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Conifer - Aspen Rust
Melampsora albertensis Arth.
Basidiomycotina, Uredinales, Melampsoraceae
Hosts: In B.C., the principal aecial hosts of Melampsora albertensis are Douglas-fir and larch, but the rust has also been shown to infect species of pine, fir, spruce, and hemlock (in order of decreasing susceptibility). Inoculation studies by Ziller (1965) demonstrated the broad conifer host range of this rust. The telial host in B.C. is trembling aspen, and in other parts of North America, eastern cottonwood.
Distribution: This fungus is widely distributed throughout B.C. where both telial and aecial hosts are present.
Identification: Conifer hosts are infected shortly after bud-break, and spermogonia and aecia appear within 2 weeks on slightly chlorotic needles (Figs. 58a, 58b). Needles become increasingly discoloured and shriveled as aeciospores mature, and soon die and are shed during the summer. Uredinia begin to appear on aspen leaves soon after aeciospore release, forming yellow leaf spots (Fig. 58c). Urediniospores reinfect aspen throughout the summer and when the rust is severe, entire leaf surfaces become yellow (Fig. 58d). In late summer, brown-coloured telia form in the place of uredinia. Necrosis of surrounding leaf tissue often accompanies the formation of telia.
Microscopic Characteristics: Spermogonia subcuticular, on all surfaces of current years needles, inconspicuous. Aecia mainly on lower surfaces of needles, round or oblong, yellow. Aeciospores with orange-yellow contents, globoid, minutely verrucose, 16-21 x 19-26 µm. Uredinia mainly on lower surfaces of leaves, orange-yellow when fresh. Urediniospores with orange-yellow contents, ellipsoid or ovoid, laterally flattened, with both smooth and verrucose sides, 15-23 x 23-35 µm. Telia on lower leaf surfaces, teliospores cinnamon-brown, smooth, 10-15 x 29-45 µm.
Damage: Melampsora albertensis can cause considerable damage to conifer hosts, particularly in nursery settings. Disease levels can be reduced if infected aspen is removed from the vicinity. Recently M. medusae f. sp. deltoides was reported on cottonwood and many hybrid poplar clones in commercial plantations in Oregon, Washington, and B.C. The presence of this pathogenic form of the rust presents a serious threat to hybrid poplar plantations and may necessitate the replacement of susceptible clones.
Remarks: There is some question as to whether M. medusae Thuem and M. albertensis are the same species. Because of the similarities in life-cycle and host preference, they are considered together in this book. Another poplar leaf rust, Melampsora occidentalis, infects similar conifer hosts but differs from M. albertensis in aeciospore size and telial hosts. Melampsora albertensis aeciospores are shorter and they infect only aspen.
References:
Ziller, W. G. 1974. The tree rusts of western Canada. Can. For. Serv., Publ. No. 1329. Victoria, B.C.
Figures
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Figure 58a: Discoloured foliage on Douglas-fir infected with Melampsora albertensis.
Figure 58b: Melampsora albertensis aecia on lodgepole pine needles
Figure 58c: Uredinia on aspen leaves.
Figure 58d: Uredinia on aspen leaves