The name "cedar" can be rather confusing. Many other species, including junipers and arborvitae, are also called cedar. Though they are different, they all share some common diseases. When planting cedar trees, it's important that they are spaced far enough apart to ensure good air flow and that you water them from below. These two cultural practices will help you avoid many problems.
Cedar-apple rust is caused by the fungus Gymnosporangium juniperi-virginianae. This disease needs an alternate host from the apple family (apples, hawthorn, mountain ash, etc.) to complete its life cycle. Symptoms include swollen growths or woody galls that appear on branches or shoots. When the fungus has matured, orange gelatinous growths emerge from the galls. In addition to the galls, the fungus can cause dieback on your cedar. Control cedar-apple rust with fungicides and by planting resistant varieties.
Port-Orford-Cedar Root Disease
Port-Orford-Cedar Root Disease is caused by the fungus Phytophthora lateralis. When a tree is infected, its rootlets appear water-soaked; they then darken and disintegrate. Eventually the tree's inner bark and tissue of the larger roots will turn a deep, cinnamon brown (as opposed to a healthy cream color). Infected trees will have foliage that is slightly lighter than normal and that wilts slightly on warm days. The foliage will soon wither and turn light brown. Bark beetles will attack unhealthy trees, which can speed death. Keep the root disease from your plant by ensuring adequate air flow between trees and planting away from low spots, swales and streams. Fungicides can control the disease, but are not always 100 percent effective.
Fungi cause Phomopsis tip blight and Kabatina tip blight, two common diseases found in states east of the Mississippi River. While the damage they inflict can be severe, they are rarely fatal to established plants. Symptoms include brown or red and dying tips of twigs. Small gray lesions may encircle branch tips. The fungus can progress down the stem, eventually killing the whole branch. Buy disease-resistant plants, make sure they have good air circulation, water only in the morning and prune out diseased branches but do not compost them.
Witches' broom produces clusters of branches that are swollen at the base with many short, lateral shoots extending from the base. The fungus that causes witches' broom causes the tree to become unsightly, but it is rarely fatal. Young, recently planted trees or trees under chronic stress are more adversely affected. While fungicides are sometimes effective, the best solutions are cultural. Cut off and destroy--do not compost--cedar brooms during the winter, and always water your trees from below to avoid wetting the foliage.
Brown Felt Blight
Brown Felt Blight is caused by the fungus Herpotrichia juniperi. When a tree is infected, brown mycelium (a network of fungus filaments) covers its leaves and twigs. The fungus is most lethal to lower, smaller branches and young trees. The fungus grows profusely on branches that are buried beneath snow, although the mycelium will appear gray when covered. Spray with a fungicide containing 5 percent zineb or maneb before the first snowfall to prevent the fungus from taking over.