Yellow witches' broom of balsam fir
Damage, symptoms and biology
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. A 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.
The disease may sometimes be present on a lot of balsam fir in some wood lots. The witches' broom deformity caused by the disease has fuelled a number of stories about woods being haunted by wicked witches. On a more serious note, pruning the witches' brooms on high-value ornamental trees can stop the disease from spreading and eliminate its effects. It is also important to remove all chickweed plants in the vicinity of these trees.
This fungus is systemic and perennial on both aecial and telial hosts. As a result, the rust may be found in areas where one of the hosts is not present. The brooming symptoms on true firs caused by M. caryophyllacearum could be confused with mistletoe infections or brooms caused by physiological abnormalities. However, the chlorotic needles in the summer and complete loss of needles in the winter are diagnostic features of the rust.
Canadian Forest Service Publications
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