Rust diseases are caused by fungi in the Basidiomycetes, and are obligate parasites of leaves, bark, flowers. The most serious rusts in timber production terms are the stem rusts.

Stem Rusts

Cronartium coleosporioides Stalactiform blister rust

Host: lodgepole pine, Indian Paint brush

Damage: causes long sunken, resinous lesions, easily confused with Atropellis canker from a distance. Can cause mortality and can grow from branch to stem.
Note: Cronartium rust cankers are often found in association with the Western pine moth(Dioryctria cambiicola) and the Sequoia pitch moth (Synanthedon sequoiae.
C. comandrae Commandra blister rust

Host: lodgepole pine, Bastard Toad flax

Damage: causes shorter, wider cankers than stalactiform rust, otherwise is very similar.
C. comptoniae Comptonia rust

Host: lodgepole pine, Myrica gale

Damage: more frequently a branch rust, but can be similar to above.
Cronartium ribicola White pine blister rust

Host: soft, or 5-needle pines

Damage: introduced from Asia, has eliminated white pine as a commercially viable species in North America.
Endocronartium harknessii Western gall rust

Host: lodgepole pine

Damage: No alternate host, causes woody galls on branches, stem infections must have originated on 1-year-old stem, cannot grow from branch to stem. Can be a sight for breakage, decay.

Needle Rusts

Melampsora occidentalis Melampsora rust

Host: Douglas-fir (aecial), cottonwood (uredial)

Damage: little on D-fir, some loss of photosynthetic capacity on cottonwood.
M. medusae melampsora rust

Host: Douglas-fir (aecial), aspen (uredial)

Damage: as above
Pucciniastrum epilobii fir-fireweed rust

Host: True firs (aecial), fireweed (uredial)

Damage: loss of photosynthetic area with fir.
Chrysomyxa ledicola Large-Spored Spruce-Labrador Tea Rust

Host: aecial hosts: white, black, Sitka, and Englemann spruce. telial hosts: Labrador-tea and northern Labrador-tea
Damage: Severe defoliation of spruce in localized area. Enhanced by moist environmental conditions.

Broom Rusts

Chrysomyxa arctostaphylli spruce broom rust

Host: Spruce (aecial), Arctostaphylos uva-ursi (uredial)

Damage: unsightly brooming, little damage caused otherwise
Melampsora caryophyllacearum fir broom rust

Host: true firs (aecial), chickweed (uredial)

Damage: can cause a decrease in growth of fir if severe enough.

Cone Rusts

Chrysomyxa pirolata C. monesis spruce cone rusts

Host: spruce, wintergreen (Pirola)

Damage: can destroy cone crops in seed orchards.

Diagrammatic representation of infection structures produced by rust fungi during the invasion of host plant leaves.