Seiridium Canker

Seiridium unicorne

Key Features

  • Death of individual branches
  • Healthy neighboring branches
  • Sunken, reddish branch canker
Close-up of Seiridium fruiting bodies and resin accumulation: Elizabeth Bush, Virginia Polytechnic Institute and State University,
Blighted branches caused by Seiridium canker: Photo courtesy of Kelly Ivors, North Carolina State University
Sunken, discolored canker (circled) on new growth: Photo courtesy of Kelly Ivors, North Carolina State University


Seiridium canker causes the death of a single branch or few individual branches. This is referred to as flagging, where a yellowed or brown branch visually stands out among healthy green branches. A reddish or purple colored sunken section of the stem (canker) can be found between the dead branch tip and the trunk. Small, black fruiting bodies may be seen on the canker surface; resin or sap may drip from or accumulate on cankers. Leyland cypress are heavily damaged, often being killed when cankers expand into the trunk. Juniper, cypress, and related species are attacked, to a lesser degree.

Seiridium canker with resin (sap) accumulating on the surface: Jennifer Olson, Oklahoma State University,


The fungus spreads from existing cankers to wounds on nearby branches or trees and infects through wounds such as cracks, pruning cuts, or holes make by borer insects. While wet weather is required for infection, symptoms generally appear in the summer when the tree is drought stressed. Growing cankers can extend back the dying branch into the trunk, killing the tree.

Management Recommendations

The best way to prevent this disease is to avoid using Leyland cypress. The next best method is to attempt to maintain good tree growth and vigor. Stressed trees are more susceptible, so water trees regularly during dry periods to alleviate drought stress. Eastern redcedar (Juniperus silicicola) or Atlantic white-cedar (Chamaecyparis thyoides) will suffer fewer environmental stresses and be more resistant to Seiridium canker if planted within their natural range. Cankered branches cannot be saved and should be pruned during dry periods or in late winter. Prune six to twelve inches below the canker or where the branch attaches to the main stem. Clean pruning tools between cuts with a 10% bleach solution or 70% alcohol. Removed branches must be burned or removed from the property. Composting branches may not kill the fungus.

Effective Pesticides

Active Ingredients include: Copper salts, Thiophanate-methyl

landscape report
Purdue Landscape Report
Plant & Pest Diagnostic Laboratory