Thuja is a coniferous genus of the cypress family. It consists of five species that include the Korean thuja, Japanese thuja, northern white cedar, Sichuan thuja and the western red cedar. These vigorously growing evergreens average about 30 feet in 30 years and can reach mature heights of up to 60 feet. These hardy trees adapt to moist soil environments and are relatively disease- and pest-free. However, thuja trees are susceptible to several diseases that can be injurious to the tree if left untreated.
Anthracnose is a fungal disease that results in the blighted appearance of the tree. The anthracnose fungus lies dormant throughout the winter on defoliated debris. The fungal spores are swept onto the tree during the windy, rainy periods of spring where they infect the newly developing twigs and foliage. Infected thujas will develop small, darkened areas on their needles. The progression of the disease will result in complete browning and deadening of the infected needles. Severely infected thuja trees will experience heavy defoliation, growth stunt, dieback, and, at times, girdling cankers around their trunks. Prevent anthracnose by keeping the thuja's area free of defoliated debris. Copper- and chlorothalonil-based fungicidal sprays are effective in treating this disease.
Phytophthora Root Rot
Phytophthora root rot is a soil-borne fungal disease. This disease can survive for several years in the ground without a host. This disease is especially present and active during long periods of warm, wet weather with saturated soils. Phytophthora root rot infects the thuja tree through its root system. Infected roots become brittle and discolored. The damage to the root system results in adverse symptoms that include dieback, growth stunt, yellowing of needles and heavy defoliation. Phytophthora root rot cannot be cured with fungicidal sprays. However, chemical treatments combined with good pruning practices can help to control the disease.
Seiridium canker is a weak fungal disease that enters trees through wounds and pruning cuts. This fungal disease infects the wood of drought-stressed thuja trees, which results in sunken and reddened areas, fungal ooze and cankers. The fungal cankers reduce the amount of nutrients and water passing through the tree, which results in dieback, growth stunt and death of the infected areas. Fungicidal treatments are ineffective in treating seiridium canker; however, you can easily prevent the disease by maintaining a regular irrigation schedule for the thuja.