White pine blister rust is an exotic, invasive disease of five-needle white pines. The disease causes cankers that usually kill the stem above the canker and often lead to tree mortality. Trees weakened by blister rust become susceptible to other damage, like a bark beetle infestation.
Asexual spore of white pine blister rust on Ribes x nidogrolaria.Bruce Watt, University of Maine,
bugwood.org Blister rust is a fungus is native to Asia and was introduced to the eastern and western coasts, of North America around the turn of the 20th century, first arriving on infected white pine seedlings grown in Europe. The disease continues to spread into new communities of white pine species causing substantial damage and mortality.
Symptoms of white pine blister rust infection may be difficult to pinpoint at first. The disease may simply appear as small yellow or red spots on a few needles. But usually within a few years, cankers will appear on the branches. These cankers will generally appear as a swollen area that may have a greenish - yellow to orange margin. As the cankers mature they will girdle the branch or stem causing death of living tissue beyond that point. The distinctive orange blisters that give the disease its name are visible in later spring and early summer.
THE GUIDE Follow these tips to prevent white pine blister rust in your operation.
Know your hosts. White pine blister rust requires an alternate host to complete its life cycle. The fungus is sometimes carried from infected deciduous shrubs, like currants and gooseberries. Pines are infected by spores produced on the alternate host. Damage is minimal on the host shrubs, but includes leaf spots and premature defoliation.
Protect the saplings. Young white pines, both saplings and pole size trees, are more severely infected and damaged than older trees for several reasons. On young trees the branches are closer to the ground, where conditions are more favorable for infection, and the needles are closer to the main trunk so that less time is needed for infections to get into the main stems and kill the trees. Infected branches do not require special disposal treatment as the pathogen will not survive in dead wood.
Reduce moisture on white pine needles. Remove lower branches gradually as trees mature. Young trees will need to be pruned slowly over the years, never removing more than 1/3 of the canopy at a time. When planting white pine trees, space plants to promote good air movement around the trees.
The facts about white pine blister rust 1. Rust signs; 2. Blisters become visible in late spring;3. Symptoms on jostaberry; 4. Signs of rust on Eastern white pine; 5. Symptoms of rust on Eastern white pine; 6. White pine blister rust on jostaberry1,3: Watt. 2: Robert L. Anderson, USDA Forest Service,
Bugwood.org ; 4, 6: Watt; 5: Minnesota Dept. of natural resources,
bugwood.org SYMPTOMS General symptoms may appear slightly different on each of the high elevation species, but include:
Branch swelling – Small branches produce a spindle-shaped swelling with a new infection. Branch flagging – Branches die distal to cankers; needles turn orange and fall off. Orange blisters – Blisters of orange spores on cankers (in spring) and branch swellings. Cankers – Diamond-shaped cankers on trunks; greenish-yellow to orange margin (mature). Resin flow – Branches and trunk on advanced infections can have streams of resin, killing the portion of the tree above the canker. Chewing – Rodents often gnaw the bark around cankers. LOCATIONS Since its introduction to the East and West coasts of the U.S. around 1900, the disease has spread to 38 states and Canada.
HOSTS Susceptible species include Eastern white pine, limber pine, whitebark pine, bristlecone pine, Southern white pine, foxtail pine, red, white and black currant, gooseberry.
LIFE CYCLE The fungus completes two of its five spore stages in the bark of pines and three stages on the leaves of certain plants of the genus Ribes (gooseberries and currants).
DAMAGE Spores from infected Ribes spp. are carried to white pine trees on cool moist air currents in late summer or fall. These spores infect pine needles if moisture is present. The fungus kills the needle and moves into the shoot or branch, traveling about 3 inches a year as the infection progresses towards the main trunk.
Once the fungus reaches the branch, a canker is formed. The canker will girdle the branch and the infection will continue down into the main trunk. Seedlings and small trees are in great danger of dying from this disease when a canker girdles the main stem.
In the first summer after infection of the pine tree, gummy orange droplets full of fungal spores may be seen on branch cankers. The second spring after infection, white blister-like structures form at the edge of the canker. These fungal structures, called aecia, crack open to release powdery yellow orange spores called aeciospores. These spores can be carried long distances on wind currents to infect Ribes, causing yellow leaf spots and sometimes leaf loss. Just two weeks after infection, the white pine blister rust fungus creates a new type of spore on the lower surface of infected leaves. These spores can only infect Ribes leaves, and their production results in new leaf spots within the plant canopy and in neighboring plants. When days begin to shorten and temperatures drop, the white pine blister rust fungus produces short hair-like structures on the lower surface of infected Ribes leaves, called telia. Telia produce yet another type of spore that is windborne to nearby white pines.
CONTROL Examine white pines each year for blister rust flags and cankers. Prune off branches with cankers at a branch union or where the branch meets the trunk. Remove at least four inches of healthy wood beyond the visible symptoms of disease.
RESISTANCE The disease arrived in the U.S. from European tree nurseries, but in Europe, pines were resistant to blister rust. U.S. pines are developing a resistance to the disease. Trees that demonstrated resistance to the rust in areas with severe rust infections have been selected as parent trees for disease resistance breeding programs. A number of rust-resistant lines have been developed. These trees are now producing seed for general distribution. There are also resistant Ribes spp. cultivars. Ask your area’s extension agent for a list.
Sources: USDA U.S. Forest Service, University of Minnesota Extension
