Fir Broom Rust

The fungus causes witches' brooms, that is, an excessive proliferation of twigs from a single point on a branch. Fir broom rust is rarely fatal, but it can result in growth reduction. The fungus lives on two hosts: the primary host is balsam fir and the secondary host is chickweed. The first type of spores is produced in spring on the two faces of needles contained in the broom. Infected needles are stunted and yellowish-green in colour. The spores form on fruiting bodies that look like small, round orange-yellow blisters. In the middle of summer, a second type of spores is produced on the lower surface of the needles. These spores become windborne and spread the infection to chickweed hosts. During the summer, three new types of spores will be produced on this alternate host. The following spring, the third type of spores ends up infecting some balsam fir, thus spreading the disease. Although the infected needles are shed each year, the disease persists in the woody tissues of the witches' broom; hence, the new shoots become infected year after year.