Leyland cypress, Cupressocyparis leylandii, is a graceful evergreen with flattened branchlets of a dark, blue-green, feathery foliage. It is increasingly popular in the American southeast as a hedge, screen or windbreak, but in recent years it has come under increased assault by a variety of fungal diseases, several of them lethal.
The most damaging fungal disease affecting Leyland cypress is seiridium canker, Seiridium unicorne. Sunken brown, purplish cankers appear on the bark, sometimes with extensive flow of resin. The cankers may appear on stems, branches, and axils of branches causing die back. Dead twigs and branches turn a bright-reddish brown. Fruiting bodies appear on the cankers as barely visible black dots.
Seiridium canker, which strikes Leyland cypress trees of all ages, is spread by water splash from irrigation or rain and by infected pruning tools. It is also spread by transported cuttings and plants. There are no recommended fungicides. Destroy pruned branches and twigs, then clean your pruning tools with alcohol.
The symptoms of Botryosphaeria canker are similar to those of Seiridium canker. The fungus Botryosphaeria dothidea causes scattered reddish-brown, dead branches and twigs that stand in stark contrast to the dark green, healthy foliage. Botryosphaeria produces long, narrow cankers on the trunk that can be more than a foot long. The growing cankers kill any branches that they encircle. The fruiting bodies just below the bark produce tiny pimples on the surface of the bark.
Botryosphaeria spores may be spread by water splash, infected pruning tools, cuttings, and transplants, and they are borne by the wind. The disease most often hits plants that are under stress or are not getting enough water for extended periods of time. There are no effective fungicides that work on botryosphaeria.
Cercospora Needle Blight
The fungus Cercosporidium sequaoiae, a needle blight common among several species of juniper and cypress, has recently begun to infect Leyland cypress. The needles turn brown next to the stem. The disease spreads upward and outward; eventually the only green needles are at the top of the plant.
Fruiting bodies, tiny greenish pustules, appear on small twigs or the upper surface of the needles. Plants usually become infected during wet weather; the spores, present throughout the spring and summer, are spread by the wind. Spray infected plants with fungicides that contain copper. Spray plants every 10 days beginning with the breaking of buds until new growth matures.
Phytophthora Root Rot
The fungus Phytophthora cinnamomi affects the roots of smaller plants, both in the nursery and landscape. The foliage yellows and some tips can die. It usually strikes plants in poorly drained soils and rarely afflicts large, established Leyland cypress trees. A laboratory analysis is ordinarily required to diagnose the fungus. Horticulturalists at North Carolina State University recommend treatment by Subdue Maxx.
Annosus Root Rot
The spores of Annosus Root Rot, Heterobasidion annosum, establish themselves on the stumps of freshly cut conifers, usually pine trees. The fungus spread to the roots of the cut stump and from there to the roots of adjacent trees and shrubs, including Leyland cypress. As the larger roots decay, the plant turns yellow and begins to decline. It can suddenly turn reddish-brown in color. A tree can die before it exhibits symptoms.
Small, irregularly shaped fruiting bodies, brown on top and white on the bottom, can form in mulch and leaf litter at the base of a tree. There are no effective chemical controls for the disease. Stumps of felled trees should be treated with dry, granular borax or removed completely.