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Western pine-aster rust

Written by B.E. Callan Revision 2025

<em>Coleosporium asterum</em>, the causal organism of pine needle rust, fruiting on red pine needles.
Coleosporium asterum, the causal organism of pine needle rust, fruiting on red pine needles.

General information and importance

Coleosporium montanum is a rust fungus that causes needle infections on pine and foliar infections on its alternate hosts, predominately species of the Asteraceae family, including aster (Aster) and goldenrod (Solidago). It is a member of the C. asterum species complex, with the more narrowly defined concept for the species limited to Japanese populations of the rust. The rust causes minor defoliation of its pine hosts, where it becomes established perennially in needle infections for several years until the needle is cast. It has an extremely large number of alternate hosts.

Distribution and hosts

Coleosporium montanum is endemic to North America and has been introduced into Asia and Europe. Lodgepole pine (Pinus contorta) and Ponderosa pine (P. ponderosa) are known conifer hosts. Many herbaceous plants host the uredinial and telial stages of this rust, including aster (Aster), Eurybia (formerly part of the genus Aster), gumweed (Grindelia), tidytips (Layia), Oclemena (aster-like flower), flat-topped goldenrod (Euthamia graminifolia), many species of goldenrod, and many species of American asters (Symphyotrichum).

Tree parts affected

Needles of pine and foliage of aster and goldenrod and related genera.

Symptoms and signs

Pine foliage with heavy infection is dotted with tiny delicate, white aecial tongues filled with orange spores. Spermogonia are produced on the lower surfaces of older pine needles (not current year), scattered, in the shape of a flattened cone, and 0.3 to 0.5 millimetres × 0.5 to 1.0 millimetres in diameter and 0.5 millimetres high. Aecia break out of the epidermis on yellowed spots on all surfaces of needles as laterally flattened tubes, 1.0 to 1.5 millimetres tall × 0.5 to 0.8 millimetres wide. These tubes are encased with a delicate colourless peridium and filled with orange-yellow aeciospores. Perennial aecial infections produce brown bands on the needle. Aeciospores are oblong, 32 to 45 micrometres long × 16 to 24 micrometres wide, and closely and coarsely verrucose (warty). Uredinia on aster and goldenrod are produced on the upper surface of leaves, initially bright orange but becoming nearly colourless with age. Urediniospores are variable in size and shape but mostly ellipsoid or oblong-ellipsoid, and warted with rods and ridges. Telia are produced on the leaf undersurface, and are rounded, waxy, and orange-red. Teliospores measure 50 to 120 micrometres × 17 to 25 micrometres, dividing into four cells internally to form an internal basidium measuring 54 to 95 micrometres × 14 to 27 micrometres with a sterile basal cell 11 to 28 micrometres long. Each basidium cell produces a single basidiospore by germination. The Asian species C. asterum differs from C. montanum in lacking the sterile cell below the basidium.

About 15 species of Coleosporium cause needle rusts of pine in North America, but many of these species occur further south in the United States, on pine species not native to Canada. The other Coleosporium species present in Canada have different alternate hosts but may be confused with C. montanum when they occur on pine (Pinus). All the Coleosporium species, except for C. montanum, are more extensively collected and reported on their non-pine hosts, making the full host range on conifers difficult to determine.

Coleosporium campanulae (Pers.) Tul. occurs in western Canada on cultivated bluebell (Campanula) but not on pine. It occurs on Scots pine (P. sylvestris), jack pine (P. banksiana), and other species in eastern Canada and the United States and Europe. Future taxonomic research is required to determine its relationship to C. tussilaginis (Pers.) Lév. and other morphologically similar species.

Coleosporium delicatulum (Arthur & F. Kern) Hedgc. & Long produces aecia on 2- and 3-needle pines. Alternate hosts are goldentop (Euthamia) and goldenrod. It is found in eastern Canada.

Coleosporium pinicola (Arthur) Arthur is autoecious (does not have an alternate host) on jack pine and has been found in Ontario, Quebec, and New Brunswick. It produces telia and basidia on yellowed year-old pine needles.

Coleosporium viburni Arthur attacks jack pine and its alternate host Viburnum.

Disease cycle

Coleosporium montanum is an obligate parasite (requires a living host to grow and reproduce). It is macrocyclic (has five different spore types: spermogonia, aeciospores, urediniospores, teliospores, and basidiospores), and heteroecious (requires alternation between two unrelated host species at different stages of their life cycle to fully complete it). It can survive perennially on both hosts. Windborne basidiospores produced on the alternate host infect pine needles of various ages in late summer to early autumn. The rust mycelium overwinters in the living needles. Infected needles remain symptomless during the summer but develop yellow spots in winter and early spring. Spermogonia develop in the yellow spots in spring. Spermatia are extruded from the spermogonia in a sweet liquid attractive to insects, which spread the spermatia to other spermogonia, thus facilitating sexual recombination. Aecia then develop and break through the epidermis to release aeciospores. The aecia weather away by the end of summer, leaving a small crater or scar in a brown band of tissue in an otherwise still green or slightly yellowed, attached needle. Aeciospores are wind-borne and those that land on the alternate host germinate and infect, first producing orange uredinial pustules on the leaf undersurfaces. Urediniospores are released and infect adjacent alternate hosts, which in turn produce more uredinia, thus increasing infection levels. Uredinial production continues until late summer and early autumn when telia start to develop subepidermally amongst the uredinia. The rust can overwinter in uredinia on perennial hosts. Teliospores produced in the telia germinate without a rest period to produce basidia and basidiospores, which are spread in wind currents back to the pine host. The aecial scars on pine needles will regrow aecia in the same spot for several years.

Damage

Heavy infections of needle rust on young pines in nurseries can occasionally cause reduced growth but they do not cause significant damage in natural forests. Occasional heavy infections cause premature needle drop, especially on lower branches of younger trees. Infections that result in whole tree mortality are rare because the current year’s needles are not affected until late in the fall, after the growing season is completed. Consecutive years of infection, accompanied by an additional stress, such as drought, could result in loss of vigour, growth loss, and whole tree mortality. Less vigorous trees are more susceptible to attack by other insects and diseases, such as bark beetles (Ips), and root disease caused by Armillaria ostoyae (Romagn.) Herink.

Prevention and management

Nurseries should not be established where the alternate hosts grow abundantly, and infected seedlings should not be transplanted from other nurseries. The alternate host plants, aster and goldenrod, should be removed within 300 metres of the pine seedlings, ideally before late summer when infected plants start to release basidiospores.

Barriers of taller non-susceptible trees around a nursery or young plantation will help intercept spores and modify air currents so that fewer spores from the alternate hosts reach the pine seedlings.

The result of several consecutive years of defoliation can reduce the merchantability of trees in the Christmas tree and ornamental tree industry. In these cases, an application of a fungicide registered for control of this pine needle rust may be warranted. The removal of any alternate host, from within 300 metres of pine plantings should also provide some level of control. Pesticides registered for use against C. montanum 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 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.

Pest management strategies for a particular pest vary depending on several factors. These include:

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.

Photos

Needle rust

Needle rust

René Pâquet

Needle rust

Needle rust

Needle rust

Needle rust

André Carpentier

Needle rust

Needle rust

Jean-Pierre Bérubé

Needle rust

Needle rust

Robert Blais

Needle rust

Needle rust

Robert Blais

Claude Monnier

Needle rust

Needle rust

André Carpentier

Needle rust

Needle rust

Robert Blais

Needle rust

Needle rust

Robert Blais

Needle rust

Needle rust

Robert Blais

Needle rust

Needle rust

Robert Blais

Needle rust

Needle rust

Robert Blais

Claude Moffet

René Pâquet

Needle rust

Needle rust

Robert Blais

Needle rust

Needle rust

Fruiting of <em>Coleosporium asterum</em>, the cause of pine needle rust, on its alternate host, goldenrod. The reddish pustules produce spores that infect pine.

Fruiting of Coleosporium asterum, the cause of pine needle rust, on its alternate host, goldenrod. The reddish pustules produce spores that infect pine.

<em>Coleosporium asterum</em>, the causal organism of pine needle rust, fruiting on red pine needles.

Coleosporium asterum, the causal organism of pine needle rust, fruiting on red pine needles.

Selected references

McTaggart, A.R.; Aime, M.C. 2018. The species of Coleosporium (Pucciniales) on Solidago in North America. Fungal Biology 122(8): 800–809. https://doi.org/10.1016/j.funbio.2018.04.007

Scharpf, R.F. 1993. Diseases of Pacific coast conifers. United States Department of Agriculture, Forest Service. Washington, D.C. Agriculture Handbook 521. 199 p.

Sinclair, W.A.; Lyon, H.H. 2005. Diseases of trees and shrubs (2nd edition). Comstock Publishing Associates, Cornell University Press. Ithaca, New York. 660 p.

Voglmayr, H.; Krisai-Greilhuber, I.; Kirisits, T. 2020. First report of Coleosporium montanum on Symphyotrichum in Austria and Europe. New Disease Reports 42(1): 24–24. https://doi.org/10.5197/j.2044-0588.2020.042.024

Ziller, W.G. 1974. The tree rusts of western Canada. Environment Canada, Canadian Forestry Service, Pacific Forest Research Centre. Victoria, British Columbia. Publication No. 1329. 272 p.

Cite this fact sheet

Callan, B.E. 2025. Western pine-aster rust. In J.P. Brandt, B.I. Daigle, J.-L. St-Germain, A.C. Skinner, B.C. Callan, and V.G. Nealis, editors. Trees, insects, mites, and diseases of Canada’s forests. Natural Resources Canada, Canadian Forest Service, Headquarters. Ottawa, Ontario.