Western Gall Rust
Basidiomycotina, Uredinales, Cronartiaceae
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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
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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.
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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.
References
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.
http://www.fs.fed.us/r6/nr/fid/widweb/wid-rust.shtml
http://www.nrc.ca/cisti/journals/tcjpp/k01-030.html