Hypoxylon canker

Written by B.E. Callan Revision 2024

• French disease name: Chancre hypoxylonien
• Other disease names: Salt-and-pepper canker, hypoxylon canker of aspen
• Pathogen name: Entoleuca mammata (Wahlenb.) J.D. Rogers & Y.M. Ju
• Kingdom: Fungi
• Phylum: Ascomycota
• Class: Sordariomycetes
• Order: Xylariales
• Family: Xylariaceae
• Partial list of synonyms:
  • Hypoxylon mammatum (Wahlenb.) P. Karsten
  • Hypoxylon pruinatum (Klotzch) Cooke
  • Nemania mammata (Wahlenb.) Granmo

General information and importance

Hypoxylon canker is an important and damaging disease of trembling aspen (Populus tremuloides) and, to a lesser degree, largetooth aspen (P. grandidentata) caused by the fungus Entoleuca mammata. Damage is especially prominent in forests that are predisposed to severe weather events or insect damage. It can infect trees of all ages but is especially damaging to aspen forests aged 20 years or younger. It causes trunk cankers that girdle and kill trees. It is responsible for millions of cubic metres of wood loss each year in Canada.

Distribution and hosts

Hypoxylon canker is endemic to North America. It is also widely established in many European countries where it was introduced, likely several centuries ago. Hypoxylon canker has been reported in Alberta, British Columbia, Manitoba, Nova Scotia, Newfoundland & Labrador, Ontario, Quebec, and Saskatchewan. It is uncommon in British Columbia, Pacific coastal regions of the United States, and the island of Newfoundland, and apparently does not occur in Alaska.

Hypoxylon canker occurs most commonly on trembling aspen and, to a lesser extent, on largetooth aspen. Both aspen hosts develop branch cankers and can be killed by trunk cankers. Hypoxylon canker also occasionally occurs in Canada on other poplars, including balsam poplar (P. balsamifera) and various hybrid poplars. Broadleaved trees in the following genera have also been reported as occasional hosts: Acer (maple), Alnus (alder), Betula (birch), Fagus (beech), Quercus (oak), and Salix (willow). On hosts other than aspen, severe cankers are not common and the pathogenic and/or saprophytic relationships with E. mammata are not well understood.

Tree parts affected

Trees of any age are potential hosts, and the fungus can infect twigs, branches, and trunks. Entoleuca mammata can also remain viable for several years as a saprophyte on dead, previously infected trees, where it causes a white rot of the heartwood.

Symptoms and signs

Cankers are perennial and may be detected any time of year. Dead leaves still attached to branches (flagging) are a crown symptom of cankers on the tree. The first symptoms of developing cankers are yellow to orange, slightly sunken areas of bark on lower trunks or branches. These are often associated with insect, woodpecker, drought, or winter damage. Cankers expand more rapidly along the longitudinal axis of the trunk than around the circumference. The surface of the bark on the canker is rough and cracked. A grey spreading layer of mycelium, also called a mycelial fan, is visible on the cambium layer just beneath the bark. The sapwood under the fans is mottled with black and cream-coloured areas. Grey, dusty pillars of fungal tissue (hyphal pegs) are visible in the bark cracks. The pillars are 1 to 2 millimetres tall × 2 to 5 millimetres in diameter. Dusty greyish masses of conidia are produced on the surfaces of the hyphal pegs. Conidia are hyaline, single-celled, and 5.5 to 8.0 micrometres long × 1.5 to 4.0 micrometres wide. Perithecia develop in whitish to greyish crusts on dark central areas of the canker, where the bark has split or been sloughed off. Mature perithecia are whitish when first developing, then turn grey and darken to almost black when they become old and weathered. Clusters of 10 to 30 globose perithecia are packed into a single crust, or stroma.  The apex of each perithecium is slightly peaked, 0.7 to 1.0 millimetre in diameter, and topped with a dark central ostiole (small circular opening). If dry stromata are cut to expose the perithecial contents, the perithecia appear hollow, with the mature hymenial layer coating the inner wall in a thin, shiny, black layer. When the perithecia are wetted, the hymenial layer swells and become viscous, filling the interior of the perithecia. Asci are hyaline, cylindrical, long stalked, contain eight ascospores, and have a prominent rectangular apical plug, which stains blue in iodine. Ascospores are blackish brown, elliptical, and single-celled. They have one flattened side that is 20 to 33 micrometres long × 9 to 12 micrometres in diameter. Under high magnification, a straight germ slit appears as a pale line on the curved side of the spore.

Disease cycle

New cankers appear on host trees after drought or other physiological stress, or wounding of bark on twigs, branches, or stems. Entoleuca mammata infects wounded areas with exposed xylem, such as those caused by boring insects tunnelling into the xylem, or insects (in particular, Saperda spp. or cicadas) that make oviposition wounds into the xylem. Areas on the trunk with woodpecker damage are also common infection sites. Infections are initiated during periods of wet weather from wind-borne ascospores, which are forcibly ejected from perithecia. Ascospores are released year-round, provided the temperature is above -4°C, but do not germinate until the temperature is above 16°C. The thick dark walls of the ascospores protect them from desiccation. Conidia, although prolific, are not considered to be a source of inoculum (they occasionally germinate in culture). They are thought to function as spermatia during sexual reproduction prior to perithecial development.

The fungus is known to produce toxins that inhibit callus formation, preventing infected wound sites from healing over, so diffuse cankers can rapidly develop. About 2 years after infection, cankered bark starts to blister and separate from the underlying wood, as hyphal pegs on the mycelial fan force the bark outward, separating it from the underlying wood. Perithecial stromata are produced on the oldest part of the canker, usually 1 to 2 years after the hyphal pegs force apart the bark.

Trunk cankers are often centred around a branch stub. Cankers initiated on branches as far as 30 centimetres from the trunk have the potential to grow down the branch and into the trunk.

Entoleuca mammata can continue to live saprophytically on old cankers on dead, fallen trees. Perithecia and ascospores may be produced for up to 2 years after the death of the tree. The fungus causes a white rot of heartwood under the cankers, resulting in branch and trunk failure.

Damage

In British Columbia and Pacific coastal regions of the United States, hypoxylon canker is uncommon and is not associated with heavy forest damage. In central and eastern North America, except for the island of Newfoundland, hypoxylon canker is an important and damaging disease of aspen. Damage is especially prominent in forests predisposed to severe weather events or insect damage. In one report from Ontario, the estimated annual loss of wood volume to hypoxylon canker was 2 million cubic metres. The same study also indicated that there is no correlation between tree diameter or crown class and canker-related mortality. In 1981, the annual wood volume loss in Quebec was estimated to be more than 1 million cubic metres.

The heaviest damage from cankers occurs in forests of aspen 20 years of age or younger. Cankers on young trees can form on the relatively thin bark of the lower trunk, resulting in girdling or breakage and subsequent death of the tree. In older trees, cankers are more likely to form higher up on the trunk where the bark is thinner. If girdling or stem breakage occurs at these locations, the trees will survive if new leaders develop from branches below the infection. There are higher levels of cankers on trees at the edge of forests and in widely spaced forests. This is likely caused by a greater number of branches on the lower trunk that can potentially develop cankers that spread to the trunk. Trees in dense forests have fewer branches due to self-pruning.

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.

Low-density forests of aspen tend to have a higher number of cankers. High stocking densities and a closed canopy in young aspen forests minimizes the incidence of cankers.

Attempts to eradicate hypoxylon canker from forests via thinning and removal of infected trees are largely unsuccessful, as many cankers on remaining trees go undetected. The fungus can produce inoculum from cankered woody debris on the ground for up to 2 years.

There is clonal variation in resistance to cankers. Aspen breeding programs have demonstrated large family differences in canker resistance. Families of trees that were better at self-pruning their branches reduced the levels of stem breakage. Increased self-pruning reduced the chances of branch infections growing into the trunk.

To prevent trunk failure of high-value trees in urban settings or high use areas, such as parks, cankered branches could be pruned to prevent the infection from spreading into the main stem.

Pesticides registered for use against E. mammata 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 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

Large canker on aspen trunk caused by the fungus Entoleuca mammata.

André Carpentier

Grey perithecia of Entoleuca mammata on a canker on trembling aspen.

Leaf wilt of trembling aspen resulting from a canker caused by Entoleuca mammata.

André Carpentier

Bark cracks on a cankered trembling aspen trunk infected by Entoleuca mammata.

Denis Lachance

Patches of grey perithecia of Entoleuca mammata.

Denis Lachance

Dead bark of trembling aspen associated with hypoxylon canker.

Eric Allen

Entoleuca mammata perithecia embedded in stromata.

Eric Allen

Breakage of trees infected by Entoleuca mammata is a common feature associated with hypoxylon canker.

Perithecial stromata of Entoleuca mammata, the cause of hypoxylon canker.

Trembling aspen infected by Entoleuca mammata, the cause of hypoxylon canker. Note the yellowish colour of the bark on the canker margin.

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.

Archambault, L. 1982. Impact du chancre hypoxylonien sur le tremble de 2 unités de gestion du Québec. The Forestry Chronicle 58(3): 139–142. https://doi.org/10.5558/tfc58139-3

Bier, J.E. 1940. Studies in forest pathology III. Hypoxylon canker of poplar. Canada, Department of Agriculture, Division of Botany and Plant Pathology, Science Service. Ottawa, Ontario. Publication 691, Technical Bulletin 27. 40 p.

Brandt, J.P.; Cerezke, H.F.; Mallett, K.I.; Volney, W.J.A.; Weber, J.D. 2003. Factors affecting trembling aspen (Populus tremuloides Michx.) health in the boreal forest of Alberta, Saskatchewan, and Manitoba, Canada. Forest Ecology and Management 178(3): 287–300. http://dx.doi.org/10.1016/S0378-1127(02)00479-6

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.

Kasanen, R.; Hantula, J.; Ostry, M. 2004. North American populations of Entoleuca mammata are genetically more variable than populations in Europe. Mycological Research 108(7): 766–774. https://doi.org/10.1017/S0953756204000334

Lavallée, A. 1990. Hypoxylon canker of aspen (revised). Forestry Canada, Quebec Region. Sainte-Foy, Quebec. Information Leaflet Laurentian Forestry Centre 21E. 5 p.

Manion, P.; Griffin, D. 1986. Sixty-five years of research on hypoxylon canker of aspen. Plant Disease 70(8): 803–808.

Ostry, M.E.; Anderson, N.A. 1998. Interactions of insects, woodpeckers, and hypoxylon canker on aspen. United States Department of Agriculture, Forest Service, North Central Research Station. St. Paul, Minnesota. Research Paper NC-331.

Pitt, D.; Weingartner, D.; Greifenhagen, S. 2001. Precommercial thinning of trembling aspen in northern Ontario, Part 2. Interactions with hypoxylon canker. The Forestry Chronicle 77(5): 902–910. https://doi.org/10.5558/tfc77902-5

Rogers, J.; Ju, Y-M. 1996. Entoleuca mammata comb. nov. for Hypoxylon mammatum and the genus Entoleuca. Mycotaxon 59: 441–448.

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.

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