Phytophthora Root Rot
HIGHPhytophthora spp.
Overview
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.
- Type
- Fungus
- Severity
- HIGH
- Source
- USFS
Symptoms
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
Affected Species
Geographic Spread
Treatments
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.
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Frequently Asked Questions
What is 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.
What are the symptoms of Phytophthora Root Rot?
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
Where is Phytophthora Root Rot currently found?
Phytophthora Root Rot is currently active in Alabama, Arkansas, California, Connecticut, Delaware, Florida, Georgia, Iowa, Illinois, Indiana, Kentucky, Louisiana, Massachusetts, Maryland, Michigan, Minnesota, Missouri, Mississippi, North Carolina, New Jersey, New York, Ohio, Oregon, Pennsylvania, South Carolina, Tennessee, Texas, Virginia, Washington, Wisconsin, West Virginia. It has been reported in 31 total states across the United States.
What plants are affected by Phytophthora Root Rot?
Phytophthora Root Rot affects Flowering Dogwood, Rhododendron, Azalea, White Oak, Red Oak, Apple. Susceptibility varies by species.
How do I treat Phytophthora Root Rot?
Treatment options for Phytophthora Root Rot include: Phosphonate Fungicide Treatment, Drainage Improvement. Treatment approaches may include chemical, biological, cultural, and mechanical methods depending on the severity and context.