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.
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.