BLISTER RUST, or white pine blister rust, a destructive forest-tree disease among white (five-needle) pines so widespread in the United States at the present time that it threatens the preservation of forest reserves. It is caused by a parasitic fungus on the foliage of currant (particularly black currant ) and goose berry bushes, both wild and domestic. It attacks a white-pine tree through the needles or the young twigs ; then, growing into the inner bark, it kills the tree by girdling.
The presence of blister rust is detected by the appearance of orange-coloured blisters, which do not, however, appear until the spring of the third or fourth year of ter infection. They are about the size of a navy bean, and contain the spores or seeds of the disease. Being small and light in weight, they are often carried great distances by the wind, but the only place of germination is on the leaves of currant and gooseberry bushes. There they grow into the leaf tissue, producing currant rust, or the summer stage of the blister rust fungus. Currant rust is detected by the appearance of orange-coloured pustules containing spores on the under side of the leaf. The summer stage gives rise at last to spores which infect the winter host, i.e., the white pine, thus com pleting the life-cycle. The black currant, however, is a favourite "host" of the blister rust disease.
The disease is apparently of Asiatic origin, and was introduced into North America on white-pine planting stock imported from Europe. It was first discovered on the North American conti nent in 1906. The disease occurs throughout eastern United States south to Virginia, and in Wisconsin, Minnesota, Iowa, Montana, Idaho, Washington, Oregon, and California ; in Canada, the follow ing provinces : British Columbia, Ontario, Quebec, Prince Edward island, New Brunswick and Nova Scotia. In New England and New York, io% of the white pines over large areas are infected, and in some smaller areas the infection ranges from so to i00%.
As is evident, blister rust has spread with such rapidity and to such an extent that its eradication is now practically impossible. It can, however, be controlled. Since this forest-tree disease must first undergo an intermediate development on the leaves of cur rant or gooseberry bushes and does not pass directly from a diseased pine tree to a healthy one, or to any other plants, the growing currants and gooseberries must be considered in con nection with its control and the preservation of white pine forests. The value of the former as food products is of relatively small importance. The value of standing white pine in the United States is approximately $soo,000,000 ; in Canada, $600,000,000. The white pine is a great national asset, the preservation of which is essential to future forestry development. To this end, the U.S. Department of Agriculture at Washington, D.C., has declared the growing of currants and gooseberries a public menace and recommends their elimination. The following States have passed laws concerning currants and gooseberries in relation to blister rust control: Connecticut, Idaho, Maine, Massachusetts, Minne sota, New Hampshire, New York, Oregon, Pennsylvania, Rhode Island, Vermont, Washington and Wisconsin. There are also Fed eral and State quarantine regulations governing the movement of currant and gooseberry plants.
Sec U.S. Dept. Agr. Bulletin 957 (Washington, 1922) ; U.S. Dept. Agr. Farmers' Bulletin 1398 (Washington, 1924) ; U.S. Dept. Agr. Misc. Circular, 4o (Washington, 1925) .