Cytospora canker
- French disease name: Chancre cytosporéen
- Other disease names: Blackstem disease (on poplar cuttings)
- Pathogen name: Cytospora chrysosperma (Pers.) Fr.
- Kingdom: Fungi
- Phylum: Ascomycota
- Class: Sordariomycetes
- Order: Diaporthales
- Family: Valsaceae
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Partial list of synonyms:
- Valsa sordida Nitschke
General information and importance
Cytospora chrysosperma causes cankers on a wide range of broadleaved tree species. It is a weak and opportunistic pathogen, and therefore, hosts must be predisposed by conditions such as drought, repeated defoliation, damage from diseases or insects, sun damage, shade suppression, frost, or nutrient deficiency for cankers to develop. Cytospora canker does not occur on healthy, vigorous tree branches or trunks. Its presence indicates an underlying cause of physiological stress. This disease can cause significant losses within stool beds (tight rows of coppiced trees cultivated to produce multiple shoots that are cut for propagation), or new plantations planted with cuttings from stool beds. The symptom that Cytospora chrysosperma causes under these circumstances is referred to as “black stem” due to the blackening of the inner tissues that can be clearly seen through the thin bark.
Distribution and hosts
Cytospora canker is found worldwide on a broad array of broadleaved hosts. In Canada, it has been reported in Alberta, British Columbia, Manitoba, Nova Scotia, Ontario, Quebec, and Saskatchewan.
In Canada, the main hosts are poplar (Populus) and willow (Salix) species. Cytospora canker has also been reported in Canada from species of maple (Acer), alder (Alnus), Saskatoon or serviceberry (Amelanchier), birch (Betula), dogwood (Cornus), hawthorn (Crataegus), silverberry (Elaeagnus), apple (Malus), mock-orange (Philadelphus), cherry and plum (Prunus), elderberry (Sambucus), and mountain-ash (Sorbus). However, there are more than two hundred different species from scores of broadleaved genera listed as hosts worldwide. Recent molecular analysis of fresh isolates of Cytospora from cankered plants located in various geographic locations have resulted in the description of many new species. As further taxonomic studies are conducted, the host associations and distribution ranges for species of Cytospora, especially C. chrysosperma, may become more narrowly defined.
Tree parts affected
Twigs, branches, and trunks of trees.
Symptoms and signs
Cytospora chrysosperma can cause spreading bark necrosis, localized lesions, or perennial target cankers. Cankers are caused by host callus formation at the perimeter of the canker during each growing season. While the tree is dormant in winter, the callus is overcome by fungal growth, causing tissue death. This repeating annual cycle of callus formation and death as the fungus grows forms concentric rings resulting in a canker that looks somewhat like a target.
Under the bark, infected tissues discolour to orange or brown, smell sour, and eventually become blackened. The early stages of the canker can also be detected on external smooth bark surfaces of aspen because they turn orange or brown as the tissues die. The conidia-producing fruiting bodies, called pycnidia, develop within the tissues of the dead bark. They are globose to irregularly lobed, with a central circular opening (ostiole) that pushes up the bark and opens at its surface. If the canker is on a branch or thin-barked stem, the pycnidia forming under the bark will cause it to become roughly pimpled. Pycnidia range from 0.5 to 1 millimetre in diameter. Their interiors are folded into a convoluted set of locules lined inside with filamentous, often branched conidiogenous (conidium-producing) cells that measure 10 to 40 micrometres long × 1 micrometre in diameter. The conidiogenous cells produce hyaline, sausage-shaped, unicellular conidia averaging 3 to 5 micrometres long × 1 to 1.5 micrometres in diameter. Masses of conidia are extruded in thin, reddish orange tendrils from the ostioles during periods of high humidity. These spore tendrils can become coiled and extend several centimetres in length.
Perithecia form after the pycnidia develop, and often replace old pycnidia in the same cankered locations on the bark. Six to twelve perithecia form in a cluster closely surrounded by a dense mass of fungal tissue called a stroma. Perithecia are conical, 0.3 to 0.5 millimetres in diameter, and dark greyish brown, with long narrow necks. At the bark surface, the necks of the perithecia collectively open into the same ostiole, which is surrounded by a pale to dark grey disc that is 0.5 to 2 millimetres in diameter. Asci, which form on the perithecial walls, are club-shaped and 30 to 45 micrometres × 5 to 7 micrometres. They have eight spores and are wider at the apical end, which is flattened and bears a hyaline ring. Asci detach from the perithecial wall and float free within the perithecium when they are mature. Ascospores are hyaline, allantoid, single-celled, 7 to 12 micrometres × 1.5 to 2.5 micrometres, and do not extrude from the ostioles in tendrils in the same way as conidia. When the contents of the perithecia swell during wet weather, asci are extruded from the perithecial ostiole and forcibly eject ascospores, which become windborne.
Disease cycle
Cankers have two sources of inoculum: conidia (spread by windblown rain and dripping water) or ascospores (forcibly ejected from the perithecia, becoming airborne). Conidia are by far the most abundant form of inoculum and are released year-round from pycnidia that develop prolifically and rapidly in recently killed tissue several weeks after infection. Pycnidia extrude massive numbers of conidia in spore tendrils, which become hard and brittle if the weather is dry. During wet weather, conidia in the spore tendrils ooze out of pycnidia. The spore tendrils dissolve and the conidia are spread by rain and splashing water.
Perithecia are less common and develop several months, or even the next year, after pycnidia first form. The ascospores are usually released by the perithecia in the spring. Young understory trees may become infected by spores produced on branch or stem cankers on adjacent or overhanging mature trees.
Conidia or ascospores can infect mechanical and insect wounds, smaller natural openings (bud or leaf dehiscence scars), and lenticels on hosts predisposed by other damaging factors. The spores can initiate infections for up to 10 days after plant tissue is wounded. Cankers often start in the dormant season when the host response in the tree to the pathogen is not as vigorous, and when wet weather is more common. There is evidence that C. chrysosperma can be present but asymptomatic on bark or in bud tissues on hosts that appear otherwise healthy. Asymptomatic branch or stem cuttings of aspen develop C. chrysosperma fruiting bodies rapidly if incubated under dry conditions. The fungus has also been isolated directly from healthy bark. Drought is a major predisposing factor for the widespread establishment of cytospora canker. Trees with drought stress are unable to properly establish resistance mechanisms to protect damaged areas from infections, such as forming callus tissues over wounds. If the host is severely stressed, the fungus becomes established and is unimpeded by host resistance responses. The fungus grows rapidly and causes widespread tissue death and a diffuse, rapidly spreading canker. Trees with a more active defence response (e.g., ones with damage limited to a wound) will develop target cankers if the wound becomes infected.
Damage
In natural forest settings, cytospora cankers contribute to the thinning of suppressed saplings and young trees and accelerate the demise of trees stressed by other biotic and abiotic factors. They can cause serious damage and losses in young plantations and horticultural settings, especially in dense, fast-growing plantings such as hybrid poplar stool beds.
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.
Any forest management protocols that reduce wounding and stress to host trees will also reduce the incidence of cytospora canker. If cuttings are used for plantings (a common practice for hybrid poplars), they should be inspected upon removal from storage for early blackstem symptoms (discoloured areas). Care should be taken to avoid drought stress and sun damage until the root systems have become established.
Pesticides registered for use against C. chrysosperma 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 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
Canker caused by Cytospora chrysosperma on the stem of trembling aspen.
Claude Moffet
Infection and canker resulting from Cytospora chrysosperma following sunscald damage to trembling aspen.
Brenda Callan
Spore tendrils of Cytospora chrysosperma on trembling aspen.
Eric Allen
Canker caused by Cytospora chrysosperma on trembling aspen.
Rod Garbutt
Canker caused by Cytospora chrysosperma on balsam poplar.
Spore tendrils of Cytospora chrysosperma on willow.
Cytospora canker caused by Cystospora chrysosperma on trembling aspen.
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.
Bloomberg, W. 1962. Cytospora canker of poplars: factors influencing the development of the disease. Canadian Journal of Botany 40(10): 181–1269. https://doi.org/10.1139/b62-120
Bloomberg, W.J. 1962. Cytospora canker of poplars: the moisture relations and anatomy of the host. Canadian Journal of Botany 40(10): 1281–1292. https://doi.org/10.1139/b62-121
Bloomberg, W.J.; Farris, S.H. 1963. Cytospora canker of poplars: bark wounding in relation to canker development. Canadian Journal of Botany 41(2): 303–310. https://doi.org/10.1139/b63-025
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.
Kaczynski, K.M.; Cooper, D.J. 2013. Susceptibility of Salix monticola to Cytospora canker under increased temperatures and decreased water levels. Forest Ecology and Management 305: 223–228. https://doi.org/10.1016/j.foreco.2013.06.002
Lawrence, D.P.; Holland, L.A.; Nouri, M.T.; Travadon, R.; Abramians, A.; Michailides, T.J.; Trouillas, F.P. 2018. Molecular phylogeny of Cytospora species associated with canker diseases of fruit and nut crops in California, with the descriptions of ten new species and one new combination. IMA Fungus 9: 333–369. https://doi.org/10.5598/imafungus.2018.09.02.07
Mclntyre, G.A.; Jacobi, W.R.; Ramaley, A.W. 1996. Factors affecting cytospora canker occurrence on aspen. Journal of Arboriculture and Urban Forestry 22(5): 229–233. https://doi.org/10.48044/jauf.1996.035
Rossman, A.Y.; Adams, G.C.; Cannon, P.F.; Castlebury, L.A.; Crous, P.W.; Gryzenhout, M.; Jaklitsch, W.M.; Mejia, L.C.; Stoykov, D.; Udayanga, D.; Voglmayr, H.; Walker, D.M. 2015. Recommendations of generic names in Diaporthales competing for protection or use. IMA Fungus 6(1): 145–154. https://doi.org/10.5598/imafungus.2015.06.01.09
Spielman, L.J. 1985. A monograph of Valsa on hardwoods in North America. Canadian Journal of Botany 63: 1355–1378. https://doi.org/10.1139/b85-190