Oregon’s Most Common Tree Diseases

Introduction

Oregon’s climate—moist winters, mild springs, and warm summers—creates ideal conditions for many fungal and bacterial tree diseases. Understanding the most common threats helps homeowners protect their landscapes and catch problems early.

1. Anthracnose

Symptoms:

• Dark leaf spots
  
  

• Early leaf drop
  
  

• Twisted or deformed leaves
  
  

• Dying shoot tips
  
  

Common Hosts:

Ash, dogwood, oak, sycamore.

Treatment:

Pruning infected limbs, improving airflow, and reducing irrigation moisture on leaves.

2. Root Rot (Phytophthora & Armillaria)

Symptoms:

• Yellowing leaves
  
  

• Sudden wilting
  
  

• Mushrooms on trunk base
  
  

• Reduced growth
  
  

Threat Level:

High — root rot can kill trees silently.

3. Leaf Spot Diseases

Affect maples, willows, cherries, and more.

Symptoms:

• Brown or black spots
  
  

• Speckled leaves
  
  

• Patchy defoliation
  
  

Most leaf spot fungi spread in spring rains.

4. Fire Blight

A bacterial infection primarily affecting fruit trees.

Symptoms:

• Blackened, “burned-looking” leaves
  
  

• Shepherd’s-crook branch tips
  
  

• Oozing bark lesions
  
  

5. Verticillium Wilt

A soilborne fungus that blocks a tree’s vascular system.

Symptoms:

• Sudden branch death
  
  

• Yellowing leaves
  
  

• Sparse canopy
  
  

• Stunted growth
  
  

Maples are particularly vulnerable.

Prevention & Treatment Overview

• Prune during dry weather
  
  

• Keep trees mulched and watered properly
  
  

• Avoid wounding bark
  
  

• Improve soil drainage
  
  

• Schedule annual inspections
  
  

Many diseases can be managed or stopped if caught early.

Vascular decay