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WHITE
PINE BLISTER RUST
(Cronartium ribicola)
Hosts:
Western white pine, whitebark pine
Quick ID:
- Branch
flagging
- Swellings
and blisters on branches; also on trunk of small trees
- Diamond-shaped
cankers on trunks
- Cankers
with greenish-yellow to orange margins
- Pustules
of orange spores on cankers and branch swellings
- Resin flow
from branch and trunk on advanced infections
 Field
Identification
Tree:
This disease does not affect species outside of the white (5-needle)
pine group (similar symptoms on lodgepole or ponderosa pine may
be caused by western gall rust). The foliage may have red or yellow
needle spots. On the branches, spindle-shaped swellings or cankers
with small cup-like indentations are frequent. Often infections
kill the branch, resulting in bright red "flags" in the
crown. On the trunk, a diamond-shaped canker with dead roughened
bark and greenish-yellow to orange margins develops.
Fungus:
A honey-colored ooze (pycnia) or pustules of powdery orange
spores (aecia) develop on the swellings or cankers in the spring.
As the disease develops, cankers will have dead centers, surrounded
by pustules of aecia, surrounded by drops of pycnia, and with a
yellowish zone outermost. The canker grows in size every year.
May be
confused with: Resin flow at the base from Armillaria root rot.
Disease
cycle: This disease is the most serious pest of 5-needle pines
in the Pacific Northwest. The rust has a five-spore cycle that requires
an alternate host, members of the Ribes family. These include species
of gooseberry and currant. On the pine in spring, pycnial spores
give rise to aecial spores, which may fly 400 to 800 miles to infect
the leaves of Ribes plants. On the plant, two spore stages follow,
then give rise to another airborne spore stage which infects the
pine again. These spores infect needles, then grow down to the twig
and from there to the branch. Branch infections within 4" of
the trunk will spread to the trunk in time. Once the trunk is infected
and a canker has developed death of the tree is inevitable, though
there is some genetic variability in rate of canker growth and some
trees have survived many years before being girdled and killed.
Once a tree gets beyond 20-30 years of age, the disease most frequently
causes only branch flagging.
Predisposing
agents: Cool, moist weather late in the summer or early fall
is required for infection of the pine host, with 48 hours of temperatures
not above 68F.
Impact:
White pine blister rust was introduced in the east from Europe between
1898 and 1908, although the disease is believed to have originated
in Asia. In 1910 it was introduced into British Columbia and has
since spread through the West. In North America as a whole, it has
caused more damage and more money has been spent to control it than
any other conifer disease. Thousands of white pine stands have been
seriously damaged and many have been entirely lost. In the Pacific
Northwest, losses are estimated at 5 million cubic feet annually.
Management:
Historically, control efforts have focused upon removing the alternate
host, members of the Ribes family, from stands of white pine. However,
this never proved to be very effective, despite the thousands of
hours of work that were invested. Likewise, effective chemical controls
have never been developed. As blister rust is an introduced species,
rather than one that evolved here, genetic resistance is limited
in the white pines. For this reason the mortality from this disease
has been extremely high. However, some naturally resistant white
pines were found and breeding programs to bulk up the seed have
been in place for some time. Resistant seedlings are finding their
way onto the market. When cutting in infected stands, any tree that
is uninfected or only lightly infected should be retained as a seed
tree, as it may be naturally resistant and pass
that resistance along. On young ornamental trees, branches with
infections may be pruned to arrest the spread of the disease. The
pruned branches should be subsequently burned.
Blister rust
resistant western white pine is a good choice for replanting root
disease centers and for incorporation in mixed-species stands. However,
it probably should not be planted in pure stands.
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