Fir broom rust

Written by B.E. Callan Revision 2024

• French disease name: Rouille-balai de sorcière du sapin
• Other disease names: Yellow witches' broom of balsam fir
• Pathogen name: Melampsorella elatina (Alb. & Schwein.) Arthur
• Kingdom: Fungi
• Phylum: Basidiomycota
• Class: Pucciniomycetes
• Order: Pucciniales
• Family: Pucciniastraceae
• Partial list of synonyms:
  • Aecidium elatinum Alb. & Schwein.
  • Melampsora cerastii (Pers.) G. Winter
  • Melampsorella caryophyllacearum J. Schröt.
  • Melampsoridium caryophyllacearum (J. Schröt.) Blanchette & Biggs

General information and importance

Fir broom rust is caused by the fungus Melampsorella elatina. This rust species causes perennial infections in both true firs (Abies) and in an alternate host, chickweed (Cerastium) and its related genera. Fir infections cause upright witches’ brooms that look like small yellowish trees on lateral branches. Perennial species of chickweed that become systemically infected by the rust are slightly stunted and yellowed. Their new shoots bear telia each spring before uredinia develop, which is an unusual feature of this rust. Apart from a few reports of light damage on fir forests in western provinces, fir broom rust generally does not cause significant losses to fir in Canada. Farther south in the western United States (Rocky Mountains and Great Basin), and in central Europe, it can cause severe timber loss. This timber loss is due to cull and mortality caused by the formation of dozens of brooms produced on nearly every tree.

Distribution and hosts

Endemic to North America, M. elatina is found in every Canadian province and territory where the host fir trees occur. Its distribution is widespread throughout the Northern Hemisphere where any of its hosts grow, including Alaska and the contiguous United States (except for southeastern states), Europe, Russia, China, and Japan. It has also been reported in South America (Chile, Argentina, and Falkland Islands). On its conifer host, the rust only occurs on true fir species. In Canada, it has been reported on amabilis fir (A. amabilis), balsam fir (A. balsamea), grand fir (A. grandis), and subalpine fir (A. lasiocarpa). Elsewhere in North America, fir hosts include white fir (A. concolor) and red fir (A. magnifica). In Europe, primary fir hosts are European silver fir (A. alba), Nordmann fir (A. nordmanniana), and Spanish fir (A. pinsapo). In Russia and Asia, Siberian fir (A. sibirica) is a common host.

The alternate hosts for M. elatina are species of chickweed, sandwort (Arenaria and other related genera in Europe), and starwort (Stellaria), all small herbaceous plants in the carnation family (Caryophyllaceae). Canadian host species include alpine chickweed (C. alpinum), field chickweed (C. arvense), Bering Sea chickweed (C. berringianum), common mouse-ear chickweed (C. fontanum subsp. vulgare [=C. vulgatum]), northern starwort (Stellaria calycantha), crisp starwort (S. crispa), long-stalked starwort (S. longipes), and common chickweed (S. media). Many other chickweed and related alternate host species are present in the United States and elsewhere in the world. The rust is frequently observed on its alternate hosts in areas where fir is not present (for example, South America). 

Tree parts affected

Systemic, perennial infections of branches and twigs (witches’ brooms) and needle blight of fir of all ages, and systemic perennial infections of chickweed plants causing leaf, shoot, and flower blight.

Symptoms and signs

The large, dense, yellow, upright, perennial, bushy witches’ brooms on branches or trunks are the most obvious symptom of the rust on its fir hosts. Brooms attain an average of about 1 metre in diameter but may grow to twice that size. The brooms are either rounded in appearance or look like small upright trees perched on lateral branches. They are completely defoliated during winter. In areas with heavy levels of infection, dozens of brooms can be found on a single tree. Infected shoots are smaller than healthy ones. They are upright and bear paler, shorter, and thicker needles. In mid-summer, the infected needles become covered with pinkish orange aecial pustules, which release dusty masses of orange spores before they are cast. The infected needles are cast in the same year they are produced, unlike healthy needles, which remain on branches for 2 or more years. The witches’ broom symptoms are similar in appearance to spruce broom rust during the summer months. However, they differ in their host specificity for fir, the yellowing (chlorosis) of the needles, and the complete annual casting of all fir needles on the broom. At the base of the brooms, the supporting branch or trunk becomes swollen, forming a gall that varies in shape from fusiform to nearly spherical. The bark covering older infected branches may become cracked and cankered. 

Spermogonia develop on both sides of needles, forming tiny subcuticular, orange hemispherical pustules less than 0.5 millimetres in diameter. Spermatia are released through irregular ruptures of the overlying cuticle. They are unicellular, colourless, oblong in shape, and range from 2 to 3.5 micrometres × 4 to 5 micrometres. Aecia are formed on the underside of all needles in the broom in two rows, one on either side of the midrib. The aecia are yellowish orange, tongue shaped to cylindrical, and measure up to 2 millimetres in length. They are covered by a thin white membrane that ruptures to release the aeciospores. Aeciospores form in orange-yellow chains in the aecia. Aeciospores have orange-yellow contents. They are unicellular and ellipsoid. They are 14 to 18 micrometres wide × 16 to 28 micrometres long and are ornamented with warts that are 1.5 to 2 micrometres long. 

Symptoms on the alternate chickweed (and related genera) hosts include yellowing of foliage and orange yellow rust pustules. Uredinia and telia develop on the shoots and all leaf surfaces. Uredinia are orange to pale yellow, hemispherical, and up to 0.4 millimetres in diameter. They are covered with a thin periderm that opens with a central pore. Urediniospores have orange-yellow cell contents. They are ornamented with small spines (echinulate) and are 12 to 18 micrometres wide × 16 to 30 micrometres long.

Telia form in the leaf epidermis and remain embedded in the leaf, producing teliospores in a single layer. The layer is one cell deep and often occurs in extensive patches that discolour the leaf. Teliospores are single-celled, smooth-walled, and colourless to pale yellow. They are short-cylindrical to polygonal in shape and are 13 to 20 micrometres wide.

Infections on perennial alternate hosts become systemic throughout the plant. New growth in subsequent years is slightly stunted and chlorotic (yellowish) compared to that of healthy plants.

Witches’ brooms on fir can also be caused by dwarf mistletoe (Arceuthobium, a genus of parasitic plants). However, the foliage in mistletoe brooms does not turn yellow during the summer. The brooms are not bare of foliage in the winter unless they are dead. The small shoots of dwarf mistletoe are also present on branches and twigs in the mistletoe brooms. Rust brooms on spruce were once thought to be caused by the same fungus, but this has been disproved and the pathogen associated with them is known as Chrysomyxa arctostaphyli.

Disease cycle

Melampsorella elatina, like all other rust fungi, is an obligate parasite and requires a living plant host to survive. It will not continue to grow or produce spores after the host (or plant part) that it is growing on dies. It is 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). Established fir broom rust infections become systemic and perennial in both conifer and alternate hosts.

Basidiospores released from chickweed (and related genera) hosts become windborne and infect nearby expanding fir buds in the spring. The basidiospores germinate to form hyphae, which penetrate the tender, new needle growth. Infections result in elongated spots the following autumn. The rust infection spreads into twigs and branches where it causes witches’ brooms and yellowing of foliage (chlorosis). The witches’ brooms grow and persist for as long as 30 years. If by then, the rust mycelium has progressed to the trunk or colonized a large supporting branch, the fungus may persist in the tree even after the broom has died.

Spermatia develop on infected current year fir needles. Sexual recombination (mating) occurs on the fir hosts when spermatia ooze out of spermogonia in a sweet, sticky liquid and are spread (usually by mites or insects) to receptive hyphae on other nearby spermogonia. After mating, the resulting rust mycelium produces aecial pustules. The aecia erupt from the epidermis and release aeciospores, which are windborne and will infect young chickweed foliage. Aeciospores cannot re-infect their conifer host. Aeciospores can only cause infections on the alternate chickweed hosts when they land on foliage. Uredinia then develop on the leaves, producing urediniospores in less than 2 weeks. Urediniospores are wind-borne and cause new infections on chickweed foliage. Uredinial spore production continues through the growing season and greatly increases disease levels on chickweed. If the infected chickweed species is a perennial species, the rust mycelium overwinters as a systemic infection in living tissues at the base of the plant.

The sequence of spore production on systemically infected perennial chickweed in the spring is unusual, because the telia are produced before the uredinia. The overwintering rust mycelium grows within and throughout the new shoots as they sprout, infecting all new foliage and stems. Patches of telia develop on the young leaves and uredinial production follows. Teliospores themselves are not a source of spread. However, they germinate in the leaves, releasing wind-borne basidiospores, which then infect new fir needles expanding from the buds.

Damage

Apart from a few reports of light damage on fir stands in western provinces, fir broom rust does not cause significant losses to fir in Canada. Farther south in the western United States on subalpine, silver, and red fir in the Rocky Mountains and Great Basin, and in central Europe (on silver fir), it can cause severe timber loss due to cull and mortality from stem infections. Apical infections of fir can result in top dieback. Stem infections can result in sections of the trunk being culled due to the production of defective, dense, brittle wood in these areas. Cracked bark in old brooms can provide an entrance point for canker pathogens and wood decay fungi. Branch brooms are not considered to cause harmful damage to the trees. Only stem infections are associated with overall tree health and loss of merchantable timber.

Prevention and management

Treatment of forests for fir broom rust is not necessary in Canada due to low levels of damage associated with this disease. Elsewhere, early culling of trees with trunk infections or branch infections close to the trunk reduces future losses in these forests. Branch cankers are also pruned in Christmas tree plantations.

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 and population level (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 any particular pest.

Pesticides registered for use against Melampsorella elatina 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 life 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

Witches' broom on balsam fir caused by the fungus Melampsorella elatina.

Aecia (fruiting bodies) of Melampsorella elatina on the underside of balsam fir needles.

Jean-Pierre Bérubé

Multiple yellow witches' brooms on balsam fir caused by Melampsorella elatina.

Close-up of a witches' broom with yellow balsam fir needles.

Brooming on fir associated with the pathogen Melampsorella elatina.

Bob Erickson

Crisp sandwort (Stellaria crispa), one of the telial hosts of Melampsorella elatina.

Eric Allen

Well-developed, upright broom with yellowed needles on fir associated with the pathogen Melampsorella elatina.

Ed Chatelle

Aecia (fruiting bodies) of Melampsorella elatina on the underside of balsam fir needles.

Ed Chatelle

Witches' broom on balsam fir, caused by Melampsorella elatina.

Distorted balsam fir needles from a witches' broom caused by Melampsorella elatina.

Selected references

Allen, E.; Morrison, D.; Wallis, G. 1996. Common tree diseases of British Columbia. Natural Resources Canada, Canadian Forest Service, Pacific Forestry Centre. Victoria, British Columbia. 178 p.

Hunter, L.M. 1927. Comparative study of spermogonia of rusts of Abies. The Botanical Gazette 83(1): 1–23.

Myren, D.T.; Laflamme, G.; Singh, P.; Magasi, L.P.; Lachance, D., editors. 1994. Tree diseases of eastern Canada. Natural Resources Canada, Canadian Forest Service, Headquarters, Science and Sustainable Development Directorate. Ottawa, Ontario. 159 p. https://ostrnrcan-dostrncan.canada.ca/entities/publication/123e1f42-369f-4c95-aded-82d95f675a52

Oliva, J.; Colinas, C. 2007. Canopy openings may prevent fir broom rust (Melampsorella caryophyllacearum) infections. European Journal of Forest Research 126: 507–511. https://doi.org/10.1007/s10342-007-0172-8

Peterson, R.S. 1963. The effects of broom rusts on spruce and fir. United States Department of Agriculture, Forest Service, Intermountain Forest and Range Experiment Station. Ogden, Utah. Research Paper INT-7. 10 p.

Peterson, R.S. 1964. Fir broom rust. United States Department of Agriculture Forest Pest Leaflet 87. 7 p.

Scholler, M.; Braun, U.; Buchheit, R.; Bubner, B. 2022. Studies on European rust fungi, Pucciniales: molecular phylogeny, taxonomy, and nomenclature of miscellaneous genera and species in Pucciniastraceae and Coleosporiaceae. Mycological Progress 21: 64 https://doi.org/10.1007/s11557-022-01810-3

Sinclair, W.A.; Lyon, H.H. 2005. Diseases of trees and shrubs. Second edition. Comstock Publishing Associates, Cornell University Press. Ithaca, New York. 660 p.

Wegwitz, E. 1993. Needle and broom rusts of true fir. Forestry Canada, Pacific Forestry Centre. Victoria, British Columbia. Forest Pest Leaflet 45. 8 p.

Ziller, W.G. 1974. The tree rusts of western Canada. Environment Canada, Forestry Service, Pacific Forest Research Centre, Victoria. British Columbia. Publication 1329. 272 p.

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