Find a Forest (NF)
or Grassland (NG)
USDA Forest Service
Bighorn National Forest
2013 Eastside 2nd Street
Sheridan, WY 82801
307-674-2600
Telephone for the
Hearing Impaired
307-674-2604
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White Pine Blister Rust
Motorists traveling along US Highway
16 in Tensleep Canyon may have noticed clusters of dying reddish or brown
colored pine trees. These trees have been infected with a disease known
as White Pine Blister Rust (WPBR). White pine blister rust, caused by
the fungus Cronartium ribicola, is a serious problem in northern states.
Needles of WPBR infected trees may have red or yellow spots. Infections
can kill the branch, resulting in bright red "flags" in the
crown, or top, of the tree.
This disease, which was introduced into the United States in 1910 from Asia, is
the most serious pest of 5-needle pines (such as western white pine, limber
pine, and whitebark pine). On the Bighorn National Forest, large stands
of limber pine trees have been infected with the fungus. This rust fungus
has a five-spore cycle that requires an alternate host, members of the
Ribes family, which includes species of gooseberry and currant plants.
The disease cannot spread from pine to pine, but is transmitted to pine
by spores produced on Ribes leaves. Pines are infected through their needles.
After a three or more year incubation period in the pines, spores are
disseminated by wind and infect Ribes plants. The fungus grows into the
wood, causing cankers (areas of dead bark) in the pines. The fungus continues to develop
on the bark, eventually forming “blisters”, which break open and release
spores. The bark becomes discolored and turns yellowish at the canker
margins.
Cankers will kill a tree if they grow into the trunk. If branches contain
cankers about four inches from the trunk, the tree will eventually die. Pruning
out limbs with cankers more than four inches from the trunk may protect a
tree. Cankers are most easily seen in the spring when orange-yellow pustules
protrude from the bark of the cankers. In late spring and early summer,
these areas will form blisters containing a sticky, yellow-orange fluid
that later turn dark and hard.
The severity of the disease is related to weather conditions. Cool, wet
weather with ambient temperatures averaging below 67 degrees fahrenheit from July
to September favors infection of pine by the rust fungus.
There are about 15,000 acres of limber pine on the Bighorn National Forest. WPBR was
known to be in the Tongue River drainage as early as the 1960s. Based
on aerial surveys, WPBR occurs at generally low levels across the Bighorn
National Forest, although it is increasing to epidemic levels in certain
areas, such as Tensleep Canyon, Shell Canyon, and the Red Grade road.
In addition, the stress caused by WPBR makes limber pine more susceptible
to attack from the mountain pine beetle, as the trees lack the energy
reserves to thwart their attacks. This has been made worse by the current
drought, causing increased mortality. Forest visitors may also see infected
branches with the bark stripped off; this can be caused by porcupines
and other rodents, which find the young cankers especially desirable.
Treatment of this disease has not been effective, as there are no fungicides
available for prevention or treatment of WPBR. The Forest Service conducted
intensive Ribes eradication efforts in Idaho in the 1940s and 1950s,
sending work crews through white pine stands to dig up or pull Ribes plants.
This proved unsuccessful. Current efforts are focusing on developing genetically
resistant strains of the 5-needle pines. The development of blister-rust-resistant
pines, the possible increase in natural resistance to the disease, the
planting of 5-needle pines in low blister rust hazard zones, and pruning
of blister rust cankers in certain high quality 5-needle pine stands can
help achieve a balance of conifer species on the forest.
If WPBR follows the pattern established in the Pacific Northwest and Lake
States with infected 5-needle pines, it would be feasible to estimate
that 75% of the native limber pine on the Big Horns would die as a result
of this pathogen. This is a significant concern to foresters as limber
pine often grows on sites that are too harsh for other plants, and there
may be no surrogate for them on these sites. The Bighorn National Forest
has only a relatively small proportion of the limber pine in the Big Horn
Mountain Eco-range (most is on BLM, private, tribal and state lands),
yet they have begun to collect seed from trees that have demonstrated
some resistance to WPBR for later sowing and out planting.
White pine blister rust causing limber pine
mortality in Tensleep Canyon.
Photo taken October 2001.
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The infection spreads down the needle
and into the twig, where slight swelling and yellowing develops.
Numerous pale yellow blisters (called aecia) as large as 6 mm (1/4
inch) across break through the infected bark in mid-April to mid-May
a year or more after the bark first becomes infected. These blisters
rupture and release large numbers of dry, yellow-orange spores. |
The above map shows the distribution of limber pine on the Bighorn National
Forest. It is located almost exclusively on sedimentary substrates.
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