Melampsora occidentalis Jacks.
Basidiomycotina, Uredinales, Melampsoraceae
Hosts: In B.C., the principal aecial hosts of Melampsora occidentalis are European, Japanese, and western larch, Douglas-fir, Sitka spruce, lodgepole, sugar, western white, ponderosa, and Monterey pine (in order of decreasing susceptibility). The telial hosts in B.C. are black cottonwood, balsam poplar, and some hybrid poplars. Telial hosts in other parts of North America include Fremont and lanceleaf cottonwoods, as well as Carolina poplar species. Conifer - Cottonwood Rust
Distribution: This fungus is widely distributed throughout the range of its hosts in B.C.
Identification: Conifer hosts are infected shortly after bud-break, and spermogonia and aecia appear within 2 weeks on slightly chlorotic needles (Fig. 60a). Needles become increasingly discoloured and shriveled as aeciospores mature, and soon die and are shed during the summer (Fig. 60b). Uredinia begin to appear on poplar leaves soon after aeciospore release, forming yellow leaf spots. (Figs. 60c, 60d) Urediniospores reinfect poplar throughout the summer and when the rust is severe, entire leaf surfaces become yellow. In late summer, brown-coloured telia form in the place of uredinia. Necrosis of surrounding leaf tissue often accompanies the formation of telia. In the spring, teliospores germinate to produce basidia and basidiospores, which infect conifer hosts.
Microscopic Characteristics: Spermagonia subcuticular, on current years needles. Aecia hypophyllous, round or ellipsoid, orange-yellow. Aeciospores with orange-yellow contents, broadly ellipsoid, slightly flattened laterally, 22-27 x 26-35 µm, walls hyaline, verrucose, 1.5-2.5 µm thick, flattened side walls 3-6 µm thick. Uredinia hypophyllous, round, orange-yellow when fresh. Urediniospores with orange-yellow contents, ellipsoid, oblong, or pyriform, 16-29 x 32-48 µm, wall laterally flattened, wall colourless, echinulate with or without smooth spots. Telia hypophyllous, small, round, waxy, cinnamon-brown. Teliospores prismatic, 10-20 x 40-64 µm, wall cinnamon-brown, 1-2 µm thick at sides, 3-5 µm thick and darker at apex.
Damage:Melampsora occidentalis, like the other Melampsora rusts, causes more serious damage to hardwoods than to its conifer hosts.
Remarks: The aecia and telia of Melampsora occidentalis are macroscopically similar to those of M. albertensis and M. medusa. However, the aeciospores and urediniospores of M. occidentalis are significantly larger.
Ziller, W. G. 1955. Studies of western tree rusts II. Melampsora occidentalis and M. albertensis, two needle rusts of Douglas-fir. Can J. Bot. 33:177-188.
Ziller, W. G. 1974. The tree rusts of western Canada. Can. For. Serv., Publ. No. 1329. Victoria, B.C.
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Figure 60a: Aecia and spermogonia of Melampsora occidentalis on Monterey pine seedling.
Figure 60b: Douglas-fir heavily infected by Melampsora occidentalis.
Figure 60c: Yellow discoloration of leaves associated with uredinia on cottonwood.
Figure 60d: M. occidentalis uredinia on cottonwood leaves.