Fungal spores are washed down the tree or splashed from tree to tree by rain or overhead irrigation. New infections develop when spores lodge in bark cracks and wounds. You should always destroy diseased plant parts and try to avoid physical damage to plants. Sanitize pruning tools between each cut by dipping in rubbing alcohol or in a solution of 1 part chlorine bleach to 9 parts water. Chemical control of the fungus has proven to be difficult but some success has been noted with a full-coverage fungicide spray at monthly intervals from April to October.
Leyland Cypress With Seridium Canker - Photo by Steve Nix and Licensed to About.com