Top Tree Diseases in California This Season: How to Spot & Prevent Them

California’s winter and early spring bring much-needed rain—but they also create the perfect conditions for tree diseases. Prolonged moisture, cool temperatures, and saturated soil allow fungi and water-mold pathogens to spread rapidly, putting both residential and commercial landscape trees at risk.

Understanding which tree diseases are most active this season and knowing the early warning signs can help homeowners take action before permanent damage occurs.

Why Winter Rains Increase Tree Disease Risk in California

During winter and early spring, California experiences:

• Frequent rainfall
  
• Cooler temperatures
  
• Poor soil drainage in many landscapes
  

These conditions allow fungal spores and soil-borne pathogens to thrive. Moisture spreads disease through splashing rain, runoff, and saturated roots, while trees are less able to defend themselves during dormancy or stress.

This is why many serious tree problems begin below ground—long before symptoms appear in the canopy.

Top Tree Diseases to Watch This Season

1. Sudden Oak Death (SOD)

High-risk trees: Coast live oak, tanoak, California black oak

Sudden Oak Death is caused by Phytophthora ramorum, a destructive pathogen that creates trunk cankers and interrupts water flow inside the tree.

Common signs:

• Dark bleeding cankers on the trunk
  
• Wilting or browning leaves
  
• Rapid canopy decline
  

Why it’s dangerous:
Once advanced, SOD often leads to complete tree failure and can spread to nearby oaks during wet seasons.

2. Phytophthora Root Rot

High-risk trees: Oaks, maples, citrus, ornamentals

This soil-borne disease attacks roots in poorly drained or overwatered soil.

Common signs:

• Yellowing or thinning leaves
  
• Wilting despite adequate watering
  
• Slow growth or sudden decline
  

Seasonal risk:
Winter rains allow Phytophthora to move easily through soil and infect healthy roots.

3. Anthracnose & Leaf Blight Diseases

High-risk trees: Sycamore, ash, oak, maple

Anthracnose fungi thrive in cool, wet weather, making them very common in late winter and spring.

Common signs:

• Brown or black leaf spots
  
• Curled or distorted leaves
  
• Early leaf drop
  

While often not fatal, repeated infections weaken trees and make them vulnerable to pests and secondary diseases.

4. Emerging Threats: Pine Ghost Canker & Foamy Bark Canker

Pine Ghost Canker

• Affects certain pine species
  
• Causes branch dieback and pale, “ghost-like” foliage
  
• Can spread through wounds and insect activity
  

Foamy Bark Canker (Oak Trees)

• Spread by bark beetles
  
• Produces wet, foamy sap oozing from the trunk
  
• Leads to branch dieback and structural weakness
  

These newer diseases are becoming more common as climate stress and pest pressure increase across California.

Symptoms Homeowners Should Look For

Early detection is critical. Contact a professional if you notice:

• Cankers or sunken areas on the trunk
  
• Oozing sap or dark staining on bark
  
• Leaf spots, blotches, or unusual discoloration
  
• Premature leaf drop
  
• Wilting or dieback despite proper watering
  
• Sudden decline in overall tree healthcare services

The earlier a disease is diagnosed, the higher the chance of saving the tree.

Proactive Tree Care & Prevention Tips

Preventive care is the most effective way to protect trees during wet seasons.

✔ Improve Drainage

• Avoid standing water around tree bases
  
• Correct grading issues
  
• Reduce over-irrigation during rainy months
  

✔ Proper Pruning

• Remove dead or infected branches
  
• Avoid tree pruning during wet weather
  
• Disinfect tools between trees
  

✔ Sanitation

• Remove fallen leaves and debris
  
• Dispose of infected material properly
  
• Avoid moving contaminated soil
  

✔ Professional Diagnosis

Many tree diseases look similar but require very different treatments. Certified arborists can accurately identify the issue and recommend safe, effective solutions.

When to Call a Certified Arborist

You should seek professional help immediately if you see:

• Trunk cankers on oak trees
  
• Persistent wilting or yellowing
  
• Rapid canopy dieback
  
• Signs of root disease or soil saturation
  
• Trees declining after storms or heavy rain
  

Delaying treatment can allow disease to spread to nearby trees and significantly increase removal costs.

Conclusion

This winter through early spring, California’s wet soil and cool temperatures increase the risk of tree diseases like Phytophthora root rot, anthracnose, and canker infections, while Sudden Oak Death remains a serious threat to oak trees statewide.

Early detection and proper seasonal care can protect your trees and prevent costly damage. If you notice any warning signs, contact Tree Doctor USA for a professional inspection. Our certified arborists provide expert diagnosis and treatment to help save your trees and keep your landscape healthy year-round