Western gall rust

Written by B.E. Callan Revision 2024

• French disease name: Rouille-tumeur autonome
• Other disease names: Pine-to-pine gall rust
• Pathogen name: Cronartium harknessii E. Meinecke
• Kingdom: Fungi
• Phylum: Basidiomycota
• Class: Pucciniomycetes
• Order: Pucciniales
• Family: Cronartiaceae
• Partial list of synonyms:
  • Aecidium harknessii (J.P. Moore) Dietel
  • Endocronartium harknessii (J.P. Moore) Y. Hiratsuka
  • Peridermium cerebroides E. Meinecke
  • Peridermium harknessii J.P. Moore

General information and importance

Western gall rust, caused by the rust fungus Cronartium harknessii, is an important rust of two- and three-needled (hard) pines (Pinus) in western Canada. It produces round galls on the trunks and branches of its host. It is especially damaging to pine seedlings and lodgepole (P. contorta var. latifolia) and jack pine (P. banksiana) stands less than 10 years old. Stem gall infections are often fatal. Known formerly as Endocronartium harknessii, the rust completes its highly reduced life cycle on pine without infecting an alternate host.

Distribution and hosts

Endemic to North America, western gall rust is found exclusively on hard (two- and three-needled) pine species. In Canada, the native host species are jack pine, shore pine (P. contorta var. contorta), lodgepole pine, red pine (P. resinosa), and ponderosa pine (P. ponderosa). It is also found on exotic pine species planted near sources of inoculum from native species. These pines are mugo pine, (P. mugo), bishop pine (P. muricata), Austrian pine (P. nigra), maritime pine (P. pinaster), Monterey pine (P. radiata), and Scots pine (P. sylvestris). In Canada, it is found from British Columbia to Nova Scotia, and north to the Yukon. In the United States it is found in coastal Alaska, and in the west as far south as southern California, east to Nebraska, the Lake States, and coastal northeastern states. Additional pine hosts of western gall rust in the United States are knobcone pine (P. attenuata), Canary Island pine (P. canariensis), Coulter pine (P coulteri), Aleppo pine (P. halapensis), Jeffrey pine (P. jeffreyi), digger pine (P. sabiniana), and Japanese black pine (P. thunbergii). In eastern North America, the distribution range of western gall rust is difficult to fully determine because it overlaps with the range of eastern gall rust (Cronartium quercuum f. sp. banksiana). On pine, the two species are morphologically identical apart from aecial germ tube structure.

Host parts affected

Stems and branches of pine; rarely cone peduncles (stems).

Symptoms and signs

The most obvious symptom of western gall rust-infected pines are rounded galls on pine trunks and branches. These galls range in size from 5 to 10 centimetres in diameter, although they may grow much larger if they persist for a long time on the trunk of a large tree. The galls die once the tree or branch dies. They will occasionally become parasitized by mould fungi, which stop or slow down spore production. Old galls on dead trees or branches are often stripped bare and marked with tiny parallel grooves caused by rodent teeth scraping off the fungal tissues. The fungus causes spherical to oblong galls on shore pine, with aecia that form in the fissures between bark scales. On ponderosa and lodgepole pine, a smooth elongated gall is produced. “Collars” form on the branch on either side of the gall as it expands and pushes back the bark. Witches’ brooms (dense proliferation of deformed branches) often develop after gall formation.

Western gall rust has only two spore-producing stages: spermogonia and aecia. The aecia produce aeciospores that infect pine rather than an alternate host. Earlier scientific literature describes these structures and spores as “peridermioid telia” and “teliospores.” This describes their function (infecting pine) instead of their appearance.

Spermogonia are infrequently observed. They can sometimes be detected by the presence of shiny, darker patches of dried exudate that form during periods of dry weather on sunken spots of the gall. The spermogonia form dense, thin patches of fungal tissue up to 150 micrometres thick. The tissue patches form below the outer layer of the bark on the gall, pushing up the bark. Under moist conditions they exude orange droplets filled with spermatia. The spermatia produced are single-celled, colourless, smooth-walled, globose or pear-shaped, and range from 2 to 2.5 × 2 to 3.5 micrometres.

Fresh aecia form pustules in bands along raised areas of the gall each year. They mature under the bark, form orange blisters, and rupture the bark the following spring to release orange powdery aeciospore masses. When aecia are present on the gall, its surface may appear convoluted, like the surface of a brain (cerebroid). When they are not producing aecia, the galls are covered in bark. Within the galled wood, rust hyphae are sparse and found mostly in the rays, but they are abundant in the bark. Before rupturing, aecia are covered with a thin, colourless layer of fungal cells called the peridium. The outermost peridial cells are verruculose (roughened with tiny warts). Inside the aecium, filaments of intercalary cells are interspersed amongst the spore masses. Aeciospores, produced in chains in the aecia, are single-celled, oblong to obovate, and measure 23 to 25 micrometres long × 14 to 24 micrometres wide. Their cell contents are filled with orange pigmented oil droplets. Aeciospore walls are covered in rough, tapered rod-like warts, but some spores have a smooth area on one side. Aeciospores germinate to form a one- to four-celled germ tube. These cells germinate and re-infect pine.

Recent phylogenetic studies examining DNA sequences from populations of Cronartium worldwide have shown that C. harknessii is genetically very closely related to eastern gall rust, which is macrocyclic (full life cycle with all spore stages) and heteroecious (two unrelated hosts, pine and oak [Quercus]). Some researchers think that the western gall rust fungus should be classified as an endocyclic subpopulation within eastern gall rust rather than as a separate species. Morphologically, the structures and the spores produced on pine are indistinguishable between the two species. However, the aeciospores of the two rusts behave very differently from each other. Western gall rust aeciospores produce relatively short germ tubes (less than 250 micrometres) that only infect pine. Eastern gall rust aeciospores produce longer, more slender germ tubes averaging (more than 500 micrometres) that only infect oak. Eastern gall rust causes galls on jack pine and Scots pine in eastern Canada (predominately southern Ontario and southern Quebec). Its distribution roughly coincides with the range of its oak hosts. It is especially damaging to pine seedlings and saplings. Its aeciospores cannot reinfect pine; they instead infect red oak (Quercus rubra) and black oak (Q. velutina), causing leaf spots and producing uredinia and telia.

Disease cycle

The life cycle of a typical macrocyclic heteroecious Cronartium rust species is complicated, with five spore stages produced on two unrelated hosts. To add a further twist, C. harknessii has a different, shortened, endocyclic life cycle, which is completed on pine without alternating to another host (autoecious). Its abbreviated life cycle is limited to the production of only spermatia and aeciospores on the pine host. These endocyclic aeciospores behave very differently than macrocyclic aeciospores of typical Cronartium species because they can re-infect pine, instead of infecting an alternate host.

Infections can occur on young growth of pines of any age. However, on most pines, the initial stem infections occur before the trees reach 10 years old. When aeciospores land on wet, succulent current-year pine tissues, such as seedlings, needles, the thin bark of year-old shoots, or the peduncles (stems) of developing seed cones, they produce germ tubes that infect the pine. Some seedlings will show reddening near the infection site as soon as 2 to 3 weeks after infection, but other tissues can remain asymptomatic until a gall forms. Even then, there may only be a slight swelling on the main stem to indicate an infection. Stem galls, which often develop into lethal infections, are initiated when the apical shoot (leader) of the tree becomes infected. This usually occurs when the trees are 10 years old or younger. If the stem is only infected on one side, it will form a trunk protrusion known as a “hip gall.” Stem or branch galls start to develop in response to the infection and 1 to 4 years later, the gall starts to produce spermogonia and aecia. The spermogonia develop in the bark of some galls in spring. Although spermatia are produced, and can sometimes be observed exuding from the bark, their function is unclear due to the reduced life cycle of the rust, especially because not all galls seem to produce spermogonia.

Once aecial production on a gall begins, new aecia are produced each year thereafter. The aecial peridium bursts and aecia start to release spores from late May until early July. During the time when the galls are not producing aecia or spores, they are covered with a smooth layer of bark. On non-rainy days, aeciospores are released primarily in daytime hours. Most are released in the morning when wind speed and temperature start to increase, and the relative humidity starts to decrease. Dry conditions facilitate the release of the spores from the spore mass. On rainy days with less change in relative humidity, aeciospores are released in lesser numbers and more evenly throughout the day. The aeciospores are spread by wind current, generally within 100 metres of their source. Under rare turbulent wind conditions, they may be carried much farther (several kilometres). Aeciospores will germinate in temperatures ranging from 10 to 30°C, but optimal temperatures for germination are 15 to 20°C. Under optimal conditions, aeciospores can begin to germinate as soon as 1 to 2 hours after landing on susceptible pine tissue, if there is surface moisture present to support the growth of the germ tube. Prolonged periods of cool, wet spring weather increase infection levels in pine stands. Years that have perfect conditions for heavy infection are rare. Years with high infection levels are known as “wave years” because they are followed by 1 or 2 years of higher-than-average infection levels.

The western gall rust fungus is an obligate parasite and requires a living host to survive. It will not continue to grow or produce spores after the tree (or galled branch) dies.

Several mould fungi parasitize the aecia of western gall rust, affording various degrees of natural control by reducing aeciospore production and viability. One of the most damaging is Scytalidium uredinicola. It was present in up to 39% of galls from 18 lodgepole pine stands in Alberta, and reduced aeciospore germination from 90% to 5.5%.

Damage

Infections in pine seedlings result in severe stunting or death. Seedlings in nurseries are more likely to die from stem infections due to the rapid girdling of the young stems by galls. Trees in dense, young stands under 10 years old are more likely to develop main stem galls, resulting in dead or deformed trees that are of low commercial quality for lumber or wood fibre. Hip and stem galls increase the chance of trunk breakage. Stem galls often kill the top of the tree or result in entire tree mortality. This is due to the restriction of water and nutrient flow in the tree, especially in young, dense stands where diseased trees are competing with healthy ones for water resources. However, not all trees with active stem galls die. There are reports of active stem galls more than 200 years old on lodgepole pine. Branch galls do not result in significant loss of growth or mortality and are considered non-lethal if they are located farther than 15 centimetres from the trunk and are pruned, or 60 centimetres away from the trunk and are not pruned. Galls are also potential sites for secondary insects or pathogens to become established.

There are numerous reports of damage caused by western gall rust across Canada. In New Brunswick, a Scots pine Christmas tree plantation was abandoned because high levels of galls caused branch and top mortality. In a survey of the Maritime provinces, 24% of jack pine in 35 different sites had one to fifty galls. No mortality was observed, but trees were stunted and malformed. In western Alberta, an estimated 15% volume loss was observed in lodgepole pine stands during a 20-year period. In British Columbia, a recent study predicted the mean wood volume losses due to stem rusts (combined comandra blister rust [Cronartium comandrae] and western gall rust) on lodgepole pine to be 7.2% at potential harvest age. Comandra blister rust was the main cause of wood volume loss due to stem rusts. 

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 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.

Unlike heteroecious rusts, western gall rust has no alternate host that can be eradicated to reduce infection levels in pine stands. Instead, silvicultural practices in juvenile stands must be used to reduce losses as the stand matures. Naturally regenerating lodgepole pine stands are often very dense. To ensure maximum productivity for wood and fibre, foresters manage them by using a spacing method. If western gall rust is present in the stands to be thinned, trees with stem galls or multiple branch galls should be selected whenever possible for removal during spacing, regardless of their size or crown position. It can be beneficial if thinning is delayed until after the stands reach 10 years of age, as resistance to infections is greatly increased at that point and continues to increase as trees reach maturity. As lower branches are shaded-out, branch galls die and inoculum in the stand is greatly reduced. However, in areas with heavy disease levels, it is not practical to aggressively thin out every diseased tree. Therefore, the number of trees remaining after thinning should be increased beyond typical stocking targets to allow for future mortality in the stand. The galls are much more visible in dense stands while they are producing bright orange aecia. Timing the thinning operations to occur in late spring and early summer will make detection of galls much easier.

In nurseries, seedlings with gall infections should be culled. Infected pines within 300 metres of the nurseries should be removed if possible. In urban or park settings, individual trees with high value can be managed by pruning branch galls. In Christmas tree plantations, pruning can also be used but will depend on infection levels and when trees go to market. Removing too many branches on Christmas trees ready for market can create “holes” in the trees’ crowns and reduce their value.

There is well-documented evidence of variation in disease resistance to western gall rust within the populations of susceptible pine species. The resistance to this disease is of sufficient importance and is now being considered as a criterion for selecting superior trees in genetic improvement programs. C. harknessii can be grown in sterile culture on pine tissue callus. This technique could be used to screen pine populations for resistant individuals to use in breeding programs.

Pesticides registered for use against C. harknessii 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 the 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.

Selected references

Aime, M.C.; Castlebury, L.A.; Abbasi, M.; Begerow, D.; Berndt, R.; Kirschner, R.; Marvanová, L.; Ono, Y.; Padamsee, M.; Scholler, M.; Thines, M.; Rossman, A.Y. 2018. Competing sexual and asexual generic names in Pucciniomycotina and Ustilaginomycotina (Basidiomycota) and recommendations for use. IMA Fungus 9(1): 75–89. https://doi:10.5598/imafungus.2018.09.01.06 

Allen, E.A.; Blenis, P.V.; Hiratsuka, Y. 1988. Axenic culture of Endocronartium harknessii. Mycologia 80(1): 120–123. https://doi.org/10.2307/3807504

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.

Chang, K.-F.; Blenis, P.V. 1989. Survival of Endocronartium harknessii teliospores in a simulated airborne state. Canadian Journal of Botany 67(3): 928–932. https://doi.org/10.1139/b89-122

Chang, K.-F.; Blenis, P.V.; Hiratsuka, Y. 1989. Mechanism and pattern of spore release by Endocronartium harknessii. Canadian Journal of Botany 67(1): 104–111. https://doi.org/10.1139/b89-015

Crane, P.; Jacubec, K.M.; Hiratsuka, Y. 1995. Production of spermogonia and aecia of Endocronartium harknessii on artificially inoculated lodgepole pine and their significance in the life cycle. Pages 101–107 in S. Kaneko, K. Katsuya, M. Kakishima, Y. Ono, editors. Proceedings of the fourth IUFRO Rusts of Pines Working Party Conference, 2–7 October 1994. Tsukuba, Japan.

Hiratsuka, Y. 1969. Endocronartium, a new genus for autoecious pine stem rusts. Canadian Journal of Botany 47(9): 1493–1495. https://doi.org/10.1139/b69-212

Hiratsuka, Y.; Powell, J.M. 1976. Pine stem rusts of Canada. Canadian Forest Service, Northern Forest Research Centre. Edmonton, Alberta. Technical Report No. 4. 103 p.

Hiratsuka, Y.; Powell, J.M.; Van Sickle, G.A. 1988. Impact of pine stem rusts of hard pines in Alberta and the Northwest Territories. Canadian Forestry Service, Northern Forestry Centre. Edmonton, Alberta. Information report NOR-X-299. 9 p.

Hopkin, A.A.; Reid, J.; Hiratsuka, Y.; Allen, E. 1988. Initial infection and early colonization of Pinus contorta by Endocronartium harknessii (western gall rust). Canadian Journal of Plant Pathology 10(3): 221–227. https://doi.org/10.1080/07060668809501728

Liberato, J.R.; Ramsfield, T.; Shivas, R.G.; Hiratsuka, Y.; Allen, E. 2006. Pine western gall rust (Endocronartium harknessii). The Pest and Disease Image Library (PaDIL). Australia Department of Agriculture, Fisheries and Forestry and Department of Primary Industries and Regional Development. Western Australia. Updated on 16 Aug 2023. Available online at: http://www.padil.gov.au

Powell, J.M.; Morf, W. 1966. Temperature and pH requirements for aeciospore germination of Peridermium stalactiforme and P. harknessii of the Cornartium coleosporioides complex. Canadian Journal of Botany 44(12): 1597–1606. https://doi.org/10.1139/b66-171

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.

Stock, A.; Duthie-Holt, M.; Walsh, S.; Turner, J.; Swift, K. 2005. Southern interior forest region: forest health stand establishment decision aids. British Columbia Journal of Ecosystems and Management 6(1): 55–73. https://doi.org/10.22230/jem.2005v6n1a308

Walla, J.A.; Lundquist, J.E.; Tuskan, G.A.; Freeman, T.P. 1991. Observations on the spermogonial stage of the western gall rust fungus. Pages 54–62 in Rusts of pine. Proceedings of the IUFRO Rusts of Pine Working Party Conference, 18–22 September 1989, Banff, Alberta. Forestry Canada, Northwest Region, Northern Forestry Centre. Edmonton, Alberta. Information Report NOR-X-317.

White, E.E.; Allen, E.A.; Ying, C.C.; Foord, B.M. 2000. Seedling inoculation distinguishes lodgepole pine families most and least susceptible to gall rust. Canadian Journal of Forest Research 30(5): 841–843. https://doi.org/10.1139/x00-013

Woods, A.J.; Nussbaum, A.; Golding, B. 2000. Predicted impacts of hard pine stem rusts on lodgepole pine dominated stands in central British Columbia. Canadian Journal of Forest Research 30(3): 476–481. https://doi.org/10.1139/x99-236

Van der Kamp, B.J. 1994. Lodgepole pine stem diseases and management of stand density in the British Columbia interior. The Forestry Chronicle 70(6): 773–779. https://doi.org/10.5558/tfc70773-6

Vogler, D.R.; Bruns, T.D. 1998 Phylogenetic relationships among the pine stem rust fungi (Cronartium and Peridermium spp.). Mycologia 90(2): 244–257. https://doi.org/10.1080/00275514.1998.12026904

Zhao, P.; Liu, F.; Huang, J.E.; Zhou, X.; Duan, W.J.; Cai, L. 2022. Cronartium rust (Pucciniales, Cronartiaceae): species delineation, diversity and host alternation. Mycosphere 13(1): 672–723. https://doi.org/10.5943/mycosphere/13/1/7

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

Cite this fact sheet

Photos

Gall on lodgepole pine caused by Cronartium harknessii, the causal agent of western gall rust.

Branch gall on jack pine caused by Cronartium harknessii, the causal agent of western gall rust.

Claude Moffet

A Scots pine Christmas tree with multiple galls caused by Cronartium harknessii.

Multiple branch galls of western gall rust on jack pine.

André Carpentier

A lodgepole pine with numerous globose galls on branches and the stem caused by Cronartium harknessii, the causal agent of western gall rust.

Bob Erickson

Stem breakage at the site of a gall caused by Cronartium harknessii.

CFS

Cronartium harknessii branch gall on jack pine with orange spores visible beneath bark.

Eric Allen

Immature gall caused by Cronartium harknessii that has never produced spores on one-year-old pine branch.

Eric Allen

Stem galls caused by Cronartium harknessii on nursery-produced jack pine seedlings.

Yasu Hiratsuka

A jack pine with numerous globose galls on branches and the stem caused by Cronartium harknessii, the causal agent of western gall rust.

Sporulating Cronartium harknessii, the cause of western gall rust, on jack pine.