Fir broom Rust (Yellow Witches' Broom of Fir)

Taxonomy

Domain	Eukarya
Kingdom	Fungi
Phylum	Basidiomycota
Subphylum	Pucciniomycotina
Class	Pucciniomycetes
Order	Pucciniales
Family	Pucciniastraceae
Genus	Melampsorella

Scientific Name

Melampsorella caryophyllacearum

Common Name

fir broom rust

Compiled by Ned Tisserat, Colorado State University:

Melampsorella caryophyllacearum

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.

Management

Trees bearing stem cankers or brooms may be removed in pre-commercial and commercial thinnings and selective harvests. Brooms may be pruned out of trees in order to reduce risk of breakage and maintain tree vigor in areas of intensive management such as recreation areas and Christmas tree plantations.

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