White pine blister rust (caused
by the fungus Cronartium ribicola J. C. Fisch. ex Rabenh.) was
introduced into the United States about 1900 and has since spread throughout
the range of white pine. The disease intensity varies throughout the range but
is normally most severe where late summers (July-September) are cool (below
67º F) and damp, conditions necessary for blister rust infection. Thus,
the farther north, the more blister rust.
The fear of blister rust has
greatly limited the amount of white pine planted in the Lake States even though
there are many suitable areas in the southern parts of the region where white
pine can be grown with no significant mortality from blister rust. Red pine has
often been substituted for white pine in the Lake States, but red pine is now
experiencing many new and serious disease and insect problems of its own. Thus,
there is renewed interest in white pine with the hope of achieving a better
balance of conifer species in the Lake States that will help minimize disease
and insect losses. The development of blister-rust-resistant white pine, the
possible increase in natural resistance to the disease, the planting of white
pine in low blister rust hazard zones, and the pruning of blister rust cankers
in certain high quality white pine stands can help achieve this balance.
Toward this end, helps for identifying white pine blister rust and
suggestions for controlling it by canker pruning are presented here.