Forest and Wildlife Extension

Forest Health Notes

A Series for the Non-Industrial Private Forest Landowner

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White Pine Blister Rust
(Cronartium ribicola)

Hosts: Western white pine, whitebark pine

Quick ID:

• Branch flagging
  
• Swellings and blisters on branches; also on trunk of small trees
  
• Diamond-shaped cankers on trunks
  
• Cankers with greenish-yellow to orange margins
  
• Pustules of orange spores on cankers and branch swellings
  
• Resin flow from branch and trunk on advanced infections

Field Identification

Tree: This disease does not affect species outside of the white (5-needle) pine group (similar symptoms on lodgepole or ponderosa pine may be caused by western gall rust). The foliage may have red or yellow needle spots. On the branches, spindle-shaped swellings or cankers with small cup-like indentations are frequent. Often infections kill the branch, resulting in bright red "flags" in the crown. On the trunk, a diamond-shaped canker with dead roughened bark and greenish-yellow to orange margins develops.

Fungus: A honey-colored ooze (pycnia) or pustules of powdery orange spores (aecia) develop on the swellings or cankers in the spring. As the disease develops, cankers will have dead centers, surrounded by pustules of aecia, surrounded by drops of pycnia, and with a yellowish zone outermost. The canker grows in size every year.

May be confused with: Resin flow at the base from Armillaria root rot.

Disease cycle: This disease is the most serious pest of 5-needle pines in the Pacific Northwest. The rust has a five-spore cycle that requires an alternate host, members of the Ribes family. These include species of gooseberry and currant. On the pine in spring, pycnial spores give rise to aecial spores, which may fly 400 to 800 miles to infect the leaves of Ribes plants. On the plant, two spore stages follow, then give rise to another airborne spore stage which infects the pine again. These spores infect needles, then grow down to the twig and from there to the branch. Branch infections within 4" of the trunk will spread to the trunk in time. Once the trunk is infected
and a canker has developed death of the tree is inevitable, though there is some genetic variability in rate of canker growth and some trees have survived many years before being girdled and killed. Once a tree gets beyond 20-30 years of age, the disease most frequently causes only branch flagging.

Predisposing agents: Cool, moist weather late in the summer or early fall is required for infection of the pine host, with 48 hours of temperatures not above 68F.

Impact: White pine blister rust was introduced in the east from Europe between 1898 and 1908, although the disease is believed to have originated in Asia. In 1910 it was introduced into British Columbia and has since spread through the West. In North America as a whole, it has caused more damage and more money has been spent to control it than any other conifer disease. Thousands of white pine stands have been seriously damaged and many have been entirely lost. In the Pacific Northwest, losses are estimated at 5 million cubic feet annually.

Management: Historically, control efforts have focused upon removing the alternate host, members of the Ribes family, from stands of white pine. However, this never proved to be very effective, despite the thousands of hours of work that were invested. Likewise, effective chemical controls have never been developed. As blister rust is an introduced species, rather than one that evolved here, genetic resistance is limited in the white pines. For this reason the mortality from this disease has been extremely high. However, some naturally resistant white pines were found and breeding programs to bulk up the seed have been in place for some time. Resistant seedlings are finding their way onto the market. When cutting in infected stands, any tree that is uninfected or only lightly infected should be retained as a seed tree, as it may be naturally resistant and pass
that resistance along. On young ornamental trees, branches with infections may be pruned to arrest the spread of the disease. The pruned branches should be subsequently burned.

Blister rust resistant western white pine is a good choice for replanting root disease centers and for incorporation in mixed-species stands. However, it probably should not be planted in pure stands.

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Forest Health Notes were prepared by Donna Dekker-Robertson, Peter Griessmann, Dave Baumgartner, and Don Hanley, Washington State University Extension. The assistance of Robert L. Edmonds and Robert I. Gara, University of Washington College of Forest Resources, is gratefully acknowledged.

Insect or disease treatments should always be part of an overall forest management planning process. It's a good idea to have a forest management plan for your property. This tool will help you prevent problems and keep your forest healthy. For information on forest stewardship educational programs, contact your local WSU Extension office. For information on technical assistance and financial incentive programs, contact the Washington Department of Natural Resources. The Natural Resources Conservation Service will assist landowners in developing conservation plans.