Hosts
Affects many of species of Abies, the true firs, including subalpine fir and white fir. Requires alternate hosts in the family Caryophyllaceae including chickweed, sandwort, and starwort.

Diagnosis and Damage
This rust causes upright, compact witches’ brooms that bear annual, yellow needles. These brooms may be mistaken for those caused by dwarf mistletoes; however, in true firs, only fir broom rust causes a marked loss of chlorophyll and annual casting of all broom needles. Infected branches and stems become swollen at the base of a broom into a spindle-shaped or nearly spherical gall. The bark on old swellings usually dies and becomes cracked, and open cankers may develop. On the leaves of the alternate host, yellow-orange spores (urediniospores and teliospores) are produced.

Damage caused as a result of this broom rust includes reduced growth, dead tops above the infection and mortality particularly in the seedling and sapling stages. Breakage may occur at the point of stem infections. Trees can survive for many years with broom rust infections.

Biology and Disease Cycle
The fungus lives systemically and perennially in both hosts and may overwinter in either host. In fir, buds and emerging twigs are infected in the spring by spores produced on the alternate host, and the fungus invades the young shoots and induces the formation of witches’ brooms. The rust produces five types of spores of which two, spermatia and aeciospores, are produced on true fir; the others are produced on the alternate host. Infection of fir requires moist and temperate weather conditions and synchrony between spore (basidiospore) release and fir shoot development.