Cause Cronartium ribicola, a fungus. The fungus attacks all five-needle pines including whitebark pine (Pinus albicaulis), which is the most susceptible, sugar pine (P. lambertiana), western white pine (P. monticola), eastern white pine (P. strobus), limber pine (P. flexilis), bristlecone pine (P. aristata) and floxtail pine (P. balfouriana), which is the least susceptible. In addition to many Ribes species that serve as alternate hosts, Castilleja (Indian paintbrush) and Pedicularis are also telial hosts.
The fungus overwinters in tree cankers. In spring, spores form in orange pustules (aecia) on cankers. Wind distributes aeciospores to currants and gooseberries, the principal alternate hosts. Uredia form on the underside of these Ribes hosts and produce urediospores. The urediospores can only reinfect more Ribes hosts and continue to do so during the rest of the growing season. Telia form in late summer where the uredia are located. The teliospores are not dispersed but germinate to form another spore type called a basidiospore. These spores are blown to white pine, causing new infections by penetrating through stomata of needles (unless stomata are occluded by epicuticular wax). Pycnia appear on the bark the following spring. Aeca erupt from the same area where the pycnia formed the previous year. Aeciospores are produced perennially until the branch dies.
The disease is favored by cool, moist habitats rather than warm, dry areas. Open stands with a thick shrub layer encourages a moist air flow and thus distribution of spores.
Similar rusts occur on other pines including Comandra blister rust (Cronartium comandrae) on ponderosa and lodgepole pines and Stalactiform rust (Cronartium coleosporoides) on lodgepole pine. Comandra blister rust alternates on bastard toadflax (Comandra sp.) while stalactiform rust alternates on chickweeds such as Cerastium sp. and Stellaria sp.
Symptoms The first symptom is a yellow or red spot on infected needles in the spring. By fall, orange hyphae may be seen in the bark at the base of these needles. Dark lesions develop in this area and continue to develop into the characteristic diamond-shape cankers. Old cankers are rough and elongate structures that develop on trunks and branches, causing dieback or "flagging." Orange spore masses burst through the margins of the cankered bark (aeca). An abundant pitch flow usually accompanies canker formation. A branch or branch stub may be near the canker's center. Generally observed in the lower portion of trees.
- Rate planting sites for blister-rust hazard, and plant resistant stock in medium- to high-hazard sites.
- Preventive pruning of lower branches is effective at minimizing risk and removing early infections.
- When cutting severely infected stands, retain uninfected trees for seed sources.
- Prune infected branches with cankers more than 4 inches from the trunk.
- In landscaping, do not plant white pine near cultivated or native currants or gooseberries.
Chemical control Protect needles in the fall when telia are produced on the alternate host.
- Armada 50 WDG at 9 oz/100 gal water. Do not use a silicone-based surfactant. Not for nursery or greenhouse use. A Group 3 + 11 fungicide. 12-hr reentry.
- Bayleton 50 T&O is registered. Landscape only, not for use on plants for sale. 12-hr reentry.
- Strike 50 WDG at 8 oz/100 gal water. 12-hr reentry.
References Maloy, O.C. 1997. White pine blister rust control in North America: A case history. Annual Review of Phytopathology 35:87-109.
Smith, J. A., Blanchette, R. A., Burnes, T. A., Gillman, J. H. and David, A. J. 2006. Epicuticular wax and white pine blister rust resistance in resistant and susceptible selections of eastern white pine (Pinus strobus). Phytopathology 96:171-177.