Common Cedar Tree Diseases: Identification and Control

Understanding "Cedar" Diversity

The term "cedar" encompasses several distinct genera—including true cedars (Cedrus), junipers (Juniperus), and arborvitae (Thuja)—each with unique characteristics but overlapping disease susceptibilities. Proper spacing for air circulation and basal watering (avoiding foliage wetting) are foundational cultural practices that prevent many common issues.

Cedar-Apple Rust

Caused by the fungus Gymnosporangium juniperi-virginianae, this disease requires an alternate host from the Rosaceae family (apples, hawthorns, mountain ash) to complete its life cycle. Symptoms begin as swollen, woody galls on branches. In wet spring weather, these galls produce distinctive orange, gelatinous spore horns. The fungus can also cause twig dieback. Management combines planting resistant cultivars with timely fungicide applications during the spring spore release period.

Port-Orford-Cedar Root Disease

This lethal condition, caused by the water mold Phytophthora lateralis, attacks root systems. Early signs include water-soaked, darkening rootlets that eventually disintegrate. The inner bark of larger roots turns a diagnostic cinnamon-brown instead of healthy cream. Foliage becomes pale, wilts on warm days, and ultimately browns. Secondary bark beetle attacks often accelerate tree death. Prevention focuses on excellent drainage, avoiding low-lying planting sites, and ensuring adequate spacing. Fungicides offer partial control but are rarely 100% effective.

Tip Blights (Phomopsis and Kabatina)

These fungal diseases, prevalent east of the Mississippi, cause browning or reddening of shoot tips with small gray lesions girdling branch tips. The infection can progress downward, killing entire branches. While rarely fatal to established plants, they cause significant aesthetic damage. Control measures include selecting resistant varieties, ensuring good air circulation, morning-only watering, and pruning out diseased branches (destroying, not composting, the debris).

Witches' Broom

This fungal disorder produces dense clusters of swollen, short lateral shoots—the "broom" appearance. Though unsightly, it's rarely fatal except in young or stressed trees. Cultural management is preferred: remove and destroy brooms during winter dormancy, and strictly avoid overhead watering. Fungicides provide inconsistent results.

Brown Felt Blight

Caused by Herpotrichia juniperi, this fungus forms a brown mycelial mat over foliage and twigs, appearing gray when snow-covered. It's most damaging to lower branches and young trees, thriving under prolonged snow cover. Preventative application of fungicides containing 5% zineb or maneb before first snowfall effectively prevents establishment.