Xylaria root rot

Written by B.E. Callan Revision 2024

• French disease name: Pourridié xylarien
• Other disease names: Black root rot
• Pathogen name: Xylaria polymorpha (Pers.) Grev. (Common name for the fungus: dead man’s fingers)
• Kingdom: Fungi
• Phylum: Ascomycota
• Class: Sordariomycetes
• Order: Xylariales
• Family: Xylariaceae

General information and importance

Xylaria polymorpha causes a root rot of various hardwood hosts. The infected roots are covered with a thin, smooth, black crust of fungal tissue, under which the decaying wood is white and interspersed with numerous zone lines. The fungus produces black, finger-like stromata (fruiting bodies) on roots, fallen tree trunks, and coarse woody debris. Trees that are crowded and being outcompeted by other larger nearby trees or otherwise stressed and growing in damp soil are most susceptible to this fungus. The pathogen is commonly observed on urban trees that have been damaged at the root collar by landscaping activities.

Distribution and hosts

In Canada, X. polymorpha is endemic but not frequently encountered. It has been reported in eastern Canada in Nova Scotia, Ontario, and Quebec. It has rarely been reported in western North America. Elsewhere in North America, it is most common in the northeastern, southeastern and northern midwestern United States. In Canada, maple (Acer) and birch (Betula) species are the most commonly encountered hosts of X. polymorpha, with sugar maple (Acer saccharum) and Manitoba maple (A. negundo) being the main species. Other less common hosts include yellow birch (Betula alleghaniensis), beech (Fagus), American beech (F. grandifolia), ash (Fraxinus) and white elm (Ulmus americana), as well as other elms (Ulmus). Elsewhere in North America, the pathogen causes root rot of apple (Malus), plum (Prunus), pear (Pyrus) and honey locust (Gleditsia). Worldwide, it has been reported from temperate areas of Europe, Asia, Africa and South America.

Xylaria species are ubiquitous endophytes, capable of living in a broad range of living plant host tissues without producing either symptoms or signs (fruiting bodies). They are often found fruiting on woody debris with advanced decay, making host determinations difficult if not impossible. For these reasons, the host range of X. polymorpha is likely broader than documented by current records of fruiting body collections.

Host parts affected

Roots and the butt of stressed trees in damp locations are affected. The fungus is also saprophytic on woody debris.

Symptoms and signs

Trees with advanced Xylaria root rot show signs of decline such as slow growth, branch dieback, and premature foliage loss at the end of the growing season. The mature stromata (fruiting bodies) of this fungus develop at the trunk base or on large roots of their hosts. They may appear to be emerging from the ground, but excavation below the stromata will show that they are attached to buried wood. When mature they are dull black, varying in shape and size but usually cylindrical with a rounded apex to club-shaped and unbranched, without a noticeable stalk but sometimes with a root-like base developing below ground. They range in size from 2 to 15 centimetres tall, and 0.5 to 3 centimetres in diameter. When they first start to develop, stromata are whitish, then tan to olive-tan, and covered with a powdery layer of conidiogenous cells and conidia (asexual reproductive spores). Conidiogenous cells form a tightly packed palisade on the surface of developing stromata, and are hyaline (colourless) to pale brown, relatively cylindrical, measuring 40 to 135 micrometres × 4 to 6 micrometres and roughened by circular scars where conidia were attached. Conidia are hyaline, smooth, varying from oblong to egg-shaped or ellipsoid, and 7 to 11 micrometres × 3 to 5 micrometres in size. Under the conidial layer, which eventually weathers away, the stroma develops a thin, dull, blackish, wrinkled crusty layer, which eventually becomes dotted with ostioles of perithecia developing beneath.

Perithecia form in a single layer directly under the black outer crust and are embedded in the white to buff-coloured corky internal tissue of the stromata. Each perithecium has a single narrow opening to the surface of the stroma, surrounded by a tiny disc. Perithecia are globose, black, 0.5 to 1 millimetre in diameter, and when the stroma is fresh and moist, mature perithecia are filled with a slimy mixture of asci and sterile hyphae called paraphyses. When the stroma dries, the perithecial contents shrink and the perithecia become hollow and coated with a black, glass-like shiny layer, which can be revived with the addition of water. Asci within the perithecia are cylindrical, with long stalks, and have a conspicuous rectangular to urn-shaped apical plug that blues in contact with Melzer’s reagent (mycological stain that contains iodine). Ascospores are smooth, brown to dark brown, ellipsoid-inequilateral (flattened on one side) to navicular (boat-shaped), and 19 to 31 micrometres × 5 to 10 micrometres, with a straight to slightly oblique germ slit that runs along one-half to two-thirds of the spore length.

Exterior portions of decayed wood are crusted with a thin black layer of fungal tissue. The fungus causes a white rot of wood, destroying the lignin and leaving cellulose behind. It also produces numerous black zone lines in the wood, giving it a spalted appearance. Advanced decay is brittle and papery.

The macabre common name for the fungus is “dead man’s fingers,” based on the appearance of the stromata as they emerge from the ground. There are many species of Xylaria known to occur in North America, but only one other causing significant root disease. Xylaria mali Fromme is pathogenic to apple and is found in eastern and south-central areas of the United States, but its distribution does not extend north into Canada. Another Xylaria species that is prevalent in Canada but not associated with root disease and frequently confused with X. polymorpha is the species X. longipes Nitschke, which has smaller ascospores measuring 13 to 15 micrometres long, with a spiralling germ slit, and a longer stem devoid of perithecia. Stromata of X. ellisii J.B. Tanney are widespread on fallen sugar maple branches in New Brunswick. This species is also frequently isolated as an endophyte of spruce (Picea), eastern white pine (Pinus strobus), and lowbush blueberry (Vaccinium angustifolium) in eastern Canada. Xylaria ellisii has smaller stromata (2 to 5 centimetres × 4 to 5 centimetres) and much smaller ascospores (8 to 10 micrometres long) than X. polymorpha.

Disease cycle

Stromata first start to develop in the spring and mature in the summer. During periods of wet weather, the perithecial contents of mature stromata swell, and asci rise to the perithecial ostioles where they forcibly eject their ascospores. The spores typically travel only a short distance but are further spread by wind and rain. Ascospores are hardy because their dark thick walls protect them from desiccation and sunlight. Ascospores in air-dried stromata stored under dry conditions can remain viable for several years. If ascospores land on a suitable host, the ascospores germinate and a hyphal tube grows out of the germ slit. Young stromata bear massive amounts of conidia, which have been observed to germinate in culture. The function of conidia in the life cycle of X. polymorpha is unknown.

Damage

Xylaria polymorpha causes symptoms typical of many root diseases: reduced growth, branch dieback in the crown of the host, early loss of foliage at the end of the growing season, and stem or trunk failure due to advanced decay. Situations that predispose trees to Xylaria root rot are high stem density that leads to competition for light, damp or water-saturated soil (either in natural settings or due to excessive irrigation in urban areas), and basal wounds such as those caused by landscaping activities. The fungus is so infrequently encountered in forests that it does not cause significant damage in such situations.

Prevention and management

In urban settings, infections can be reduced by protecting the root base from damage during landscaping activities and ensuring adequate soil drainage. Xylaria species are known to persist on buried root fragments for many years, so replacing a dead tree with another susceptible host tree at the same site is undesirable.

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.

Selected references

Callan, B.E.; Rogers, J.D. 1993. A synoptic key to Xylaria species from continental United States and Canada based on cultural and anamorphic features. Mycotaxon 46:141–154.

Ibrahim, A.; Tanney, J.B.; Fei, F.; Seifert, K.A.; Cutler, G.C.; Capretta, A.; Miller, J.D.; Sumarah, M.W. 2020. Metabolomic-guided discovery of cyclic nonribosomal peptides from Xylaria ellisii sp. nov., a leaf and stem endophyte of Vaccinium angustifolium. Scientific Reports 10:4599. https://doi.org/10.1038/s41598-020-61088-x

Proffer, T.J. 1988. Xylaria root rot of urban trees caused by Xylaria polymorpha. Plant Disease 72(1): 79.

Rogers, J.D. 1986. Provisional keys to Xylaria species in continental United States. Mycotaxon 26: 85–97.

Rogers, J.D.; Callan, B.E. 1986. Xylaria polymorpha and its allies in continental United States. Mycologia 78(3): 391–400. https://doi.org/10.1080/00275514.1986.12025261

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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Photos

Fruiting bodies of Xylaria polymorpha on a fallen, rotting white birch stem.

André Carpentier

Fruiting bodies of Xylaria polymorpha emerging from the forest floor duff layer.

André Carpentier

Fruiting bodies of Xylaria polymorpha emerging from a rotting log.

André Carpentier

Fruiting bodies of Xylaria polymorpha on a fallen, rotting white birch stem.

André Carpentier