Spruce Broom Rust (Chrysomyxa arctostaphyli)

Compiled by Ned Tisserat, Colorado State University:

Host
Several species of spruce (Picea species): primarily affects Engelmann spruce (Picea engelmannii) and Colorado spruce (Picea pungens) in the [High Plains] region. Requires alternate host in the genus Arctostaphylos including kinnikinnick (bearberry) and manzanita.

Symptoms
This rust causes conspicuous, perennial witches’ brooms, which bear annual, yellow-green needles. Spermagonia appear in summer as small reddish eruptions that release a putrid odor detectable from several meters. Broom needles appear yellow-orange in mid-summer when spore (aecial) pustules are formed. These brooms may be mistaken for those caused by dwarf mistletoes; however, in spruce, only spruce broom rust causes a marked loss of chlorophyll and annual casting of all broom needles. On the lower surface of the leaves of the alternate host, telia appear as localized reddish-brown spots.

Impact of Disease
Damage caused as a result of this broom rust includes reduced growth, stem deformation, dead tops (spiketops), and tree mortality. Breakage may occur at the point of stem infections, which act as infection courts for decay fungi.

Life Cycle of the Pathogen
The fungus lives systemically and perennially in both hosts and may overwinter in either host. In spruce, 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 current year’s needles of true fir; the others are produced on the alternate host.