Leyland cypress (Cupressocyparis leylandii) is a soft-textures evergreen with greenish-blue foliage. This pyramid-shaped ornamental and landscaping tree increases the appeal of commercial and residential areas, maintaining privacy while demarcating a particular boundary. Despite its moderate tolerance to most pests and diseases other cypress trees are susceptible to, certain diseases adversely affect the health of a Leyland cypress tree.
Cercospora needle blight, Botryosphaeria (Bot) and Seiridium canker, Phytophthora root rot and Annosus root rot are all diseases that affect the Leyland cypress.
The Cercosporidium fungus causes the Cercospora needle blight. Bot and Seiridium cankers are caused by Botryosphaeria dothidea and Seiridium Unicorne respectively. Annosus root rot originates from Heterobasidion annosum, while Phytophthora root rot is caused by Phytopthora cinnamomi.
Cercospora needle blight spreads by fruiting bodies that resemble small pastules, usually green in color, on the upper parts of twigs, branches or needles. The infection occurs during wet weather. Bot canker is identified by narrow, 1-foot long cankers on tree trunks. According to North Carolina State University, Seiridium canker is the most damaging disease of Leyland cypresses. These cankers resemble sunken patches on the bark and cause infected parts to turn a deep red, forming a striking contrast against the rest of the tree. These cankers emit a heavy golden resin. Fruiting bodies, usually at the base of the tree, that are white underneath but brown above indicate the presence of Annosus root rot. Phytophthora root rot causes the roots of the infected cypress tree to turn black.
Cercospora needle blight causes the needles in the lower crown of the Leyland cypress to turn brown. Most of the outward symptoms of Seiridium and Bot canker are the same, such as browning of limbs and some dieback; the only difference is the needles that do not separate form the trees infected with Bot canker. Change of foliage color to brown, immediate decline or sudden death of the cypress indicates the presence of Annosus root rot. Foliage changing in color from green to yellow is an indication of Phytophthora root rot.
Bot cankers occur on highly stressed Leyland cypress trees. Water frequently, especially during long periods of drought, and mulch heavily to keep roots cool. Avoid over-fertilization and aggressive pruning. Control the spread of Seiridium canker from an infected tree to another by washing and disinfecting pruning equipment and avoiding water stress.
Leyland cypress trees infected with Cercospora needle blight need repeated applications of fungicides that contain copper. Spray every 10 days after buds break until they are established. Effective chemical control of Bot canker, Annosus root rot and Seiridium canker do not exist. Apply a systematic fungicide to trees infected with Phytophthora root rot.