Leyland cypress trees (Cupressocyparis leylandii) grow in hardiness zones 6 to 10A and are not native to North America. They reach 35 to 50 feet in height and have an oval or pyramidal shape. The trees can experience a range of diseases that detract from their appearance. While homeowners can treat some diseases, most have no known chemical control.
Two types of root rot occur on Leyland cypress: Phytophthora root rot and Annosus root rot. Initial symptoms occur underground--the roots rot--so cannot be readily detected. Foliage turns yellow or brown. Sometimes, trees collapse without displaying outward signs. Gardeners cannot control root rot, and affected trees will eventually die, though it may take several years. To prevent this disease, plant only in well-draining soils.
North Carolina State University terms Seiridium canker the worst disease afflicting Leyland cypress trees. Trees develop dark brown or purple sunken cankers on the bark; some cankers ooze a sap or resin. Branches throughout the tree develop rusty brown foliage. There is no treatment for this disease aside from pruning off affected branches. North Carolina State University advises cutting down severely affected trees.
Botryosphaeria canker, caused by the fungus Botryosphaeria dothidea, displays symptoms similar to Seiridium canker. However, its cankers are long (sometimes as much as 1 foot) and narrow and rarely leak resin. As with Seiridium canker, no chemical control exists for this disease, and homeowners should prune away affected branches.
Cercospora Needle Blight
Cercospora needle blight turns the needles of a Leyland cypress brown. First the base of the needles discolors. Then, as the disease progresses, the center and tip of the needles turn brown and the disease spreads upward and outward. Severely afflicted trees bear green needles only at the top. Gardeners can treat affected trees with fungicides containing copper.