Scots Pine Blister Rust

**This organism is not known to be present in the United States but poses a threat.

Scots pine blister rust, a disease caused by the fungus Cronartium flaccidum, is not known to be present in the United States but may pose a serious risk to our forests and Christmas tree industry if introduced. Close relationships between known host trees and native hard pines raise concern that our trees could be highly susceptible to this exotic rust.


Native to Europe and Asia, Scots pine blister rust is an aggressive fungus that attacks a wide variety of hard pine species. Scots pine blister rust damages vascular tissue and kills the cambium of infested trees, leading to girdling and eventual death. It is more virulent than white pine blister rust, which has already cost over $1 billion dollars to manage in the U.S. This fungus decimated two-needle pine forests of the Mediterranean region in the 1960s and 70s and infected or killed half of a 10,000 square kilometer pine forest in Greece over a six-year period. Potential impacts in the U.S. are considered incalculable.


Scots pine blister rust could travel into the U.S. on infected plant material, seedlings, and nursery stock. Japanese black pine (Pinus thunbergii), mugo pine (Pinus mugo) or other hard pines, commonly imported as live trees for use where dwarf conifers are desired for landscape design, pose some of many significant threats for introduction of the pathogen.

Once established, the disease could spread great distances by wind and on infected plant material, making it difficult to contain.


Tree hosts of Scots pine blister rust include over 15 hard pines with 2- or 3-needle clusters, but it is unknown how susceptible native North American pines would be. Several common and economically important trees in the U.S. may be potential hosts. Additionally, the Scots pine, which has been widely planted and naturalized in the U.S., is the main host in northern Europe. If the pathogen were introduced, it might have or gain the capacity to infect native pine species alongside known, introduced hosts.

Over the course of its life cycle, Scots pine blister rust also infects a long list of herbaceous hosts. The wide range of herbaceous host species includes many common garden plants, wildflowers and weeds.


While Scots pine blister rust is difficult to impossible to detect during some parts of its life cycle, the spore stage presents recognizable signs and symptoms. Lesions spread along and around branches and stems of pines, growing into resinous cankers which girdle the tree and cause the crown to die. Several years after initial infection, the fungus produces spores in conspicuous orange blisters emerging from the bark. These blisters appear for a month each year until the tree dies, and while they are present, the disease is relatively easy to diagnose.

Signs and symptoms on 2- and 3-needled pine species:

  • Stem swelling/cankers
  • Brightly colored orange spores in "blisters" on trunk
  • Flagging (death of individual branches)
  • Top-kill


Think you've spotted this pest?

If you think you've found this pest in your landscape contact your local extension office to see about sending in a sample.
Find your local extension office here.