White pine blister rust
White pine blister rust, caused by the fungus Cronartium ribicola, occurs on both eastern white pine (Pinus strobus) and currants or gooseberries (Ribes spp.). This fungus was introduced into North America on white pine seedlings from Europe in the 1890's. The complicated life cycle of white pine blister rust involves five reproductive (spore) stages, two on the white pine host and three on the alternate host, currant or gooseberry (Ribes spp.).
Spores produced on the undersides of infected Ribes leaves are wind blown to white pine needles during cool, wet periods from July to October. During the next twelve to eighteen months, the fungus grows within the needle, into the twig, and enters the branch. A canker, swollen resinous lesion, develops and continues to grow as long as the branch remains alive. Yellowed needles on infected branches are observed early in the infection period. If a canker girdles a branch, the movement of water and nutrients is blocked and the branch dies from that point outward.
White pine blister rust
Photo: Robert Blanchette
The first reproductive structures, pycnia, appear on infected pine branches 2 to 4 years later. Pycnia exude sticky drops of yellow fluid. The next reproductive structures, aecia, develop in the same region in the spring 3-6 years after initial infection. Aecia are pale yellow to bright orange blisters. Aecia produce spores which are wind blown to the alternate hosts.
Aeciospores infect the underside of currant or gooseberry leaves where the third type of reproductive structures, uredia, develop. Spores produced by uredia can infect more Ribes leaves throughout the season, thus increasing the number of infections. During late summer and early fall, brown hair-like reproductive structures called telia develop near the uredia. Teliospores germinate and form another reproductive structure called basidia. Basidiospores are wind blown to white pine from July to October, causing new infections.
There are no fungicides available for prevention or treatment of white pine blister rust. Cultural control methods should include proper pruning and inspection of pine trees. The majority of infections occur on pines within a few feet of the ground. Less air movement and higher relative humidity in the lower story facilitate spore germination and infection. As a preventive measure, prune lower branches of young white pines so that the lower third of the tree is free of branches and prune large trees up to the nine-foot level. Inspection of pine trees should be performed each spring. The most commonly observed symptoms are branches with red-brown needles, swollen resinous lesions centered around a dead branch stub, or yellow to orange blisters on branches or the trunk. Infected branches should be removed to prevent infections from reaching the main stem. If an infection reaches the trunk, it will eventually kill the tree.
Removal of nearby Ribes may reduce the incidence of disease on pine trees, but is not usually feasible. However, it is important to be sure that Ribes plants are completely removed including the roots. All native Ribes spp. are susceptible to white pine blister rust, but some of the introduced and cultivated species are resistant. Resistant white pine may be an option in the future.
Chad Behrendt, Crystal Floyd