A fast-growing, pyramid-shaped evergreen with a soft texture and blue-green foliage, the Leyland cypress (Cupressocyparis leylandii) is a landscaping tree grown in residential and commercial areas alike. Its dense foliage forms a suitable barrier and privacy screen. Although more disease- and pest-resistant than other trees, Leyland cypress is susceptible to a few diseases that cause problems.
Seiridium canker, Botryosphaeria (Bot) canker, Annosus and Phytophthora root rot and Cercospora needle blight are fungal diseases that affect Leyland cypress trees.
Seiridium canker, the most damaging disease for the Leyland cypress, according to North Carolina State University, is caused by Seiridium unicorne, which affects plants of all ages and sizes. Botryosphaeria canker is caused by a fungus called Botryosphaeria dothidea. Heterobasidion annosum causes Annosus root rot and Phytopthora cinnamomi causes Phytophthora root rot. Cercospora needle blight is caused by a fungus botanically known as Cercosporidium sequoiae.
Seiridium canker resembles a sunken patch on the bark of the trunk, stem or twig that emits a deep golden-brown resin and causes infected parts to turn deep reddish-brown. The fungus that causes Bot canker produces narrow cankers on the trunk that sometimes extend over 1 foot in length, but they do not emit any resin. Annosus root rot is identified by small irregular fruiting bodies that are brown on top but white underneath. They usually occur at the tree's base. Leyland cypresses infected with Phytophthora root rot have black roots. Fruiting bodies of the Cercospora needle blight appear as small green pustules on twigs or needles.
Seiridium canker causes infected stems, twigs or branches to turn deep brown and die back. The external symptoms of Bot are similar to those of the Seiridium canker, but if the needles remain intact and do not fall off after you run your hands over them, the canker is Bot. Symptoms of the Annosus root rot include foliage color change from green to brown or decline of a Leyland cypress followed by death. Early signs of the Phytophthora root rot include yellowing of foliage and tip die-back. The needles on Leyland cypress trees infected with Cercospora needle blight exhibit brown needles in the lower crown.
Seiridium canker spreads from one part of the cypress to another, or to a different Leyland cypress through infected irrigated water and pruning tools. Spores of Botryosphaeria dothidea spread through the same means as those of Seiridium unicorne, along with being carried by wind. Annosus spreads through spores that penetrate freshly exposed stumps. The spores then reach the roots and also infect roots of adjacent trees through root contact. Phytophthora root rot infects nursery and landscape cypresses that stand in poorly drained soils. Cercospora needle blight spreads in severely stressed Leyland cypress trees.
Although preventive measures exist, there are no chemical treatments to control trees infected with Seiridium canker, Bot canker and Annosus root rot. Treat trees affected with Phytophthora root rot with Subdue Maxx. Spray Kocide or other fungicides that contain copper to treat Cercospora needle blight.