Cronartium ribicola

Cronartium ribicola is a species of rust fungi in the family Cronartiaceae that causes the disease white pine blister rust.

Like many other rusts, C. ribicola is heteroecious, meaning it requires two host species to complete its lifecycle that includes five spore stages. The aecial hosts are white pines (Pinus subgenus Strobus, family Pinaceae) and the telial hosts include currants (Ribes, family Grossulariaceae), and two genera of the Orobanchaceae, Pedicularis and Castilleja. Species of both telial and aecial hosts have varying levels of resistance or immunity to infection. This rust is believed to be native to Asia or Europe and was subsequently introduced to North America. Some European and Asian white pines (e.g. Macedonian Pine, Swiss Pine, Blue Pine) are mostly resistant to the disease, having co-evolved with the pathogen.

It was accidentally introduced into North America about 1900, where it is an invasive species causing serious damage to the American white pines, which have little genetic resistance. Mortality is particularly heavy in Western White Pine, Sugar Pine, Limber Pine and Whitebark Pine. Efforts are under way to select and breed the rare resistant individuals of these species; resistance breeding is concentrated at the United States Forest Service Dorena Genetic Resource Center in Oregon.

Some limited silvicultural control of the disease is possible. If bark blisters are found on branches over 10-15 cm from the bole, those branches may be pruned off, which will stop the spread of the disease to the rest of that tree. If the main trunk is affected then no control is possible, and the tree will die once the infection girdles the tree. Infected trees are often identified by "flagging", when all the needles on a branch turn brown and die. Infections often occur on low branches close to the ground on young trees, so pruning of white pine can also be effective in multiple ways, as it improves the quality of timber by creating more knot-free timber, and reduces the likelihood of infection from the blister rust to a small extent. Another form of control practiced in some areas is to diligently remove Ribes plants from any area near white pines. Because the infection moves from currant plants, to pines, and back again, it cannot continue to exist without its alternate host. Although effective in theory, removal of currants is rarely successful in practice, as they readily re-grow from small pieces of root left in the soil, and the seeds are very widely spread in birds' droppings.

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