Sweetfern Blister Rust

Cronartium comptoniae Arthur

Basidiomycotina, Uredinales, Cronartiaceae

Hosts: Cronartium comptoniae is restricted to pines, specifically the two- and three-needle, hard pines. In B.C. it is found on lodgepole, ponderosa, and jack pine in natural forests, and on any of the introduced hard pines including bishop, mugo, Monterey, Scots, and Austrian pines. The alternate (telial) host present in B.C. is sweet gale (Fig. 33a); elsewhere in North America, sweet fern is a host.

Distribution: This fungus has been found throughout the province but is restricted to sites where both pine and sweet gale are present. Sweet gale is found in moist habitats, mainly at low elevations.

Identification: Sweetfern rust first appears as a fusiform swelling on stems and branches of the pine host. The swellings are generally elongate (up to four times longer than broad), diamond-shaped, and often girdle smaller stems and branches. Bark tissue may be thickened resulting in fusiform swelling. During late spring and summer, aecia appear as white-orange blisters that release orange aeciospores (Fig. 33b). During other times of the year, C. comptoniae infections may be identified by their size, shape, and by sunken, dead bark and resinosis associated with the cankers. Porcupines and other rodents often preferentially chew on cankers during the winter, leaving exposed wood.

Microscopic Characteristics: Spermatia and aecia on pine caulicolous, spermatia ellipsoid. Aecial filaments on inner surface of peridium continuous (non-stalactiform), extending from the top to the base of the peridia. Aeciospores orange, short-ellipsoid, 16-24 x 24-3 m, coarsely verrucose with a conspicuous smooth spot and warts up to 3 µm high (Fig. 33c).

Urediniospores on Myrica gale oval-obovate, 16-21 x 23-31 µm, orange-yellow, sparsely echinulate. Teliospores oblong, 13-17 x 28-56 µm, walls faintly coloured, smooth.

Damage:Very little damage has been documented in natural stands, but severe losses have occurred in plantations of susceptible pines located near swampy habitats of sweet fern. Rust cankers are reported as entry points for decay fungi that cause further damage to infected trees.

Remarks:Cronartium comptoniae can be distinguished from C. comandrae by its ellipsoid aeciospores (cf. pear-shaped aeciospores of C. comandrae). Differentiating between C. comptoniae and C. coleosporioides is more difficult. The aecial filaments of C. comptoniae are continuous, whereas those of C. coleosporioides are pendant. In addition, longitudinal hypertrophied ridges produced by C. comptoniae (Fig. 33d) are not produced on stems infected with C. coleosporioides. Identifying the presence of the alternate hosts will also help distinguish between these two rusts.


Anderson, G. W. 1970. Sweetfern rust on hard pines. USDA For. Serv., Pest Leaf. No. 79. Washington, D.C.

Peterson, R. S. and F. F. Jewell. 1968. Status of American stem rusts of pine. Ann. Rev. Phytopathol. 6: 23-40.

Ziller, W. G. 1974. The tree rusts of western Canada. Can. For. Serv., Publ. No. 1329. Victoria, B.C.


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Figure 33a: Myrica gale, the telial host of C. comptoniae.







Figure 33b: Aecia of Cronartium comptoniae on the main stem and branches of Monterey pine.




Figure 33c: Aeciospores of C. comptoniae.







Figure 33d: Hypertrophied ridges characteristic of C. comptoniae cankers.