Forest Health
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 listers 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 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 Bighorn 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.
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.
Pine Beetle - A Disturbance in the Forest
Ponderosa pine is likely the first tree species visitors are likely to encounter upon entering the Bighorn National Forest. This stately conifer is widely distributed throughout the west and grows in nearly pure stands along the eastern flanks of the Bighorn Mountains.
Mountain pine beetle is a widespread and important insect in various pine forests throughout the Rocky Mountains. Adult beetles attack trees in July and August, when they bore through the bark into the phloem region, or inner bark. The females build a network of pathways in the phloem, called galleries, where they lay their eggs. In addition to depositing eggs, the beetles carry a fungus that causes a blue-gray staining of the wood. The combined action of the beetle galleries and the bluestain fungi inhibits the flow of water in the tree. The eggs quickly hatch, and the larvae overwinter in the trees, then mature and fly from their home tree the following summer to commence another cycle.
An excellent opportunity to witness ponderosa pine forest dynamics lies a short distance from Buffalo, Wyoming. Driving west on Highway 16, the climb up the mountain begins as the short and mid-grasses of the Great Plains fade in the rearview mirror and are quickly replaced by ponderosa pine forest.