Seiridium canker of Leyland cypress is a fungal disease that produces oozing brown cankers and causes yellowing and browning of tree branches.
The disease primarily affects the Leyland cypress, a fast-growing evergreen tree. Trees of all sizes and ages are susceptible.
Drought stress or cold injury favor the development of the seiridium canker fungus. Symptoms typically appear in early spring but can be seen year-round. Fungi enter the tree through cracks or wounds in the bark, and form brown cankers that leak a thick, sticky substance called resin. Leaves at the tips of affected stems or branches turn yellow, then brown. The disease rapidly progresses in wet weather, and, left untreated, can lead to full branch discoloration or death.
Leyland cypress trees are commonly planted close together as hedges, which helps to advance the spread of seiridium canker once a specific tree has been affected. Severely diseased trees should be removed to preserve the health and vitality of nearby plants. Generally, trees should be removed if multiple branches randomly dispersed throughout the canopy have turned yellow or brown. If a single branch is discolored and infected, it can be pruned and destroyed to discourage the further spread of the disease. Talk to a professional arborist about proper pruning of diseased branches to ensure that all infected plant parts are removed. Additionally, keep trees well-watered during dry periods to minimize drought stress and build a natural defense against seiridium canker.