Spruce broom rust
- French disease name: Rouille - balai de sorcière de l'épinette
- Other disease names: Yellow witches’ broom of spruce, yellow witches’ broom rust, broom rust of spruce
- Pathogen name: Chrysomyxa arctostaphyli Dietel
- Kingdom: Fungi
- Phylum: Basidiomycota
- Class: Pucciniomycetes
- Order: Pucciniales
- Family: Coleosporiaceae
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Partial list of synonyms:
- Peridermium coloradense (Diet.) Arth. & Kem
General information and importance
Spruce broom rust is a native, fungal plant pathogen that alternates its life cycle between two hosts (heteroecious). This pathogen, although rarely the direct cause of tree death, impacts the visual quality of trees and can affect timber productivity in managed forests. This pathogen robs its host of nutrients that are needed for tree growth and reproduction.
Distribution and hosts
Spruce broom rust can be found in every Canadian province and territory and throughout the entire United States wherever the pathogen’s hosts are found. It occurs more frequently and in higher numbers in the Rocky Mountain and boreal forest regions of North America.
Spruce broom rust is a heteroecious parasite, meaning that part of the life cycle of the parasite is spent on one host and part is spent on a secondary host, so the parasite can complete its life cycle. The primary host of this rust species is spruce (Picea), and the secondary host is bearberry or kinnikinnick (Arctostaphylos). The most common primary hosts are black spruce (Picea mariana), white spruce (P. glauca), Engelmann spruce (P. engelmannii), red spruce (P. rubens), and Sitka spruce (P. sitchensis). It is also found on two introduced tree species: Colorado blue spruce (P. pungens) and Norway spruce (P. abies). All three species of bearberry (A. alpina, A. rubra and A. uva-ursi) can serve as secondary hosts for the pathogen. Manzanita (also in the genus Arctostaphylos) is an occasional secondary host of C. arctostaphyli.
Tree parts affected
Spruce: Needles, bole, twigs and branches
Bearberry/kinnikinnick: Leaves
Symptoms and signs
Once the pathogen becomes systemic in woody tissues, C. arctostaphyli infections stimulate branch proliferation, resulting in abnormal bushy-like growths known as “witches’ brooms”. Twigs in brooms are shorter and thicker than normal. Brooms grow perennially and can be more than 30 years old and up to 2 metres in diameter. Spruce needles in the brooms are smaller than normal and chlorotic (yellowed). The entire broom appears yellow when aecia are fully mature on the spruce needles within the broom. These yellow brooms stand out in striking contrast to the surrounding healthy green branches on the tree. When the fungus is sporulating, brooms also have a distinctive, pungent odour. All the needles in the brooms are cast at the end of the growing season and brooms appear dead.
Spermogonia are produced below the epidermis on all surfaces of the needle. They are yellow at first and darken to brown with age. They are flattened, flask-shaped, 110 to 150 micrometres in diameter, and have a slightly protruding central pore at their opening. Spermatial characteristics are not reported in published descriptions. Aecia are blister-like and form in rows on the needle undersurface on either side of the mid-vein. They are 1 to 2 millimetres long × 0.8 millimetres tall, covered at first with a thin white peridium (layer of fungal cells), which ruptures along the mid-line to expose the orange-yellow aeciospore masses within. Aeciospores are single-celled, ellipsoid to globose and 26 to 35 micrometres long × 16 to 25 micrometres wide. They have a colourless wall that is 2 to 3 micrometres thick and densely covered with an even layer of coarse warts. Oily contents of the aeciospores are orange when fresh and become colourless with age.
The rust causes purplish leaf spots on the bearberry host. Telia form in the spots on the undersurface of year-old foliage, pushing through the epidermis in reddish-brown waxy cushions. Teliospores are formed in chains in the telia. They are single-celled and colourless. They have smooth thin walls and measure 23 to 64 micrometres long × 13 to 18 micrometres wide. Basidiospores are single-celled, with orange cell contents. They are smooth-walled, broadly ellipsoid to subglobose, and measure 8.5 to 9.5 micrometres × 7.5 to 8.0 micrometres.
Several species of dwarf mistletoe (Arceuthobium, a parasitic plant genus) also cause witches’ brooms on spruce and may be confused with spruce broom rust. However, brooms caused by dwarf mistletoe retain their needles and remain green year-round. The small shoots of dwarf mistletoe are also present on branches and twigs in the mistletoe brooms.
Disease cycle
Spruce broom rust is an obligate heteroecious parasite that must alternate between conifer and bearberry hosts to complete its full 2-year life cycle. However, unlike most other Chrysomxya species, it does not have a uredinial stage on its non-conifer host. Initial infections on young spruce needles are from basidiospores carried by wind from nearby bearberry plants. As a result of infection from germinating basidiospores on the spruce needles, spermogonia develop and extrude spermatia in droplets that produce a strong, fetid odour. Insects are attracted to the odour. They visit the spermogonia and spread the spermatia to receptive hyphae on adjacent spermatia, fertilizing them. After germination, the infection spreads to young needles and aecia start to develop. The rust mycelium in spruce needles also invades the twigs and branches, becoming systemic and perennial, eventually causing formation of witches’ brooms. In spring of the following year, aecial blisters rupture from these brooms and release massive numbers of aeciospores that are carried to the bearberry leaves by wind. Aeciospores cannot reinfect spruce; they can only infect bearberry. If they land on the bearberry leaves and the aeciospores germinate, the resulting infection causes leaf spots, which appear in spring on this infected overwintered foliage. In the fall, telia begin to develop on the leaf spots. The following spring, around the time the new spruce needles are expanding from buds, teliospores in the telia mature and germinate to produce basidia that release basidiospores. The teliospores remain in the leaf tissue. They are not a dispersal mechanism. Basidiospores cannot reinfect bearberry. However, they become windborne and infect young, developing spruce needles.
Damage
This pathogen infects spruce of all ages, causing reduced growth, deformity and sometimes tree mortality. The extent of the damage depends on the number and size of brooms within individual trees, as well as their location on the tree. When brooms occur upon the primary trunk of the tree, this can lead to top kill, which occurs when the portion of the tree above the broom dies. Weakened trees are more vulnerable to secondary pest attacks and are predisposed to decay fungi. These decay fungi can decrease the volume of merchantable wood obtained from spruce trees. This greatly reduces the trees’ marketable value. Internal wood decay also causes host trees to be structurally unstable and more vulnerable to windstorm and snowfall breakage. These compromised trees can pose hazards in residential and recreational areas. Trees with spruce broom rust show a 30% reduction in growth and have three times greater mortality during the course of 10 years when compared to healthy trees. One incidental benefit of the large, dense broom clusters is that they can provide shelter and nesting areas for many different birds and small mammals.
Spruce broom rust can provide extra fuel for forest fires. Large brooms are filled with small twigs and dead needles that can provide a fuel ladder for upward spread of fire into tree crowns. Crown fires are far more damaging than surface fires. Brooms that are broken during winter storms accumulate around the base of infected trees and increase available fuel.
Prevention and management
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.
For spruce broom rust, most infections on individual or a small number of spruce trees can be removed mechanically by pruning out brooms or cutting trees with deformed trunks. This approach works well for small parcels of private land and smaller woodlots where regular monitoring can be completed and new infections can be removed as they develop. This method is effective in preventing the spread of reproductive spores.
There are no chemical pesticides that have proven to be effective against spruce broom rust, while remaining economically viable.
Photos
A spruce broom caused by Chrysomyxa arctostaphyli that has killed the top of its spruce host. During the winter months, the broom appears dead with no needles.
Alan Stewart
A spruce broom caused by the fungus Chrysomyxa arctostaphyli with sporulating aecia on needles.
Eric Allen
A small spruce broom caused by the fungus Chrysomyxa arctostaphyli with sporulating aecia on needles.
Eric Allen
A close-up of infected yellowed spruce needles with the sporulating aecia of the fungus Chrysomyxa arctostaphyli.
Eric Allen
Bearberry (or kinnikinnick), the telial host of Chrysomyxa arctostaphyli. Discoloured spots on the upper surface of the leaves indicate that telia are present on the lower surface.
Eric Allen
Telia of Chrysomyxa arctostaphyli on the lower surface of bearberry leaves.
CFS
Selected references
Allen, E.A.; Morrison, D.J.; Wallis, G.W. 1996. Common tree diseases of British Columbia. Victoria, Canada: Natural Resources Canada, Canadian Forest Service, Pacific Forestry Centre. Victoria, British Columbia. 178 p.
Burleigh, J.; Ebata, T.; White, K.J.; Rusch, D.; Kope, H. (editors). 2014. Field guide to forest damage in British Columbia (3rd revised edition). British Columbia Ministry of Forests, Lands and Natural Resource Operations. Victoria, British Columbia. Joint publication. 17
Parks, C.G.; Bull, E.L. 1997. American marten use of rust and dwarf mistletoe brooms in northeastern Oregon. Western Journal of Applied Forestry 12(4): 131–133. https://doi.org/10.1093/wjaf/12.4.131
Peterson, R.S. 1961. Host alternation of spruce broom rust. Science 134(3477): 468–469. https://doi.org/10.1126/science.134.3477.468
Singh, P. 1978. Broom rusts of balsam fir and black spruce in Newfoundland. European Journal of Forest Pathology 8(1): 25–36. https://doi.org/10.1111/j.1439-0329.1978.tb00611.x
Unger, L.S. 1995. Spruce foliage and broom rusts. Natural Resources Canada, Canadian Forest Service, Pacific Forestry Centre. Victoria, British Columbia. Forest Pest Leaflet 39. 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.