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Forest Pathogens: Western Gall Rust

Western Gall Rust infection in Ponderosa Pine

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.  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 or sugar 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

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.

Control

Unfortunately there is no “silver bullet” that eradicates Western Gall Rust. However, there are management techniques that can control its spread.

The first line of defense is “sanitary salvage” in which infected trees are removed from the stand. Salvage is a hunt-and peck method of logging that takes out the dead and dying trees over the landscape to improve forest health and is well-suited to removing those infected with WGR.

The second management option is pruning and thinning the stand. This is more labor-intensive than salvage logging, but depending on the situation it can be effective in controlling the pathogen. Care has to be taken in timing the operation correctly, because of the potential of spreading the pathogen during its spore releasing phase.

For high-value stands such as test plots used in scientific studies, a systemic fungicide can be used to minimize the effects of gall on the plantation. Because of the high cost, this method is used sparingly and only on selected stands.

Finally, as often the case, prevention is the best cure. Planting multiple species is likely the best way to prevent the spread of WGR in newly-planted stands. Soper-Wheeler often plants up to five species in new units in order to add biodiversity and pathogen resistance to the forest. The most common species we plant are Ponderosa Pine, Sugar Pine, Douglas Fir, White Fir, and Incense Cedar.

Click on photos to enlarge.

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