This disease is caused by the fungus Didymascella thujina, and is the most damaging to Western Red Cedar (Thuja plicata). Initial symptoms appear as one or more whitish, bleached areas on individual scale leaves, usually on the upper surface. Later, these spots join, turn brown, and cover the entire scale leaves.
As the spores form inside the leaf, a distinctive cover flap opens when wet, allowing their spread into surrounding plants. Eventually, the infected foliage turns brown and drops, leaving the branches bare. If stem infections are numerous, they can girdle a main branch or main stem resulting in death of a major section of the tree. Mature trees are less likely to be severely affected, but small trees can be killed.
To minimize the damage caused by this disease, we recommend an application of Bordeaux Mix to the foliage to prevent the further spread of this disease. For best results, we recommend two applications from early spring to early summer.