White Pine Blister Rust
Cronartium ribicola J.C. Fisch.

Hosts:  Southwestern white pine is the only species currently affected in the Southwest; limber pine and bristlecone pine are also susceptible. Currants and gooseberries (Ribes spp.) are alternate hosts.

Symptoms/Signs:  Tapered branch swellings or stem swellings (on young trees) are an early symptom on pine. The characteristic white blisters (aecia) appear on mature cankers in late spring. After the blisters disintegrate, cankers have a dark, roughened appearance. Flagging (i.e. recently killed branches with red needles) occurs several years after initial infection.

Biology:  Spores produced on Ribes leaves are wind dispersed and infect pine needles. The fungus grows into the inner bark, forming a canker that eventually girdles the branch or stem. Blisters erupt through the bark of the canker, releasing the spores that infect Ribes.

Effects:  This nonnative disease is one of the most damaging tree diseases in North America. Trees of all sizes can be affected, although smaller trees are killed more rapidly than larger trees.

Blister rust was found near Cloudcroft, New Mexico in 1990, and is now widespread throughout the Sacramento and adjoining White Mountains. It is most common on moist, mixed-conifer sites above 2,450 m. Blister rust also occurs in the nearby Capitan Mountains and on Gallinas Peak (Cibola National Forest, near Corona, NM). It was found for the first time on the Gila National Forest (5 km from the Arizona border) in 2005, and in several additional locations of the Gila in 2007. The disease was detected for the first time in nothern New Mexico (Santa Clara Pueblo) in 2006.

Similar Diseases:  Atropellis canker, which is fairly common on white pines in the Southwest, forms slight swellings with roughened bark. It produces small black fruiting bodies near the canker margin. Dwarf mistletoe causes branch swellings that can look remarkably similar to blister rust cankers. Sunscald and mechanical injuries can superficially resemble blister rust damage. Ribes leaves are often infected by other rusts, such as Coleosporium ribicola, that can be mistaken for white pine blister rust.

References:  26, 70, 92