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United States

Department

of Agriculture

Forest Service

Intermountain

Research Station

Research Note

INT-406

August 1992

How To Recognize

Blister Rust Infection

On Whitebark Pine

Ray J. Hoff

White pine blister rust is a disease of five needle

pines. The fungus (Cronartium ribicola) infects

trees through the needles. It grows down the interior of the needle and into the stem, producing

fusiform cankers. Susceptible white pines that are

exposed to the disease become infected with one or

more cankers. Most die, with just a few remaining

canker-free.

The disease is native to Eurasia. It was introduced into western North America in 1910 near

Vancouver, BC. Foresters did not notice it until the

autumn of 1921. Within 25 years the disease had

expanded over most of the range of western white

pine (Pinus monticola), whitebark pine (P.

albicaulis), limber pine (P. flexilis) and sugar pine

(P. lambertiana).

The fungus needs two hosts. It is perennial in the

inner bark of white pines, and annual on the leaves

of currant and gooseberry bushes (Ribes spp.). The

fungus moves from currants and gooseberries to

pine by a very small spore, too small to be visible

without a microscope. These spores usually move

during the fall, or anytime during extended cool,

moist conditions. The fungus moves from the pine

to currants and gooseberries in the spring by

another type of spore that is small, but can be seen

with a hand lens.

The first blister rust canker recorded on

whitebark pine was in 1922 at the University of

British Columbia arboretum. Infection of natural

stands was soon observed. Foresters quickly realized that whitebark pine was several times more

susceptible to blister rust than western white pine.

Nonetheless, a few whitebark pine trees do not

become infected.

The following photos and descriptions of blister

rust on whitebark pine (figs. 1 through 19) are to

help identify the disease. Forest workers armed

with this knowledge can find canker-free trees to

start developing a new variety of whitebark pine

that is resistant to blister rust. At times some

resistance reactions can even be observed (figs. 18

and 19).

Abstract—Color photographs show how white pine blister

rust can be identified. In addition, the photographs show how

pines resistant to the fungus could be identified. Such trees

could be used to develop a new variety of whitebark pine that

is resistant to blister rust.

Keywords: disease resistance, Cronartium ribicola,

Pinus albicaulis, plant diseases, fungal

diseases

Ray J. Hoff is principal geneticist located at Intermountain

Research Stations Forestry Sciences Laboratory, Moscow, ID.
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Figures 1 and 2—These red-brown branches (flags) are evidence that blister rust is present.

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Figures 3 and 4—The first signs of a stem infection are these yellow-orange patches on the stem.

These infections are about 1 year old.

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1
3

Figure 5—(Above left) This canker is 4

to 5 years old. A yellow-orange margin

is visible at the base. The center is

rough and broken due to fruiting of the

fungus in previous years. The white

areas in the lower right side are

remnants of fruiting in the current year.

Figure 6—(Above) The fungus within

this canker has not fruited, so the bark

is still smooth. However, there is a hint

of yellow-orange color, especially in the

upper one-half of the canker. If this

portion were rubbed with a little water,

the color would become visible.

Figure 7—(Left) High amounts of

sugars are associated with blister rust

cankers. Ants, grasshoppers, mice, and

other rodents feed on these sweet

cankers.

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6
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Figure 8—The white material on this canker is

the remnant of fruiting during the current year.

The fruiting structures originate within the stem.

As they grow, the bark ruptures.

Figure 9—This is the lower part of the canker

in figure 8. The area has been rubbed with a

little water to reveal the yellow-orange

margin—a sure sign of a blister rust infection.

Figure 10—Two cankers are visible here.

The upper one is still living. The lower one

has caused the branch to die. As the fruiting

structures of the fungus grow, living cells

within the stem are broken, crushed, or dried.

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10 11

Figure 11—This canker has killed the branch,

which has become a “flag”. A rodent appears to

have made a meal of the sweet, succulent

tissue.
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Figure 12—(Above left) Fruiting lags a

year or two behind the growing margin

of the canker. The stem, or top of a

tree, does not die until the fungus has

grown and fruited completely around it.

Figure 13—(Above) This photo shows

the canker that killed the top of the tree

in figure 12. Typically, the dead bark is

still present and a lot of pitch is

associated with the dead portion of the

canker. Pitch usually drips and runs

down the stem.

Figure 14—(Left) The top of this young

tree has been killed by blister rust. A

large stem canker is still visible. Several

flagged branches and a couple of

branch cankers are visible on the

middle-right side of the tree.

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12 13
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Figure 17—Four dead cankers 17 are visible. Can you find them?

15 16

Figure 15—A fairly old canker. This canker

has fruited. However, it doesn't seem to be

very active. No yellow-orange margin is visible.

Figure 16—An old, dead canker.
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Figures 18 and 19—Cankers appear to have been suppressed by a defense reaction in the tree.

These two trees are resistant to blister rust. Living stem tissue in the area shown is dead. This

reaction is similar to that observed in western white pine. The death of host cells may be related

to the amount of fungus mycelium present, or to the physical environment. The tree builds a

wound periderm around the infection. The death of the pine cells and development of the

periderm starve the fungus.

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Intermountain Research Station

324 25th Street

Ogden, UT 84401

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