Arborvitae are blue-green cypress trees or shrubs sometimes grown as privacy screens because of their broad, spreading branches and evergreen foliage. When the foliage starts to turn brown or black, you must take immediate action to solve the health problem that has led to the discoloration. Discolored arborvitae are unattractive and may spread disease to other parts of the garden.
Bacterial Diseases
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According to the North Dakota State University website, black branches and foliage on arborvitae may be the result of bacteria that thrive in excessively moist, humid environments. Water that sits on the branches of the arborvitae creates a place for bacteria to thrive, leading to discolored, slimy patches on the tree or shrub. Avoid overwatering the arborvitae and ensure that the soil around the base of the tree drains well to prevent unhealthy soil conditions.
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Twig Blight
Phomopsis juniperovora is a disease that causes twig blight of arborvitae trees and shrubs. Twig blight disease causes the tips of branches to turn dark brown, gray or black as it progresses. Later in the growth stages of the disease, according to Penn State, small black pustules form; these are spore-producing sites that spread the disease further. Branches that are girdled by the disease experience browning of all foliage. Chemicals containing thiophanate methyl are effective at controlling twig blight. Prune affected branches.
Gray Mold
Botrytis cinerea and other species of botrytis cause a condition called gray mold on arborvitae. As the gray mold grows, the foliage of the arborvitae turns black or gray and exhibits a layer of mold. Botrytis is a serious problem for arborvitae and can prove fatal, especially for young or stressed plants. There are no chemical controls for this disease; remove and burn or bury affected branches and keep the growing environment of the arborvitae clean and well-drained. Avoid injuring the plant during pruning and do not use excess fertilizer.
Dieback
Pestalotiopsis is a genus of fungal diseases that affect arborvitae, especially when the plant is weak, stressed or subjected to other disease and pest problems. Dieback causes branches and foliage to turn dark brown or black, especially near the base of the plant. Both foliage and smaller twigs are killed. Dieback is usually a minor cosmetic problem that is symptomatic of greater environmental concerns. Fungicides and careful pruning, as well as management of the moisture and fertilizer content of the soil, are key to controlling dieback.