Western cedar trees are evergreen trees with needled foliage that are popular both on their own and planted closely together as screens. They are relatively low-maintenance once they are mature, but you still need to keep an eye out for western cedar diseases that can easily move from one infected tree to another or take out a landscape accent tree. Being aware of the signs and symptoms of these problems can help you avert problems before they become serious.
Cedar-apple rust fungus moves back and forth between cedar trees and apple or crabapple trees. The fungus needs both trees to survive and propagate. On the cedar tree, you will notice large, bulbous, brown sacks full of fungal spores attached to the branches. Use sterile pruning techniques to remove the branches holding the spore sacks and dispose of them in sealed garbage bags or by burning. In most cases, this should break the cycle. If not, you can treat the fungus with a fungicide or remove one of the two trees--apple or cedar--from the area to interrupt the host-cycle.
Bagworms infest western cedar trees in the winter, but their presence is not usually apparent until spring when the small larvae hatch and start eating the cedar, resulting in browning, defoliation and the formation of unsightly silken bags on the tree. If the infestation is minor, you can try to remove the bagworms by hand. During the winter, remove the females and their eggs, which is the best way to control an infection. However, you can also remove the adult males during late summer and fall. Help your tree combat the damage with nitrogen fertilizer and, if necessary, treat the bagworms using an insecticide.
Spider mites are easiest to spot by the webbing that they leave in the crooks of leaves and twigs. These small, red mites can eventually brown your foliage and even kill your tree if left unchecked. Remove dead foliage from the tree using sterile pruning techniques and then dispose of it by burning or in a sealed bag. Then treat the tree with a horticultural oil like neem oil to make it untenable for the spider mites. You can also spray your tree down with a hose to remove the mites, but you will need to treat it in order to prevent their return.