White pine blister rust—Cronartium ribicola
The fungus can infect all species of white (five-needle) pines and usually kills infected trees. Hosts include foxtail, limber, singleleaf pinyon, sugar, western white, and whitebark pines.
Identification
Initial symptoms include tiny, yellow spots and yellow mottling on pine needles, which later die and remain hanging on the tree. Infected branches swell into spindle-shaped galls. Infected bark develops raised blisters and becomes rough and cankered. A honey-colored ooze, then pustules of powdery orange spores develop on cankers in the spring.
Infections are most common near the ground where humidity is highest. At sites with more fog, more of the cankers are higher in trees.
On the pathogen's alternate hosts, currant and gooseberry (Ribes spp.), the fungus causes slightly raised, yellowish spots on the underside of leaves and young stems.
The fungus needs two hosts to survive, spending part of its life on five-needled pines and the other on Ribes. Viable spores that infect pines originate from the alternate hosts, Ribes.
Trees are infected through a windborne spore landing on a water droplet on a live needle fascicle, or group of pine needles. The spore germinates and infects the tree by growing through the base of needles to the inner bark. Once in the stem or branch, the pathogen causes canker formation, and eventually girdling, which kills the plant parts outward or upward from the canker.
Damage
Infected pine foliage turns yellow, then brown and dies and remains hanging on the tree. Diseased tissue increasingly encircles (girdles) the infected limb or trunk and eventually kills it.
Leaves of infected Ribes may drop prematurely. Ribes generally tolerate this damage.
Solutions
To avoid white pine blister rust, plant resistant or nonhost pine species. Cutting off infected limbs may increase tree longevity. Promptly remove seriously diseased or hazardous trees. Do not move Cronartium-infected pines or wood off-site because some parts of California are still free of this introduced pathogen.
Removing alternate hosts of the fungus (Ribes spp.) within about 500 feet of high-value white pines may help to reduce infections if trees are relatively isolated from other white pines. Ribes removal is generally not recommended because of the extensive labor required. It is difficult to completely eliminate all the alternate host plants, and Ribes regrow rapidly from seed when the soil is disturbed mechanically or after fire.
Fungicides have not been found to be effective in managing this disease. Consult the California Forest Insect and Disease Training Manual and White Pine Blister Rust in California for more information.
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