Phytophthora Root Rot in California
HIGHFungus
Phytophthora Root Rot is currently active in California. Watch your susceptible trees closely and act early.
About Phytophthora Root Rot
A group of water-mold diseases that attack the roots and root collar of many tree species. Phytophthora thrives in wet, poorly drained soils. It destroys fine roots and can girdle the trunk at the soil line. Affected trees show a general decline that is often misdiagnosed as drought stress. Oaks, dogwoods, rhododendrons, and many other species are susceptible.
Symptoms to watch for
Crown thinning and reduced leaf size, leaves may appear pale green or chlorotic, dark water-soaked bark at the base of trunk near soil line, cankers at root collar that ooze dark sap, fine roots are brown and rotted rather than white, gradual decline over one to several years, tree may lean or become unstable
Trees at risk in California
Tracking one of these? Add it to My Yard to get alerts when threats go active near you.
Treatment options
Phosphonate Fungicide Treatment
Apply phosphonate fungicides (Agri-Fos, Reliant) as a soil drench or trunk injection to suppress Phytophthora. Most effective as a preventive treatment or in early stages of infection.
Drainage Improvement
Improve soil drainage around affected trees, avoid overwatering, reduce soil compaction, and avoid piling mulch against the trunk. Phytophthora thrives in wet conditions.
Recommended Products & Tools
Helpful supplies for managing Phytophthora Root Rot. Always read and follow label directions, and confirm a diagnosis before treating.
Broad-spectrum protectant for many fungal leaf and bark diseases.
Preventive spray for needle cast, leaf spots, and blights.
Sharp, clean cuts to remove affected wood — a tree-care essential.
As an Amazon Associate, TreeIssues may earn a commission from qualifying purchases made through these links — at no extra cost to you.
Need help with Phytophthora Root Rot in California?
Connect with ISA Certified Arborists serving California:
Browse all California arborists →Frequently asked questions
Is Phytophthora Root Rot in California?
Phytophthora Root Rot is currently active in California. Watch your susceptible trees closely and act early.
What does Phytophthora Root Rot look like?
Crown thinning and reduced leaf size, leaves may appear pale green or chlorotic, dark water-soaked bark at the base of trunk near soil line, cankers at root collar that ooze dark sap, fine roots are brown and rotted rather than white, gradual decline over one to several years, tree may lean or become unstable
What trees does Phytophthora Root Rot affect in California?
It affects Flowering Dogwood, Rhododendron, Azalea, White Oak, Red Oak, Apple. Susceptibility varies by species.
How do I treat Phytophthora Root Rot?
Options include: Phosphonate Fungicide Treatment, Drainage Improvement. Confirm the diagnosis before treating, and consider a certified arborist for high-value trees.