Stem and Cone Rusts of Pine
FIELD GUIDE TO INSECTS AND DISEASES OF ARIZONA AND NEW MEXICO FORESTS

Fir Broom Rust
Melampsorella caryophyllacearum J. Schröt

Hosts: White and subalpine fir

Spruce Broom Rust
Chrysomyxa arctostaphyli Dietel

Hosts: Engelmann and blue spruce

Figure 231. Yellow to pale-green witches' brooms are conspicuous even at the forest level.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.

Figure 232. Expanding foliage is pale green.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

Figure 233. Foliage yellows in late summer.
Figure 234. Needles die and drop, leaving the broom devoid of foliage during winter.
Figure 235. Chickweeds are alternate hosts for fir broom rust.
Figure 236. Topkill caused by spruce broom rust.
Figure 237. Infected needles release spores in mid to late summer.
Figure 238. Kinnikinnick is the alternate host to spruce broom rust.