Shepherd’s crook of aspen
- French disease name: Brûlure des pousses du peuplier
- Other disease names: Venturia blight of aspen
- Pathogen name: Venturia moreletii Rulamort
- Kingdom: Fungi
- Phylum: Ascomycota
- Class: Dothideomycetes
- Order: Venturiales
- Family: Venturiaceae
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Partial list of synonyms:
- Fusicladium radiosum var. lethiferum (Peck) Ritschel & U. Braun
- Pollaccia lethifera (Peck in Sacc.) Morelet
- Pollaccia radiosa (Lib.) Baldacci & Cif.
- Venturia macularis (Fr.) E. Müller & Arx
- Venturia tremulae Aderh. var. grandidentatae M. Morelet
General information and importance
Shepherd’s crook of aspen is named after the characteristic appearance of dead shoots that are caused by infections of Venturia moreletii. The fungus, which is endemic and widespread in North America, infects tender, young shoots in the spring, causing the tips to shrivel and curl over into a blackened hook (often referred to as an ‘old shepherd’s crook’).
Distribution and hosts
Venturia moreletii is endemic to North America and occurs on aspen throughout Canada, in every forested province and territory where trembling aspen (Populus tremuloides) and largetooth aspen (P. grandidentata) are found. It also occurs in Europe, where white poplar (P. alba) is a host. There are infrequent records of the fungus on other poplar hosts in Canada, including narrowleaf cottonwood (P. angustifolia), balsam poplar (P. balsamifera), and black cottonwood (P. trichocarpa). However, the related Venturia populina (Vuill.) Fabric. is by far the most common and damaging species on balsam poplar and narrowleaf cottonwood. Venturia inopina is the most common and damaging Venturia on black cottonwood and its hybrids.
Tree parts affected
Foliage and young tender shoots of aspen are susceptible to Venturia moreletii. Trees under 3 metres in height are more likely to sustain heavy damage.
Symptoms and signs
Growing, tender, young shoots and leaves are infected in spring by airborne ascospores or rain-splashed conidia. Pseudothecia (overwintering fruiting bodies producing asci and ascospores) are produced late in the growing season on fallen dead leaves and old shepherd’s crooks, the latter often remaining attached to the tree until the following spring. Pseudothecia are black, globose, and smooth to setose (bearing short, dark, stiff bristles). They measure 0.08 to 0.14 millimetres in diameter and have an apical ostiole that is 0.02 to 0.05 millimetres in diameter, erupting through the host epidermis when mature. When they are present, setae average 30 to 50 micrometres in length. Asci, which form inside the pseudothecia, are cylindrical to oblong, 42 to 63 micrometres long × 10 to 12 micrometres in diameter, bitunicate (having two-layered walls), and contain two, four, or eight olive-brown ascospores. These ascospores are faintly rough-walled to smooth and elliptical to club-shaped. They are 15 to 22 micrometres long × 7 to 11 micrometres in diameter at their widest point, with one cell larger than the other. After infection by ascospores or overwintered conidia, diseased tissues wilt and turn black. The dead areas become covered with an olive-green, velvety coating of conidium-producing (conidiogenous) cells and conidia by early summer. Conidiogenous cells are cylindrical, olive green to brown, non-septate, and 8 to 12 micrometres long × 4 to 6 micrometres in diameter. Conidia are olive-brown and ellipsoid to cylindrical. They are 12 to 42 micrometres long × 6 to 11 micrometres in diameter, straight or slightly curved, and have 0 to 2 cross walls.
Heavy levels of infections result in a stunted, bushy tree when most of the new shoots in the crown are infected. The curved ends of the shepherd’s crooks are devoid of foliage, and beneath them, clusters of blackened dead leaves remain attached to their branches.
Venturia borealis occasionally causes leaf spots on trembling aspen in northern British Columbia and the Yukon Territory. It sometimes produces lesions adjacent to V. moreletii on the same leaf. It is distinguishable by infections that are limited to small purplish spots that detach from the leaf, leaving “shot holes” in their place. Conidia of V. borealis are cylindrical, light brown, and single-celled. They measure 15 to 22 micrometres long × 4 to 5 micrometres in diameter and are significantly narrower than those of V. moreletii.
Venturia populina and V. inopina G. Newc. are both associated with shepherd’s crooks and leaf blight of cottonwoods and balsam poplar in North America.
Disease cycle
Each spring, primary inoculum is initiated from two sources. The primary source is windborne ascospores forcibly ejected from overwintered pseudothecia embedded in either shepherd’s crooks on the trees or on leaves on the ground. Rain-splashed overwintered conidia on shepherd’s crooks are a second source of early inoculum. The first black leaf spots of the season develop just a few days after infection and are often adjacent to the shriveled grey remnants of last year’s shepherd’s crooks.
Infections begin as dark lesions on shoots, leaves and petioles. These lesions spread rapidly and cause death of the entire shoot. Conidia are produced on the dead tissue and are spread by rain and wind, causing new infections throughout the growing season during wet weather. The infections are limited to developing, tender shoots and leaves, and do not spread into woody twigs or branches.
Damage
Disease damage is heaviest and can reach epidemic levels in years with mild, wet spring weather. Under these conditions, most shoots in young aspen stands naturally regenerated by sprouting may die. Repeated infections by V. moreletii may result in small, stunted, deformed and bushy crowns. Terminal shoots are the most affected because they continue to produce susceptible new growth as they elongate throughout the growing season, unlike lateral branches, which harden off. Heavily infected trees become crooked because of repeated attacks on the terminals. The dead terminals result in new growth of lateral branches that bend upward in place of the dead terminals. Terminal shoot replacement, followed by its infection and death, can happen several times in one season. Saplings shorter than 3 metres are the most susceptible due to their proximity to spring inoculum on the ground, and overstory inoculum if nearby mature aspen are present in the stand. Heavy disease levels can reduce the crown height by one-third in the year of infection.
Once trees reach a height of 5 metres or more, the impact of the disease lessens.
Prevention and management
In nurseries, removal and destruction of fallen dead leaves in the autumn would reduce but not eliminate inoculum in the spring. This is because the shepherd’s crooks remaining on the trees would still produce spores. There is considerable genetic variation in the resistance of aspen to V. moreletii, which indicates that breeding programs could incorporate selected resistance traits into progeny.
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 (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 a particular pest.
Photos
Wilted trembling aspen shoots caused by the fungus Venturia moreletii.
André Carpentier
Venturia moreletii.">
Necrotic shoots and leaves of trembling aspen with characteristic "shepherd's crook" symptoms caused by the fungus Venturia moreletii.
Bob Erickson
Venturia moreletii.">
Necrotic shoots and leaves of trembling aspen with characteristic "shepherd's crook" symptoms caused by the fungus Venturia moreletii.
Bob Erickson
Upper crown of young trembling aspen with multiple shoots affected by Venturia moreletii.
Shepherd's crook of aspen caused by the fungus Venturia moreletii.
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.
Barr, M.E. 1989. The Venturiaceae in North America: revisions and additions. Sydowia 41: 25–40.
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.
Dance, B.W. 1961. Spore dispersal in Pollaccia radiosa (Lib.) Bald. and Cif. Canadian Journal of Botany 39(6): 1429–1435. https://doi.org/10.1139/b61-123
Funk, A. 1985. Foliar fungi of western trees. Agriculture Canada, Ministry of State for Forestry, Pacific Forest Research Centre. Victoria, British Columbia. Information Report BC-X-265. 159 p.
Funk, A. 1989. Observations on an aspen leaf spot disease and associated fungus, Pollaccia borealis. Canadian Journal of Plant Pathology 11(4): 353–356. https://doi.org/10.1080/07060668909501079
Gross, H.L.; Basham, J.T. 1981. Diseases of aspen suckers in northern Ontario. Canadian Forestry Service, Great Lakes Forest Research Centre, Sault Ste. Marie, Ontario. Information Report O-X-329. 16 p.
Holeski, L.M.; Vogelzang, A.; Stanosz, G.; Lindroth, R.L. 2009. Incidence of Venturia shoot blight in aspen (Populus tremuloides Michx.) varies with tree chemistry and genotype. Biochemical Systematics and Ecology 37(3): 139–145. https://doi.org/10.1016/j.bse.2009.02.003
Rulamort, M. 1986. Remarques taxinomiques et nomenclaturales sur quelques micromycètes. Bulletin de la Société Botanique du Centre-Ouest 17: 191–192.
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.