Fir Broom Rust
Melampsorella caryophyllacearum J. Schröt

Hosts: White and subalpine fir

Spruce Broom Rust
Chrysomyxa arctostaphyli Dietel

Hosts: Engelmann and blue spruce

Symptoms/Signs:  Both of these diseases appear very similar on their respective hosts. The yellow to pale green brooms are dense and compact. Stem or branch swellings may also occur near the point of infection.

Biology:  Windblown spores produced on an alternate host are needed to start new infections on trees. Chickweeds (Cerastium and Stellaria spp.) are alternate hosts for fir broom rust, while bearberry or kinnikinnick (Arctostaphylos uva-ursi) is the alternate host for spruce broom rust. Once a tree is infected, the fungus stimulates bud formation, leading to broom development. The brooms shed their needles in the winter and grow new ones in the spring.

Effects:  Broom rusts can be found throughout much of the Southwest on their respective hosts. They typically occur at low levels, but are abundant in some locations. Infection typically results in deformity, which is most significant on young trees. Stem infections sometimes result in topkill and/or stem breakage.

Similar Diseases:  Broom rusts are sometimes mistaken for dwarf mistletoe witches’ brooms. However, the former are more dense and compact, and lack mistletoe shoots. Dwarf mistletoes of true firs and spruces have very limited distributions in the Southwest.

References:  92, 116