Ink spot of aspen
- French disease name: Tache d'encre
- Other disease names: Inkspot
- Pathogen name: Ciborinia whetzelii (Seaver) Seaver
- Kingdom: Fungi
- Phylum: Ascomycota
- Class: Leotiomycetes
- Order: Helotiales
- Family: Sclerotiniaceae
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Partial list of synonyms:
- Ciborinia bifrons Whetzel
- Sclerotinia bifrons Whetzel
General information and importance
Ink spot is widespread on poplars (Populus) throughout their natural range. The disease causes brown patches of leaves that are dotted with black round fungal spots that often fall out, leaving holes in the leaves. Ink spot disease rarely causes significant widespread defoliation of aspen but is more likely to be heavier in dense stands of young trees.
Distribution and hosts
Ink spot is endemic and widespread across Canada, following the distribution of its primary hosts, trembling aspen (P. tremuloides) and largetooth aspen (P. grandidentata). Balsam poplar (P. balsamifera), eastern cottonwood (P. deltoides), and black poplar (P. nigra) are occasional hosts, but symptoms are neither widespread nor severe.
Tree parts affected
Ink spot is a foliar disease, often limited to the leaf blade and leaving the petiole green.
Symptoms and signs
The first symptom of foliar infection is leaf browning, often visible as irregular patches of dead brown leaves in the canopy of the tree. The leaf petiole often remains green long after the leaf turns brown. Leaf browning is followed one month later by the formation of sclerotia, which are black structures resembling ink spots on the leaf blades, between veins. The sclerotia do not contain or produce spores, they serve as a resistant overwintering repository of fungal tissue that stores energy for apothecial production in the spring. Sclerotia are 2 to 8 millimetres in diameter, roughly circular to oval in shape and the same thickness, or slightly thicker, than the leaf, with a thin rind of blackened tissue covering an internal palisade layer (tightly packed row) of fungal cells. Not all sclerotia remain in the leaf tissue throughout the growing season until natural leaf drop. Some detach from the leaf and fall to the ground, leaving the leaf with a shot-hole appearance. In the spring, just after bud break, the upper surfaces of overwintered sclerotia resting in areas of moist soil or forest duff will produce apothecia, which look like tiny, beige cups resting on long stalks. The apothecia range from 2 to 10 millimetres in diameter, and their stalks are slender, 5 to 25 millimetres long. The upper surface of the apothecia is formed from a tightly packed layer of microscopic asci, which are club-shaped, 160 to 180 micrometres × 11 to 12 micrometres, each with an apical plug that stains blue in Melzer’s reagent (mycological stain that contains iodine). Asci contain eight ascospores, which are single-celled, smooth-walled, egg-shaped, hyaline (uncoloured), and measure 7 to 10 micrometres × 3 to 4 micrometres. The apothecia are short-lived and infrequently observed, due also to their small size and neutral colour.
Ciborinia pseudobifrons, which causes similar symptoms on aspen, is less frequently encountered. Its sclerotia are smaller in size (3 to 5 millimetres in diameter), and they are located on the petiole or leaf veins rather than on the leaf blades.
Disease cycle
In the spring, overwintered sclerotia on the forest soil begin to sprout apothecia around the time that new leaves are expanding on host trees. During periods of rainy weather apothecia forcibly eject ascospores into the air, where they are spread by wind to land on and infect tender new leaf tissues. Infected leaves start to turn brown about one month after infection, and sclerotia start to develop one month after leaves brown. Sclerotia start to detach from leaves and fall out in early summer, creating “shot holes”, but some sclerotia remain on foliage until leaf dehiscence in autumn.
Damage
Ink spot occasionally causes early defoliation of aspen, especially in dense stands of young trees where the closed canopy contributes to higher humidity levels, and the inoculum sources (apothecia) are closer to the susceptible foliage. Prolonged wet weather during bud break and leaf expansion also contributes to high disease levels. Low levels of infection do not cause significant damage.
Prevention and management
In a forest setting, there is no practical method of managing the disease. Raking and removing fallen leaves and mulching under aspen growing in urban settings would help to reduce the number of overwintering sclerotia under trees.
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.
Photos
Sclerotia of the fungus Ciborinia whetzelii on aspen.
Al Funk
Ink spots caused by the fungus Ciborinia whetzelii. The spots are the overwintering stage of the fungus.
Trembling aspen infected by Ciborinia whetzelii, the causal pathogen of ink spot of aspen.
Selected references
Baranyay, J.A.; Hiratsuka, Y. 1967. Identification and distribution of Ciborinia whetzelii (Seaver) Seaver in western Canada. Canadian Journal of Botany 45(2): 189–191. https://doi.org/10.1139/b67-015
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.
Groves, J.W.; Bowerman, C.A. 1955. The species of Ciborinia on Populus. Canadian Journal of Botany 33(6): 577–590. https://doi.org/10.1139/b55-046
Kohn, L.M. 1979. A monographic revision of the genus Sclerotinia. Mycotaxon 9(2): 365–444.
Pomerleau, R. 1940. Studies on the ink-spot disease of poplar. Canadian Journal of Research 18c(5): 199–214. https://doi.org/10.1139/cjr40c-022
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.