Cronartium ribicola

Cronartium ribicola (White Pine Blister Rust) is a species of rust fungi in the family Cronartiaceae.

It is a heteroecious rust with two alternating hosts, a white pine (Pinus subgenus Strobus, family Pinaceae) and a currant (Ribes, family Grossulariaceae), and five spore stages. It is native to Europe and Asia, where the European and Asian white pines (e.g. Macedonian Pine, Swiss Pine, Blue Pine) are mostly resistant to the disease, having evolved alongside it.

It was introduced accidentally into North America in 1909, where it is an invasive species causing serious damage to the American white pines, which have little or no resistance to it. 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; research 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.