White pine blister rust
White pine blister rust (WPBR) is a devastating disease of five-needled pines and the only rust fungus that attacks the stems of white pine in North America. This disease can kill branches, upper shoots, creates stem cankers and can result in eventual death of a tree. White pine blister rust can be found throughout Minnesota but infection of white pine trees is most common in northern and eastern Minnesota where cool moist conditions in late summer favor infection.
The fungus Cronartium ribicola.
Eastern white pine (Pinus strobus) and other five-needle pines
Currants and gooseberries (Ribes spp.), indian paintbrush (Castilleja spp.), and lousewort (Pedicularis spp.)
Eastern white pine (Pinus strobus)
- All needles on one or more individual branches first turn yellow then rusty red.
- The branch with the dead needles will have a canker, a swollen area with discolored and cracked bark.
- Cankers on the main trunk are oval or diamond-shaped; often with a dead branch in the center.
- Sticky clear to white sap exudes from the canker and drips from the branch or runs down the trunk.
- In spring, white to yellow blisters form at the edge of the canker and release powdery orange spores.
- Gummy orange droplets containing spores may be seen along the canker in summer.
Ribes plants (Red, white and black currant, gooseberry)
- Angular yellow leaf spots that are contained by leaf veins can be seen on the upper leaf surface.
- Raised orange pustules on the underside of the leaf spot.
- By late summer or early fall, orange or brown hair-like tendrils form amongst the orange pustules on the lower surface of the leaf.
- Severely infected leaves or leaves on highly susceptible cultivars may fall off during the growing season.
The white pine blister rust fungus, Cronartium ribicola, needs to infect both white pine and a Ribes spp. to complete its lifecycle. 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, travelling 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. Girdling stem cankers on older trees result in top-killing and the death of branches but this is usually not life-threatening.
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 spp.
Aeciospores infect the leaves of Ribes spp. 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, called a urediniospore on the lower surface of infected leaves. Urediniospores 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, known as a basidiospore. Basidiospores are somewhat fragile and need cool moist air currents to carry them to nearby white pines. Basidiospores of white pine blister rust only infect five-needle pines.
'Paton's Silver Splendor' is a variety of eastern white pine that is resistant to White Pine Blister Rust. This variety was released by the University of Minnesota and has been available in nurseries for home landscapes since 2011. It is hardy to zone 3, distinctly upright and pyramidal when young. The tree has a silvery appearance, a fast growth rate and is 100 ft tall and 35 ft wide at maturity.
Many cultivars of gooseberry and currant with resistance to white pine blister rust are now available (Table 1). Resistance may be complete and no symptoms of disease develop, or resistant plants may become infected but disease remains very minor and does not progress as rapidly as in susceptible cultivars.
Table 1. Ribes spp. with resistance to white pine blister rust
|Red & white currant
Ribes sativum or Ribes rubrum
Ribes uva-crispa or Ribes hirtellum
|Black currant x gooseberry
R. x Nidigrolaria
Doch Siberyachki (Daughter of Siberia)
- 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 4 inches of healthy wood beyond the visible symptoms of disease.
- Infected branches do not require any specially 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. At least 9 feet of trunk should be visible between the ground and lowest branches of mature trees.
- When planting white pine trees, space plants to promote good air movement around the trees.
- Redirect lawn sprinklers or irrigation systems to avoid wetting pine needles.
- Do not plant eastern white pine and currants or gooseberry together in the landscape unless resistant cultivars are used.
- Avoid planting white pine in low lying areas or cold pockets.
- Plant new white pine trees underneath the canopy of older trees, but allow enough light for growth of the planted trees. The combination of shade and some sunlight will shelter the planted pine from the evening dew and help to promote growth of the newly planted trees.