Cause Seimatosporium berckmansii (formerly Coryneum berckmansii), a fungus. The fungus produces spores in late September, which are washed and splashed by rains to infect new foliage within the plant. These small spores are also easily carried by insects or by air currents to other shrubs. This is a foliage disease confined to the imbricated or bract-like leaves and small twigs where the fungus overwinters. Large woody stems are not attacked, but infected plants eventually die because of continued defoliation.
Berckmann's arborvitae-Platycladus orientalis L. var. conspicua 'Berckmannsii'-is particularly susceptible to infection. Most cultivars of the species orientalis, including the golden arborvitae and several green forms of globular or pyramidal habits, are susceptible to the disease as well. American and European species seem to be immune. Pyramidal arborvitae (Thuja occidentalis 'Pyramidalis') is not generally susceptible, thoughthe WSU-Puyallup Plant Clinic found one exception.
Symptoms Infections begin in young, tender foliage at branch tips and develop downward. Color changes from the normal green to a grayish cast. As the fungus moves down small branchlets to larger ones, some of the latter become girdled; foliage beyond these injuries then changes from the normal green to a reddish brown. This brown discoloration is especially conspicuous above zones where branches are girdled. Eventually much of the infected foliage falls and exposes unsightly masses of gray stems. Tiny black fruiting bodies (acervuli) of the causal fungus may be found on infected gray foliage, especially where branchlets are girdled.
- Establish plantings with disease-free plants propagated from cuttings.
- Cut out and destroy dead and dying twigs and branches.
Chemical control Spray twice in fall (late September or early October, again in early November). Spray again in early spring (February to March) if disease is severe.
- Basic copper sulfate at 1 lb/50 gal water. Washington only. 24-hr reentry. O
- CuPRO 2005 T/N/O at 0.75 to 3 lb/A (or 1 to 3 Tbsp/1000 sq ft) dormant or at 0.75 to 2 lb/A when new growth is present. 24-hr reentry.
- Monterey Liqui-Cop at 3 tablespoons/gal water. H
Reference Pirone, P.P. 1978. Diseases and Pests of Ornamental Plants, 5th ed. New York: John Wiley & Sons, Inc.