens spread through airborne spores 
• Survey above-ground symptoms first 
• Minimize soil disturbance 
• Limit movement from infected zones 

Management thresholds were also discussed, including removal when a tree is less than 30% functional and treatment options when canopy loss is still moderate. Determining functionality requires evaluating structural stability, canopy density, and overall vigor. Waiting too long can increase hazard risk, especially in high-traffic areas.

This is particularly important on redevelopment sites where previous tree removals have occurred. Pathogens such as laminated root rot can persist in soil and woody debris for years, creating infection centers that impact future plantings.

(Applicable to Oregon areas only) 

Do not move material out of quarantine zones. Firewood movement continues to be one of the primary pathways for invasive pest spread. Even small amounts of transported material can introduce destructive species into new regions.

 Work windows for certain operations: Oct 1 – April 30 

The conference also addressed climbing safety in declining ash. While ANSI does not formally prohibit climbing, heavily declining ash trees are widely considered unsafe. 

Urban tree health requires awareness of both symptoms and visible pathogen signs to prevent widespread spread in commercial corridors and residential neighborhoods.

Speakers revisited the classic disease triangle: host, pathogen, environment. This simple framework remains one of the most effective tools for understanding why outbreaks occur.

Common symptoms and signs were highlighted:

Symptoms: yellowing, dieback 

Signs: fruiting bodies, sooty bark 

Urban spread pathways continue to be a major concern, especially through: 

• Distribution centers
• Box stores
• Rest stops
• RV parks
• Truck stops
• Green-waste sites
• Nurseries
• Firewood movement

 Urban landscapes function as hubs of movement. Trees in commercial corridors, apartment complexes, and retail centers are often exposed to greater introduction risk than those in remote areas. 

High ant activity often indicates aphids or scale Spider mites thrive in dusty conditions. These small observations often provide faster answers than expensive testing, and can speed up field diagnosis significantly. 

Invasive Pests: Current Threats 

Invasive tree pests in Oregon continue to threaten both residential landscapes and commercial properties. Two invasive insects continue to demand close attention from arborists and land managers.

 Mediterranean Oak Borer – has since been confirmed as a significant white oak killer in affected regions

• First detected in 2018 
• Confirmed white oak killer by 2022 
• Detections remain specific to Oregon 
• Signs include dieback, frass, tiny brown beetles, and black galleries 
• Property owners should monitor for dieback, fine frass, and small brown beetles emerging from black galleries beneath the bark.

 Emerald Ash Borer – poses severe risk to all native and ornamental ash species

• Larvae cause the most damage 
• All common ash species are susceptible 
• Adults are active for roughly six weeks per year 
• Look for D-shaped exit holes, bark splits, and serpentine galleries Avoid planting ash in affected regions. Communities that once relied heavily on ash as a street tree are now facing large-scale canopy loss and significant replacement costs.

Communities across the Midwest have experienced widespread canopy loss due to Emerald Ash Borer infestation, leading to substantial municipal and private replacement costs.

Field Reminder for Arborists 

In fast-paced field environments, having a consistent diagnostic workflow prevents assumptions and missteps. A simple but powerful workflow: 

Document  → Photograph → Sample  → Verify → Consult

As pest pressure and root disease continue to affect Pacific Northwest tree health, routine tree inspections and early diagnostics are becoming critical components of responsible property management. Whether managing a single residential landscape or a multi-acre commercial property, working with a certified arborist ensures accurate tree disease identification, informed treatment planning, and long-term protection of landscape assets. 

The depth of expertise these speakers share underscores the importance of continuing professional education in a rapidly changing landscape.

Thomas Whitney, Technical Advisor, Davey Tree Expert Company 

Meg McConnell, Senior Diagnostician, Bartlett Tree Research Laboratory 

Rachel Brooks, Forest Pathologist, Washington Department of Natural Resources 

Kat Bethea & Christine Buhl, Oregon Department of Forestry 

Margaret Parks, Director of Operations and Technical Support, Evergreen Growers

Joey Holbert, Research Assistant Professor, WSU Urban Forest Health Lab 

Bett Levin, Senior Associate, Rincon-Vitova Insectaries