Conifer-aspen rust
- French disease name: Rouille des feuilles du peuplier
- Pathogen name: Melampsora medusae f. sp. tremuloidis Shain
- Kingdom: Fungi
- Phylum: Basidiomycota
- Class: Pucciniomycetes
- Order: Pucciniales
- Family: Melampsoraceae
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Partial list of synonyms:
- Caeoma faulliana Hunter
- Melampsora albertensis Arthur
General information and importance
Melampsora medusae forma specialis tremuloidis is an obligate parasite, meaning it cannot survive without a living host. The rust alternates between a conifer host and an aspen host to complete its complex life cycle, producing five morphologically and pathologically different spore stages. The rust causes minor to moderate defoliation of its aspen (Populus) hosts. It can be very damaging to delicate conifer seedlings, especially those planted in bareroot nurseries close to aspen trees.
Distribution and hosts
Conifer-aspen rust occurs in all forested provinces and territories, following the natural distribution of both its aspen host species. The main aecial hosts are seedlings and saplings of larch (Larix), pine (Pinus), and Douglas-fir (Pseudotsuga menziesii), as they are most susceptible to this rust. The main uredinial hosts are trembling aspen (Populus tremuloides) and largetooth aspen (P. grandidentata).
Tree parts affected
Infections are primarily foliar (leaves of aspen, current year needles of conifers), but tender young shoots of conifers (especially seedlings) can also be affected.
Symptoms and signs
The first signs of Melampsora medusae f. sp. tremuloidis occur in summer on trembling aspen and largetooth aspen as small, rounded yellowish-orange uredinial pustules on the undersurfaces of infected leaves. They may initially appear as powdery dots on the leaves, less than 1 millimetre in diameter. As the season progresses, the entire undersurface of the leaf may appear orange. Heavy infection levels cause the foliage to yellow and prematurely drop. The uredinia contain masses of urediniospores that are ellipsoid to obovoid (slightly tapered at one end, like an egg) and 15 to 23 micrometres × 23 to 35 micrometres in size. They have yellowish-orange contents and lateral walls that are slightly thickened on opposite sides. Urediniospores are ornamented with minute spines, except for an equatorial smooth area (also called a bald spot). Interspersed among the urediniospores are capitate paraphyses (sterile hyaline hyphal structures with rounded apices). Telia are found on attached aspen leaves near the end of the growing season and on fallen leaves. They are waxy in appearance, developing under the epidermis on the lower side of the leaf. They are orange at first and become reddish brown as the leaves fall. Teliospores are prismatic (rectangular from the side, square from above), 20 to 45 micrometres × 10 to 15 micrometres in size, thick-walled, and brown. In the spring, as the conifer host needles emerge from buds and begin to expand, the teliospores germinate and form a septate basidium bearing four basidiospores.
Young needles on conifers infected with basidiospores develop microscopic volcano-shaped spermogonia under the cuticle of the needle. Shortly thereafter, aecia start to develop on the undersurfaces of needles. Aecia are round to oval, yellow, and 0.5 to 1 millimetre in diameter. Aeciospores are produced in chains in the aecia. They are globose, 19 to 26 micrometres × 16 to 21 micrometres in size and have minutely spined spore walls that are thicker at the equator.
In eastern Canada, a second rust species, Melampsora abietis-canadensis (Farl.) C.A. Ludw., also occurs on largetooth aspen, producing the same symptoms and signs on its foliage. M. abietis-canadensis alternates between poplars and eastern hemlock (Tsuga canadensis). On aspen, the urediniospores of the latter species are ellipsoid to globose, and sometimes slightly flattened along the long axis. They have sparse, fine warts and are smaller, measuring 16 to 24 micrometres × 13 to 18 micrometres in size. The aecial stage is restricted to eastern hemlock. In this stage, the rust attacks needles, green cones, and tender, young shoots.
Melampsora medusae f. sp. deltoidis infects conifers, eastern cottonwood (Populus deltoides), and other poplars that are intersterile with aspen. It is likely that these two formae speciales are in fact separate species. Some references use the name Melampsora albertensis for the conifer-aspen rust for this reason.
Disease cycle
Melampsora medusae f. sp. tremuloidis is an obligate parasite (requires a living host to grow and reproduce). It is also macrocyclic (has five different spore types: spermatia, aeciospores, urediniospores, teliospores, and basidiospores), and heteroecious (requires alternation between two unrelated host species at different stages of its life cycle to fully complete it). Basidiospores from overwintered telia on the fallen aspen leaves become windborne and infect nearby young conifer foliage in the spring. The rust colonizes the conifer needles and produces spermogonia. The spermogonia exude spermatia. Sexual recombination (mating) occurs when spermatia ooze out of spermogonia in a sweet, sticky liquid and are spread (usually by mites or insects) to receptive hyphae on another nearby spermogonium. The resulting union produces aecial pustules about 2 weeks after the initial basidiospore infection. The aecia erupt from the epidermis and release aeciospores, which become windborne and infect young aspen foliage. Aeciospores cannot re-infect their conifer host. Aeciospores can only cause infections on young aspen foliage.
Uredinia resulting from the aeciospore infections are then produced on the aspen foliage in abundance, releasing large amounts of urediniospores throughout the summer. Urediniospores become windborne and cause new infections on aspen foliage. Uredinial spore production continues throughout the growing season and greatly increases disease levels on aspen. At the end of the growing season, uredinial production slows down. Uredinia are gradually replaced by telia, which develop on the dying aspen leaves. The teliospores are tightly packed in the telia. They are not a source of spread. In the spring, they germinate on fallen leaves to produce basidia and basidiospores.
Damage
Conifer-aspen rust causes low to moderate defoliation on its aspen host. The rust causes the most damage in natural forests in years with mild spring temperatures and higher than average rainfall. Severe damage to very young (1- to 2-year-old) conifer seedlings can occur in reforestation plantations and bareroot nurseries located in forested areas where aspen (the alternate host) is present.
Prevention and management
The impact on the aspen hosts is relatively minor and occurs primarily in natural forests, so no management for the rust on this host is warranted. In conifer bareroot nurseries, disease incidence can be reduced by removal of nearby aspen trees. If that is not possible, the removal and destruction of fallen aspen leaves will also work. This will break the disease cycle by greatly reducing basidiospore production from the overwintering teliospores on the dead leaves.
Pest management strategies for a particular pest vary depending on several factors. These include:
- the population level of the pest (i.e., how numerous the pest is on the affected host[s]);
- the expected damage or other negative consequences of the pest’s activity (either to the host, property, or the environment);
- an understanding of the pest’s life cycle, its various life stages, and the various natural or abiotic agents that affect population levels;
- how many individual host specimens are affected (an individual tree, small groups of trees, plantations, forests);
- the value of the host(s) versus the costs of pest management approaches; and
- consideration of the various silvicultural, mechanical, chemical, biological, and natural control approaches available and their various advantages and disadvantages.
Decisions about pest management strategies require information about each of these factors for informed decision-making. These various factors should then be weighed carefully in terms of costs and benefits before action is taken against a particular pest.
Pesticides registered for use against Melampsora medusae f. sp. tremuloidis under specific situations may change from year to year. Therefore, please search Health Canada’s Pesticide Product Information Database for currently registered pesticides and product information for use against this pathogen. The application of any registered product should be based on population size and applied only when necessary and against the approved disease stage. It is also recommended to consult a local tree care professional. Pesticides may be toxic to humans, animals, birds, fish, and other beneficial insects. Apply registered products only as necessary and follow all directions and precautions noted on the manufacturer’s label. In some jurisdictions and situations, only a licensed professional can apply pesticides. Consulting relevant local authorities to determine local regulations that are in place is recommended.
Photos
Yellowish-orange uredinial pustules of Melampsora medusae f. sp. tremuloidis measuring less than 1 millimetre in diameter on the undersurface of an infected aspen leaf.
Aspen leaves with uredinia (fruiting bodies) of Melampsora medusae f. sp. tremuloidis.
Robert Blais
The undersurface of an aspen leaf with uredinia (fruiting bodies) of Melampsora medusae f. sp. tremuloidis.
André Carpentier
Aspen leaves with uredinia (fruiting bodies) of Melampsora medusae f. sp. tremuloidis.
RT
Melampsora medusae f. sp. tremuloidis aecia on lodgepole pine needles.
FIDS
Discoloured foliage on Douglas-fir infected with Melampsora medusae f. sp. tremuloidis.
Leo Unger
Uredinia of Melampsora medusae f. sp. tremuloidis on aspen leaves.
Leo Unger
Selected references
Allen, E.A; Morrison, D.J.; Wallis, G.W. 1996. Common tree diseases of British Columbia. Natural Resources Canada, Canadian Forest Service, Pacific Forestry Centre. Victoria, British Columbia. 178 p.
Callan, B.E. 1998. Diseases of Populus in British Columbia: a diagnostic manual. Natural Resources Canada, Canadian Forest Service, Pacific Forestry Centre. Victoria, British Columbia. 157 p.
Hamelin, R.C.; Shain, L.; Thielges, B.A. 1992. Influence of leaf wetness, temperature, and rain on poplar leaf rust epidemics. Canadian Journal of Forest Research 22(9): 1249–1254. https://doi.org/10.1139/x92-166
Vialle, A.; Frey, P.; Hambleton, S; Bernier, L.; Hamelin, R. 2011. Poplar rust systematics and refinement of Melampsora species delineation. Fungal Diversity 50: 227–248. https://doi.org/10.1007/s13225-011-0129-6
Ziller, W.G. 1965. Studies of western tree rusts VI. The aecial host ranges of Melampsora albertensis, M. medusae, and M. occidentalis. Canadian Journal of Botany 43(2): 217–230. https://doi.org/10.1139/b65-026
Ziller, W.G. 1974. The tree rusts of western Canada. Environment Canada, Canadian Forestry Service. Victoria, British Columbia, Canada. Publication No. 1329. 272 p.