Conifer-poplar rust
- French disease name: Rouille des conifères et du peuplier de l'Ouest
- Other disease names: Conifer-cottonwood rust, leaf rust of cottonwood
- Pathogen name: Melampsora occidentalis Jacks.
- Kingdom: Fungi
- Phylum: Basidiomycota
- Class: Pucciniomycetes
- Order: Pucciniales
- Family: Melampsoraceae
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Partial list of synonyms:
- Caeoma occidentalis Arth.
General information and importance
Melampsora occidentalis is an obligate parasite, meaning it cannot survive without a living host. The rust alternates between a conifer host and a poplar host, most commonly a cottonwood species, to complete its complex life cycle. It produces five morphologically and pathologically different spore stages, which exclusively occur on either the conifer or poplar host. The rust causes minor to moderate defoliation of its poplar host. It can be very damaging to delicate conifer seedlings, especially those planted in bareroot nurseries close to cottonwood trees.
Distribution and hosts
The distribution of conifer-poplar rust follows its main uredinial host, black cottonwood (Populus trichocarpa), in western North America. It was also introduced to the central United States, likely by experimental black cottonwood plantations. At the periphery of the natural range of black cottonwood, balsam poplar (P. balsamifera) and narrowleaf cottonwood (P. angustifolia) have also been reported as hosts. Conifer species in the following genera that either occur naturally or are planted in the same areas as black cottonwood are aecial hosts: true fir (Abies), larch (Larix), spruce (Picea), pine (Pinus), Douglas-fir (Pseudotsuga menziesii) and Rocky Mountain Douglas-fir (P. menziesii var. glauca). In Canada, the pathogen is endemic in Alberta, British Columbia, Saskatchewan, and the Northwest Territories.
Tree parts affected
Infections are primarily foliar (leaves of poplar, current year needles of conifers), but tender young shoots of conifers (especially seedlings) can also be affected.
Symptoms and signs
The first signs of M. occidentalis occur in summer on poplar trees as small, rounded yellowish 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 broadly ellipsoid to oblong to pyriform (pear-shaped) and 30 to 50 micrometres × 17 to 30 micrometres in size. They have yellowish contents and lateral walls that are slightly thickened on opposite sides. Urediniospores are ornamented with minute spines over their entire surface, a feature that distinguishes M. occidentalis from M. medusae and its hybrids, whose urediniospores usually have bald spots. Interspersed among the urediniospores are capitate to club-shaped paraphyses (sterile hyaline hyphal structures with rounded apices). Telia are found on attached poplar 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 orange-brown as the leaves fall. Teliospores are prismatic (rectangular from the side, square from above) and 40 to 66 micrometres × 10 to 20 micrometres in size. They have thick-walled apices and are yellow-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, 26 to 35 micrometres x 22 to 27 micrometres in size, and have minutely warty spore walls that are thicker at the equator.
Most of the hybrid poplars commercially grown in western North America are resistant to M. occidentalis. However, this endemic rust has hybridized with the introduced M. medusae f. sp. deltoidis in western North American hybrid poplar plantations. The hybrid rust, Melampsora x columbiana, has morphological features that are intermediate between both parent rusts and is particularly pathogenic to hybrid crosses of P. trichocarpa and P. deltoides.
Disease cycle
Melampsora occidentalis 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 poplar leaves become windborne and infect nearby young conifer foliage in the spring. The rust colonizes the conifer needles and produces microscopic volcano-shaped spermogonia under the cuticle of the needle. 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 cannot re-infect their conifer host. If the aeciospores land on young poplar foliage, they will infect them. Uredinia resulting from the aeciospore infections are then produced on the poplar foliage in abundance, releasing large amounts of urediniospores throughout the summer. Urediniospores become wind-borne and re-infect poplar foliage. Continuous production of new urediniospores can result in high stand levels of foliar infection by the end of the growing season. At the end of the growing season, uredinial production slows down. Uredinia are gradually replaced by telia, which develop on the dying poplar 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 septate basidia and basidiospores.
Damage
The rust causes the most defoliation on cottonwoods 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 poplar (the alternate host) is nearby. Larch seedlings are the most susceptible to M. occidentalis, followed by Douglas-fir, pine, true fir, and spruce.
Prevention and management
The impact on the cottonwood hosts is minor to moderate in wet years and occurs primarily in natural forests, so no management for the rust on these hosts is warranted.
In conifer bareroot nurseries, disease incidence can be reduced by removal of nearby cottonwood trees. If that is not possible, the removal and destruction of fallen poplar 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 M. occidentalis 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
Yellow discoloration of leaves associated with uredinia of Melampsora occidentalis on cottonwood.
Brenda Callan
Melampsora occidentalis uredinia on cottonwood leaves.
CFS
Aecia and spermogonia of Melampsora occidentalis on Monterey pine seedling.
Wolf Ziller
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.
Newcombe G.; Stirling B.; McDonald S.; Bradshaw H.J. 2000. Melampsora x columbiana, a natural hybrid of M. medusae and M. occidentalis. Mycological Research 104(3): 261–274. https://doi.org/10.1017/S0953756299001665
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. 1955. Studies of western tree rusts. II. Melampsora occidentalis and M. albertensis, two needle rusts of Douglas-fir. Canadian Journal of Botany 33(2): 177–188. https://doi.org/10.1139/b55-013
Ziller, W.G. 1974. The tree rusts of western Canada. Environment Canada, Canadian Forestry Service. Victoria, British Columbia, Canada. Publication No. 1329. 272 p.