Conifer - Aspen rust
Damage, symptoms and biology
In natural forests, the rust causes only slight damage and tree mortality is rare, except during severe infections. A first type of spores is produced in the spring on tamarack or pine needles; these spores infect the leaves of poplar. During summer, golden-coloured pustules smaller than 1 millimeter form on the surface of poplar leaves and produce spores that spread the infection within the tree. In fall, a crusty structure ranging in colour from brown to black forms on the lower surface of the leaves. The spores produced by these structures over winter in the ground and, once spring arrives, induce new infections on host conifers. During the spring after a severe infection is initiated, bud break in affected poplars may be delayed or irregular.
Melampsora medusae can cause serious damage in nurseries. Infection rates can be reduced by removing infected trembling aspen growing in the vicinity.
In some cases, the fungus causes as much damage to its conifer hosts as it does to poplar trees. When planting poplars, be sure to choose rust-resistant varieties and plant the trees far away from pine and tamarack stands.
M. medusae f. sp. deltoïdes has recently been reported on black cottonwood and on many hybrid poplar clones in commercial plantations in Oregon, Washington and British Columbia. The presence of this form of the rust poses a serious threat to hybrid poplar plantations and may necessitate the replacement of susceptible clones.
Another poplar leaf rust, Melampsora occidentalis, infects the same conifer hosts but can be distinguished from M. medusae by the size of the aeciospores and the hosts of the teliospore stage. The aeciospores of M. medusae are shorter and infect only trembling aspen.