Canker girdling main stem of young tree
Steven Katovich, USDA Forest Service, Bugwood.org
White Pine Blister Rust
White pine blister rust, caused by the fungus Cronartium ribicola, was introduced into the U.S. around 1900 and has since spread throughout the range of white pine.
Keywords: fungus, Pinus strobus, spores, discolored bark, swelling, cankers, currant bush, gooseberry bush
|Distribution Map||Distribution Source Image|
White pine blister rust distribution
The disease intensity varies throughout the range but is normally most severe where late summers are cool and damp, conditions necessary for blister rust infection; thus, the farther north, the more blister rust. Symptoms vary with different stages of spore and canker development. The fungus has five spore forms in its life cycle: two (pycniospores and aeciospores) occur on pine, and the others (urediospores, teliospores, and basidiospores) occur on the alternate hosts, currant and gooseberry bushes. The fungus cannot spread from pine to pine. Basidiospores produced on the currant or gooseberry host during cool, wet weather in late summer and fall infect the white pine through stomata on needles or young stems. Initial infections cause patches of browning bark bordered by a yellowish discoloration the first year and a typical spindle-shaped swelling by the second year after. Cankers on the main stem will usually kill the tree above the canker. No control is possible. Cankers on the branches at least 4 in. from the main stem (trunk) should be removed from the tree to prevent the canker from reaching the main stem.