Arborvitae (Thuja spp.) cedar trees and shrubs are nothing if not versatile. Close plantings of densely needled arborvitaes make sound-absorbing privacy hedges. Sculpted specimens become striking topiaries. Varieties including spherical, dwarf "Golden Globe," columnar, 20-foot "DeGroot's Spire" and conical, 40-foot eastern white cedar (Thuja occidentalis) ensure an arborvitae for every garden space, no matter its size. As cedars, arborvitaes share susceptibility to a range of fungal infections.
Conditions Favorable for Fungus Diseases
Many arborvitae-attacking fungi thrive in moist conditions. Rainy spring weather that keeps branches and foliage wet invites needle and twig-blight infections. Cypress canker fungus and Pestalotiopsis tip blight move in when an arborvitae's bark is damaged as well as wet. Too much -- or too little -- water leaves arborvitaes vulnerable to Pythium and Phytophthora root rot fungi, although the latter seldom harms western red cedar (Thuja plicata) varieties. Taking preventive measures when conditions are favorable for fungal infection may save your arborvitae.
The greatest disease threat to arborvitaes, especially to western red cedar varieties, comes from Keithia blight fungus. The disease surfaces as tiny black holes on the flat leaf scales that bleach as the foliage browns. Infected leaves may drop in fall; those that remain become ash-gray. Seedlings and young plants sometimes succumb to defoliation. Cercospora blight browns arborvitae foliage from the stems outward. In its advanced stages, only the arborvitaes' highest branch tips remain green. Pestalotiopsis tip blight targets environmentally stressed arborvitae, progressing from the leaves’ tips to their bases. Affected twigs and foliage become dark brown. The plants' heavy lower growth may die. Repeat springtime applications of copper-based fungicide are standard treatment for this fungal trio. Apply the spray according to the manufacturer's specifications.
Cypress canker (Seiridium cardinale) fungus attacks arborvitae twigs and branches, producing cankers, or sunken areas of dead tissue. The infected limbs' mature foliage browns in spring before their new needles fade and die. Water-borne fungal spores enter the plants through insect-damaged or wounded bark. Removing cankered branches 1 to 2 feet below the damaged tissue controls the disease's spread. Between pruning cuts, sanitize your tools for two minutes in a solution of 1 part bleach to 9 parts water and dry them.
Soil-borne Phytophthora, Pythium and Armillaria root rot fungi attack and slowly decimate arborvitae roots. Phytophthora- or Pythium-infested roots blacken and lose their flexibility. They struggle to transport water and nutrients. Aboveground Phytophthora and Pythium symptoms include stunted growth, dying branches with yellowing, dropping foliage and wet cankers at the soil line. Cracked, resin-oozing bark revealing rotting wood around the base of the trunk, a classic Armillaria symptom, may take years to develop. Root-rot management begins with well-draining soil and proper irrigation to discourage the moisture-loving fungi. Use timely fertilization and supplemental water during drought to slow Armillaria's advance. Apply a systemic fungicide to the soil for additional protection against Phytophthora and Pythium attacks. Don't expect it, however, to eliminate root-rot pathogens from already-infected tissues.
- Monrovia Plant Catalog: Arborvitae
- University of California Integrated Pest Management Pests in Gardens and Landscapes: Arborvitae
- British Columbia Ministry of Agriculture: Dying Cedar Hedges -- What Is the Cause?
- Pacific Northwest Plant Disease Management Handbook: Cedar, Western Red (Thuja Plicata) -- Leaf Blight (Keithia Blight)
- North Carolina State University Plant Pathology Extension: Diseases of Leland Cypress
- Missouri Botanical Garden: Pestalotiopsis Tip Blight on Arborvitae
- Pacific Northwest Plant Disease Management Handbook: Cypress (Cupressus Sp.) -- Canker
- University of California Marin Master Gardeners: Prudent Pruning
- Nursery Management: Phytophthora and Pythium Root Rot
- University of Missouri Extension: The Stealthy Tree Killer -- Armillaria Root Rot
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