Thuja (Thuja spp.) is an evergreen belonging to the cypress family. Other common names for thuja include eastern arbor vitae, arbor vitae, white cedar and American arbor vitae. Thuja grows as an upright pyramidal, columnar, or rounded shrub or tree ranging in height from 3 to 50 feet tall depending upon cultivar. Thuja shares a number of common diseases and pests with cypress trees that result in stunted growth, damage to foliage and rotting of the interior tree wood.
Leaf blight (Fabrella thujina) is one of the most destructive diseases of giant arbor vitae in the northwest areas of the United States, according to Pascal Pirone, in "Diseases and Pests of Ornamental Plants." The disease also occurs in ordinary arbor vitae as well. Leaf blight is characterised by the appearance of one or more irregularly shaped brown to black circles on new foliage in spring. Gradually foliage starts to brown and the damage intensifies as though leaves are fire scorched and by fall the leaves drop entirely. Control option include the application of copper sprays on smaller sized trees during the middle of summer and early autumn.
Juniper blight (Phomopsis juniperovora) is a common disease of juniper and red cedar which also afflicts thuja. The fungal disease is a progressive dying back of branches and twigs and is caused by Phomopsis juniperovora, Kabatina juniper or Sclerophoma phthiophila. The disease is most destructive in younger trees and hedges. Early symptoms of juniper blight appear on the lower branches and dieback starts at the shoot tips. If left unchecked, the disease can cause the death of the entire plant. Management options include pruning the infected tree parts and adequately spacing plants to allow good air circulation. Fungicides with the active ingredients copper, potassium bicarbonate or propiconazole are recommended as chemical treatment options by Cornell University Extension.
Pestalotiopsis Tip Blight
Pestalotiopsis tip blight is a fungal disease of thuja that usually afflicts foliage already weakened and injured due to environmental stress or unfavourable growing conditions. As the infected needles start to die, the disease spreads to the twigs. The colour of the affected leaves turn from green to yellow and eventually to dark brown. Pestalotiopsis is more common during spring, according to the Missouri Botanical Garden's Kemper Center for Home Gardening. Disease management includes pruning infected tree parts as soon as disease is evident. Chemical control includes the use of recommended copper based fungicides.
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