May 2014 Plant Disease Update
Seiridium canker is a common fungal disease on Leyland cypress. The pathogen causes cankers on branches and the trunk. Cankered areas may be swollen, sunken, or cracked, and exude resin (Fig. 1a & 1b). Typically the first noticeable symptom of Seiridium canker is scattered branch dieback (Fig. 2).
Leyland cypress is predisposed to this disease by water stress, so ensuring plants are irrigated adequately when rainfall is less than 1” per week is recommended. There are no effective control measures once trees are colonized by this pathogen, other than pruning out dead branches back to healthy tissue and avoiding water stress (irrigating may help diseased trees recover from the disease). The best samples for accurate diagnosis of the disease are branch sections approximately 2” in diameter or more that are cankered and have areas of resin-flow.
Because the disease is lethal when cankers girdle the trunk and irrigation may be impractical for large trees, consider other evergreens for new plantings. Green Giant arborvitae (Thuja standishii x plicata) and Steeplechase arborvitae (Thuja x) are fast growing evergreens that are relatively disease-free and do not brown out during the winter like some other arborvitae species.