White Pine Blister Rust (WPBR)

White pine blister rust is a deadly disease that affects the branches and trunks of white (5-needled) pine trees.  It is especially damaging in the Western U.S., where it kills rare and important pines in fragile mountain habitats. Fortunately, Nate Green is on the scene to protect the pines from this killer fungus.

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  • When white pine blister rust infects a branch, it often kills the phloem all the way around it, causing the needles to turn brown and the branch to die.  Photo:  Joseph O'Brien, USDA Forest Service, Bugwood.org

  • Pine branches infected with white pine blister rust (Cronartium ribicola) often become fatter, or swollen, where the fungus is causing damage.  The "blisters" caused by this fungus can also be seen here.  Photo: USDA Forest Service Archive, USDA Forest Service, Bugwood.org

  • Close-up of the blisters caused by white pine blister rust (Cronartium ribicola) on a pine branch.  These blisters break through the tree's bark when the fungus is ready to reproduce. The yellow parts in the blisters are the spores of the fungus.  Photo:  Joseph O'Brien, USDA Forest Service, Bugwood.org

  • This currant (Ribes sp.) leaf carrying the white pine blister rust fungus appears to be covered in rust!  Photo: Robert L. Anderson, USDA Forest Service, Bugwood.org

  • Close-up of the bottom side of a Ribes leaf infected with the white pine blister rust pathogen.  The hair-like orange structures in the picture release spores of the fungus, which can infect white pines.  Photo: USDA Forest Service, Wikimedia Commons, www.commons.wikimedia.org

  • Photos: A, B, C, D. Steven Katovich, USDA Forest Service, Bugwood.org  E. USDA Forest Service  F. Joseph O'Brien, USDA Forest Service, Bugwood.org  G. Robert L. Anderson, USDA Forest Service, Bugwood.org

  • White pine blister rust is a disease of white pines.  These trees can easily be identified because they hold their needles in groups of five.  Other pines, such as Virginia pine (top), have different numbers of needles per bundle. Photo: Annemarie Nagle, American Public Gardens Association.

  • Cones, seeds, and leaves of eastern white pine (Pinus strobus), the only species of pine in the eastern U.S. that suffers from white pine blister rust. Photo: Zelimir Borzan, University of Zagreb, Bugwood.org

  • Western white pines (Pinus monticola) are majestic trees that were once very important for timber.  Almost 90% of them outside of California have been killed by white pine blister rust.  Photo: Chris Schnepf, University of Idaho, Bugwood.org

  • Close-up of the cones and short needles of Pinus flexilis.  The common name is "limber pine" because the branches are so flexible they can be tied in knots! Unfortunately, this tree is very susceptible to white pine blister rust.  Photo: Dave Powell, USDA Forest Service, Bugwood.org

  • Leaves and fruit of a gooseberry currant (Ribes montigenum), an alternate host of white pine blister rust.  The fungus requires this plant or another alternate host to complete its lifecycle, but on Ribes it only infects the leaves, which does not harm the plant very much.  Photo: Dave Powell, USDA Forest Service, Bugwood.org

  • Gooseberry currants (Ribes montigenum), alternate hosts of white pine blister rust, are small to medium shrubs with red or purplish fruit.  Photo: Dave Powell, USDA Forest Service, Bugwood.org

  • Indian paintbrush (Castilleja sp.), a small flowering plant native to the U.S., was recently discovered to be an alternate host for the white pine blister rust fungus! Photo: Harlan B. Herbert, Bugwood.org

  • They might look pretty, but louseworts (Pedicularis spp.) were recently discovered to be alternate hosts for the white pine blister rust fungus!  Photo: Mary Ellen (Mel) Harte, Bugwood.org

  • An eastern white pine (Pinus strobus) infected with white pine blister rust.  Dead tops are common on infected trees because the pathogen can girdle the trunk, cutting off water and nutrients from the portion of the tree above the infection site.  Photo: Minnesota Department of Natural Resources Archive, Minnesota Department of Natural Resources, Bugwood.org

  • Many of these pines have been killed by white pine blister rust (Cronartium ribicola).  The reddish-brown, dead needles often caused by this disease can be seen here.  Photo: USDA Forest Service - Ogden Archive, USDA Forest Service, Bugwood.org 

  • White pine blister rust (Cronartium ribicola) has killed most of the pines on this hillside.  Photo: USDA Forest Service - Ogden Archive, USDA Forest Service, Bugwood.org 

  • Whitebark pine (Pinus albicaulis) is a very important 'keystone species' in the high mountains of the western U.S.  It may not be very abundant, but many animals and plants depend on it for survival.  This species is highly threatened by white pine blister rust and mountain pine beetle.  Photo: Dave Powell, USDA Forest Service, Bugwood.org 

  • Pine trees are very important parts of their ecosystems.  They provide food for many different animals, including red squirrels like this one.  Photo: Terry L Spivey, Terry Spivey Photography, Bugwood.org 

  • This is a Clark's nutcracker (Nucifraga columbiana).  They depend on seeds from the whitebark pine to survive, and bury large seed stashes in the ground!  The pines depend on this special bird to plant their seeds so new trees can grow.  Both whitebark pine trees and Clark's nutcrackers are in danger because of white pine blister rust!  Photo: Vern Wilkins, Indiana University, Bugwood.org 

  • In the high mountains of the western U.S., nutritious pine seeds are a very important food for grizzly bears. If white pine blister rust kills all the pine trees, what will the grizzlies eat?!  Photo: Terry L Spivey, Terry Spivey Photography, Bugwood.org 

  • Programs to eliminate alternate hosts of white pine blister rust (Ribes spp.) occurred in both the eastern and western U.S.  Here, workers from the North Carolina  Forest Service cut out Ribes shrubs in Mitchell County, 1974. Photo: North Carolina Forest Service Archive, Bugwood.org 

  • These pine seedlings are being tested for their ability to fight off white pine blister rust (Cronartium ribicola).  Scientists hope to find pines that can resist the fungus and plant these trees in the wild. Photo: Joseph O'Brien, USDA Forest Service, Bugwood.org

Scientific Name:Cronartium ribicola

 

Description:  White pine blister rust is a serious disease of pine trees that is caused by a fungus.  The fungus has a very complicated life cycle, and needs to infect a plant in the Ribes genus (gooseberry or currant) in order to mature and be able to make pine trees sick.

 

Native Range:  Central Siberia, near the Ural Mountains, and east into Asia.

 

Introduced Range:  WPBR is found throughout the northeastern U.S., the states around the Great Lakes, and throughout the pine-growing regions of the western U.S. It has also spread throughout southern Canada and Europe.

 

Distribution:

 

WPBR Distribution

 

Map Courtesy of U.S. Forest Service

 

Habitat: WPBR is found in natural forests, pine plantations, and nurseries.  It prefers to live in areas where summers and autumns are cool and moist.

 

Host Trees: WBPR can infect all North American white pines.  These are pine trees whose needles grow in groups of five.  In the eastern U.S, eastern white pine (Pinus strobus) can be infected.  In the western U.S, there are six species of pine that can be infected: whitebark pine (P. albicaulis), Rocky Mountain bristlecone pine (P. aristata), foxtail pine (P. balfouriana), limber pine (P. flexilis), sugar pine (P. lambertiana), and western white pine (P. monticola).

 

The Facts: Have you ever had a blister?  They are no fun, right?  Well, trees don’t like getting blisters either.  White pine blister rust is named for the bumps or “blisters” that the fungus forms under the bark on infected branches and trunks.  These blisters are filled with “rusty” colored spores, or fungal particles that allow this bad guy to move from one host species to another.  The spores are very tiny and easily carried in the air, where they float along until they land on an unsuspecting  currant or gooseberry plant (these plants are called ‘alternate hosts’ and are required for the fungus to complete its lifecycle).  The fungus can infect these plants, but only the leaves, which doesn’t make the plant very sick.  The problem, though, is that WPBR can make LOTS more spores on the currant leaves.  These spores float off in the air before the currants drop their leaves in the fall to—you guessed it—find some pine trees to infect!

 

When spores first land on pine trees, they cause little yellow spots on the pine needles.  The fungus grows from the needles and into the branch, where it sets up shop, causing the branch to swell up and killing the bark and phloem (the part of the tree that moves nutrients).  The dead area created by the fungus is called a ‘canker’.  The edges of the canker are where the whitish “blisters” form to spread the fungal spores.  If the fungus kills the bark all the way around the branch (called ‘girdling’), the needles on that branch all turn reddish-brown and die. Sometimes, the fungus moves from the branch and into the tree’s trunk.  It can girdle the trunk too, causing the top of the tree, or often the whole tree, to die.

 

WPBR was brought to North America around 1900 on eastern white pine seedlings (Pinus strobus) that were grown in Europe and shipped to the U.S. and Canada for planting during reforestation efforts.  In the humid forests of the eastern U.S., WPBR spread quickly.  Its spread has been slower and patchier in the harsh mountain environments in the Western U.S., but its impacts are much worse.

 

Because WPBR needs Ribes plants in order to cause pines to get sick, early control efforts focused on destroying all currants and gooseberries in pine forests.  These Ribes control efforts cost tons of money, had to cover thousands of acres of land, and required a lot of work.  In eastern forests, these efforts were somewhat effective, but unfortunately in western forests, they weren’t very good at preventing pines from getting sick.  Today, WPBR is managed by cutting off infected pine branches before the fungus reaches the trunk (in plantations), or by planting pine trees that are not harmed as badly by the fungus (these trees are said to be ‘resistant’).  

 

In the mountain regions of the western U.S., WPBR is a big problem, and threatens very special ecosystems.  The whitebark pine (Pinus albicaulis) grows where few other trees can in very cold, snowy, windy, and rocky places.  It provides lots of seeds for birds, grizzly bears, and squirrels, and is very easily killed by WPBR.  If whitebark pine is lost from these ecosystems, many other species will be threatened too (it is called a ‘keystone species’).  Fortunately, scientists are working hard to find resistant whitebark pines and save the beautiful Rocky Mountain forests!