White pine blister rust, caused by _Cronartium ribicola_, was introduced
to North America on nursery stock about 1900. It is the most important
disease on white pine in the United States. In the South, the disease is
found on eastern white pine in the Appalachian mountains.
[Illustration: Fruiting cankers showing yellow-colored spores of
blister rust on eastern white pine.]
[Illustration: (cont.)]
The disease is caused by a fungus that attacks both white pine and wild
and cultivated currant and gooseberry bushes, called _Ribes_. Both hosts
must be present if the fungus is to complete its life cycle. Attack by
the disease is followed by the development of cankers on the main stem
or branches. Infected pines die when a canker completely girdles the
main stem or when many of the branches are killed by girdling. The most
conspicuous symptoms of the disease are the dying branches or crowns
(“flags”) above the girdling cankers, and the cankers themselves.
Initially, a narrow band of yellow-orange bark marks the edges of the
canker. Inside this band are small irregular dark brown scars. As the
canker grows, the margin and bank of dark scars expand and the portion
formerly occupied by the dark scars is now the area where the spores
that infect _Ribes_ are produced. During the months of April through
June white sacs or blisters containing orange-yellow spores (called
aeciospores) push through the diseased bark. The blisters soon rupture
and the orange-yellow spores are wind-dispersed for great distances.
Generally, there is some tissue swelling associated with the canker,
which results in a spindle-shaped swelling around the infected portion
of the stem.
Loss of white pines from blister rust can be prevented by destroying the
wild and cultivated _Ribes_ bushes. Bushes may be removed by uprooting
by hand, grubbing with a hand tool, or with herbicides. Pruning infected
branches on young trees will prevent stem infections and probably tree
mortality.