Leyland cypress is a natural cross of Monterey cypress and Alaskan cedar and has been available for more than 100 years as a landscape plant. It is extremely tough and salt tolerant, making it popular for coastal settings where other trees will not grow. It is popular, in part, because it is extremely resistant to pests and disease. There are some serious diseases that attack Leyland cypress and should be addressed immediately upon discovery.
Seiridium canker is caused by a fungus called Seiridium unicorne. The canker appears as a localized, sunken or cracked area on the trunk or branches of the tree. The area has a definite edge and is dry and dead-looking, and often oozes a purplish or dark brown resin. The disease causes the affected branches to die back, discoloring the foliage above the infection site.
The affected branches should be removed by pruning. The tools should be disinfected at each cut to reduce the risk of spreading the infection. Because the disease is spread by water, irrigation should be around the base of the tree, rather than on the leaves and branches. A chemical control of thiophanate-methyl fungicide can be applied to the entire tree monthly from April to October to prevent to disease, but not to cure it.
Phytopthora Root Rot
The tree is also susceptible to root rot from the fungus Phytophthora. This disease is frequently related to wet soil conditions, particularly in young plants. Phytopthora lives naturally in the soil and usually infects wounds on the root system. As the disease progresses, the fungus will destroy the root system. The tree will begin to display signs of illness including stunted and sparse growth, color changes and the eventual death of branches and then the tree itself.
This is a serious disease that has no treatment. Soil can be fumigated prior to planting with a fungicide to reduce the risk of infection. Organic material can be added to soils that tend to retain water. Infected trees need to be removed and destroyed. It is important to remove the entire root system.
The fungus Botryosphaeria causes leader trunks or major branches to die back. When dissected, the inside of the branch reveals a deeply sunken canker that is often cracked around its edges. There is dark discoloration under the bark. The disease is usually triggered by stresses caused by excessive heat, drought or transplanting.
The dead branches should be pruned back below the dead wood using the same hygienic methods used in Seiridium canker. Chemical control with thiophanate-methyl fungicide can also be used on the entire tree.