Capable of growing 4 feet a year when young, Leyland cypress (x Cupressocyparis leylandii) is a popular ornamental evergreen for large landscapes. Pyramidal form and feathery, dense blue-green foliage make it valuable as a specimen tree or in privacy screens. Leyland cypress, however, is susceptible to a number of parasitic pest and diseases. Left unchecked, they can ruin its appearance and significantly shorten its lifespan.
Leyland cypress is vulnerable to both insect and fungal parasites. Most serious is the Seiridium uncorne canker fungus, according to former Clemson Cooperative Extension horticulture agent J. McLeod Scott and colleagues. Phytophthora root rot and Botryosphaeria dieback fungi also attack the tree. Insect parasites include bagworms (Thyridopteryx ephemeraeformis), juniper scale (Carulaspis juniperi) and spruce spider mites (Oligonychus ununguis).
Symptoms of Infection
Seiridium canker results in dead, discolored branch and stem sections. Bark oozes resin, developing purple or brown splotches. Black dots on bark are fungus spores. Root rot causes stunted, thin, discolored foliage. Botryosphaeria dieback produces cracked branches, stem cankers and dark areas on interior wood.
Bagworms spin white cocoons as they feed. Completed cocoons resemble cypress cones. Small yellow spots on the trees' needles indicate the presence of spider mites. Webbing is a sign of heavy mite infestation. Juniper scale insects appear as tiny bumps on needles' undersides. They eventually cause needle yellowing or browning and twig die back.
Uncontrolled Seiridium canker can spread from branches to the trunk, killing the tree. Root rot kills trees by destroying their root systems. Botryosphaeria dieback kills the branches' tissue from their canker and circle areas to their tips. Light bagworm infestations cause needle drop; heavy ones can defoliate a tree in a matter of weeks. Spruce spider mites and juniper scale insects suck fluids from needles, depriving the trees of nutrients. Continuing infestations are fatal.
Cankers attack trees stressed from freezing, drought or wounds. Frost protection, adequate water, and quick wound treatment reduce canker risk. Root rot retention requires complete removal of all infected trees. Early spring removal and destruction of bagworm cocoons prevents another generation of larval infestation. Predator mites and ladybugs feed on spruce spider mites. High-pressure water spray also removes them. Winter application of horticultural oil suffocates female scale insects overwintering on the trees before they lay eggs.
Applications of thiophanate-methyl fungicide control the cankers. No fungicides work on root rot. BT-based (Bacillus thuringiensis) insecticide kills bagworms. Miticides destroy spruce spider mites while limiting damage to beneficial insects. Malathion-based insecticides control both the mites and juniper scale insects.