In Georgia, seiridium canker is most often caused by a fungus called Seiridium unicorne. This plague to Leyland Cypress trees forms on the stems and branches. Dark oval to elongated lesions are dry and look like a healing wound on a human with raised edges. Multiple lesions will form around the branch choking it by cutting off water flow. Resin may leak through cracks but is not a determining factor of seiridium canker as this occurs in healthy Leyland Cypress trees as well. A small incision near the canker should reveal a reddish to brown color and will be sticky if the tree is stricken with seiridium canker. Limbs affected will turn yellow to grayish or reddish-brown on random areas throughout the tree. Leaves that have turned will easily fall off as your hand touches it. If the fungus gets into the main trunk it will kill the tree.
The environmental stress of drought and a late freeze in the spring can help aid the development of this fungus. Small black dots present on the cankers (only visible with a magnifying glass) can easily be spread throughout the tree. Rain and overhead sprinklers can disperse these spores throughout the tree or even to nearby trees. Improper pruning techniques can also spread the growth of seiridium canker.
Prevention and Treatment
When planting as a screen, provide enough space (a minimum of 12 to 15 feet) between trees for good air circulation and to minimize stress as they mature and enlarge. For a more readily available screen try staggering trees. Avoid over-fertilization. Place mulch under trees to at least the drip line (and preferably further) to reduce water evaporation and competition for water as well as potential damage to trees from lawnmowers and string trimmers.
Drought-stricken trees are significantly more susceptible to infection. Some of the trees we have worked with were able to heal when we implemented a ground-level irrigation system over 1-2 years. As such, make sure Leyland cypresses are irrigated during drought or semi-drought conditions. Apply water at the base of trees to keep branches dry and reduce disease spread. If overhead irrigation is necessary, it should be applied very early in the morning. Remember overhead watering and rain can spread the disease.
Prune out and destroy diseased branches as soon as possible. Make pruning cuts 3 to 4 inches below diseased tissue. Sterilize pruning tools between each cut by dipping in a 10 percent bleach solution (1 part chlorine bleach to 9 parts water). Severely affected trees should be removed and destroyed. Calling a professional tree service to perform this work is highly recommended.
Fungicides are not considered an effective or practical means for controlling canker diseases in general or Seiridium canker in particular.