Western Gall Rust
Basidiomycotina, Uredinales, Cronartiaceae

Western Gall Rust - Fruiting Bodies
Image Source: James W. Byler, USDA Forest Service

Pest Status
Western gall rust (WGR) is most destructive in tree farms, plantations and nurseries. WGR can directly and more rapidly infect because it has no alternate host. The aeciospores clearly can reinfect pine. There are no telia or basidia spores. Conditions optimal for infection occur every several years, resulting in "wave years" of infection and gall formation. Airborne spores infect the succulent tissue of young shoots and cause the wood to swell into galls (globe-shaped clumps). WGR is a disease that occurs on 2-and 3-needle pine trees. This disease does not affect 5-needle pines such as western white pine. WGR is found in eastern North America as well, the common names "pine-to-pine gall rust", "globose gall rust" or Woodgate gall rust are sometimes used.

Primary Hosts and Distribution

Western Gall Rust - Branch Gall
Image Source: Susan K. Hagle, USDA Forest Service

WGR is widespread throughout the Pacific Northwest affecting susceptible trees throughout their range. And despite its name, it does occur in the East too. In the Pacific Northwest, it is found on lodgepole, ponderosa, and jack pine in natural forests, and on any of the introduced hard pines including bishop, mugo, Monterey, Scots, Austrian pines.

Life Cycle
WGR has a two-year life cycle, from infection to the time when spores are first produced. Galls are generally globose, they may be asymmetrical and are sometimes deeply fissured. In May-July, climate dependant, orange spores form in blisters beneath the bark of the galls. Spores become airborne and infect new shoots. Small galls are visible on branches about 1.5 to 2 years later and are able to produce spores. Galls continue to grow and release spores each spring until they girdle the host stem or branch. Infection occurs through the succulent tissue of elongating shoots, so all galls are initially formed on one-year-old growth. Moist conditions promote spore release and infection. Galls continue to increase in diameter as the host tree grows, and typically reach sizes of 5-10 cm in diameter (although larger galls sometimes develop on main stems). The fungus and the tree may survive for 200 years. Galls become inactive with the death of the branch or stem, or are often killed by hyperparasitic fungi, but the woody swellings remain on the tree.

Detection and Damage
Look for the following signs and symptoms:
Globe-shaped perennial galls but sometimes very young galls can be spindle-shaped. Galls of this disease are woody and are perennial, producing spore blisters each year until the death of the tree.
Orange-yellow spores become exposed as the bark falls off.

Management and Impact
Practically all of the lodgepole pine stands in the Pacific Northwest are infected to some degree. WGR is impossible to eradicate, as its spores may spread for miles and directly infect other pines. Remove trees showing numerous galls, especially stem galls. During thinning operations, favor lightly infected trees without stem galls, and if it's practical, prune off branch galls on "leave" trees. Severe infection causes stem malformation (and loss of value), breakage, and tree killing, particularly of seedlings. Because of the hazard associated with wind breakage, watch for large stem infections on pines growing near structures and, if desired, prune off galls to increase vigor and improve form. Loss in cubic feet per year is not known.

Western Gall Rust on Ponderosa Pine
Image Source: Linda Haugen, USDA Forest Service

Biological Control
Scytalidium uredinicola is a destructive hyperparasite of WGR, which reduces the inoculum potential. This mycoparasite breaks down spores and the active rust sori of WGR, making the infected areas appear clumpy and stringy. Infected sori are a yellowish-green to whitish-gray in appearance, depending on the stage of infection. Hyphae of the mycoparasite are able to penetrate the wood tissue of the galls and destroy rust hyphae to a depth of 300 um below the sori. Infections begin early in the season and are found in the developing sorus under the peridium, thus providing control before spores are released. S. uredinicola may be able to completely deactivate older galls once an infection is established. The difficulty in promoting the natural control of WGR by S. uredinicola lies in efficient means of application for the biological control agent. Often mycoparasites, despite having considerable potential as biocontrol agents for plant pathogens, have not been successfully implemented in field situations, which is likely a failure in application.

Dissemination of the mycoparasite Scytalidium uredinicola
In western North America, there were 78 species of invertebrates collected from WGR. It was found that Epuraea obliquus (Coleoptera: Nitidulidae) was the most abundant invertebrate species on gall rust, with much of its life cycle occurring on the gall during sporulation. Since S. uredinicola is also present on the sporulating tissue during the same period, it has a close temporal association with the beetle. It has been established that viable spores of this mycoparasite can be cultured from the beetle’s frass. The close association between these two combined with the abundance of the beetle on WGR makes the beetle and important vector for the mycoparasite.

Currie, C.R., and Hiratsuka, Y. 1996. Evaluating insect-mediated dispersal of Scytalidium uredinicola for the biological control of western gall rust. Can. J. For. Res. 26: 1754-1760.
Moltzan, B.D., and Blenis, P.V. 1999. Effects of gall age, gall size, and rust severity on the incidence of the mycoparasite Scytalidium uredinicola. Can. J. For. Res. 29: 319-343.