Sooty bark disease
- French disease name: Maladie de la suie de l’érable
- Pathogen name: Cryptostroma corticale (Ellis & Everh.) P.H. Greg. & S. Waller
- Kingdom: Fungi
- Phylum: Ascomycota
- Class: Sordariomycetes
- Order: Xylariales
- Family: Xylariaceae
-
Partial list of synonyms:
- Coniosporium corticale (Ellis & Everh., 1889)
General information and importance
Cryptostroma corticale is a canker-causing fungus that affects maple trees (Acer species), most notably sycamore maple (A. pseudoplatanus), Norway maple (A. platanoides), and bigleaf maple (A. macrophyllum). Due to the pathogen’s ability to invade the bark and sapwood of its hosts, wilt and dieback occur due to vascular system disruption. The disease caused by C. corticale can lead to the death of its host within three years of initial symptom onset.
Distribution and hosts
The natural range of C. corticale is believed to be limited to the Great Lakes region in Ontario. Chryptostroma corticale is endemic to its native host, sugar maple (A. saccharum), in the Great Lakes-St. Lawrence region and does not cause disease. The pathogen is currently thought to be introduced to urban and natural forests in coastal British Columbia and neighbouring Washington state where it infects maple trees, especially the native bigleaf maple and the introduced sycamore maple and Norway maple. Cryptostoma corticale is also an invasive species that causes disease to its hosts throughout Europe. It was first noticed in the United Kingdom in 1945 and has since spread to other countries where sycamore maple is the main tree species affected, such as Austria, Czech Republic, France, Germany, Slovenia, and Switzerland.
Tree parts affected
Symptoms and signs of the pathogen are visible on the stem and branches of host maples of any age.
Symptoms and signs
The cracking and detachment of bark are the most obvious symptoms of this disease. The fungus sporulates in patches (stromata) beneath the bark, which ranges in colour from black or dark brown to light grey or white. In spring or early summer, dark sticky fluid may exude from cracks in the infected stem or branches, or from pruning wounds. Cankers may appear as raised bark with elongated blister-like growths, running vertically along the stem or branches. Host trees will also undergo reduced vigour and crown decline, which is characterized by the shortening of twigs, shoot mortality, wilting or curling of leaves, defoliation, and self-pruning of lower branches. Suckers and sprouts may also appear near the tree’s base as a stress response to the infection. Infected trees may show a dark greenish-brown to yellow stain in the heartwood.
Mature stromata (singular: stroma) consist of two distinct layers: roof and floor stromata. Black stromatic columns support roof stromata. The resulting space is filled with a powdery spore mass. The spores (conidia) are produced from a continuous layer of short, erect, unbranched conidiophores that are crowded on the surface of the floor stroma and absent from the roof stroma and stromatic columns. The conidiophores are 3- to 5-celled, hyaline at the base, and brown toward the apex. Conidia are ovoid to occasionally cylindrical, hyaline at first but becoming brown with age, smooth-walled, 5 × 4 micrometres, and formed from phialides in dry chains that easily disarticulate.
Several other diseases and fungi can be confused with sooty bark disease, such as brittle cinder fungus (Kretzschmaria deusta), leopard spot fungus (Biscogniauxia bartholomaei), charcoal canker (Biscogniauxia mediterranea), and bacterial wet wood. If dark discolouration only occurs on the bark surface and not beneath detached bark pieces, it is not sooty bark disease.
Disease cycle
Cryptostroma corticale can produce a large quantity of conidia (asexual spores) on infected trees. A host tree with a stem diameter of 30 centimetres can produce an estimated 3.4 x 108 conidia per square centimetre of stromatal surface. These conidia can easily spread from one maple to another via wind dispersal. The conidia that develop beneath the bark of an infected tree can travel through the air if the bark surrounding the stromata is disturbed. The pathogen’s spores penetrate new hosts through wounds or natural openings in the bark’s surface, including bark fissures and lenticels (small pores that allow gas exchange between the atmosphere and internal plant tissues). The pathogen can act as an endophyte in healthy living hosts but becomes pathogenic when the host tree is weakened by unfavourable environmental factors. Cryptostroma corticale initially infects young shoots and branches, which leads to twig and branch dieback. Soon after, mycelia spread through the sapwood. Initial symptoms typically appear one year after the host becomes infected, with tree death usually occurring within three years.
Damage
Trees of all ages and sizes are prone to infection by this disease. However, maples located at the edge of woodlands, along footpaths, and near highly frequented areas are often most affected due to the stress resulting from soil compaction from higher foot traffic. Cryptostroma corticale spreads faster within hosts experiencing unfavourable conditions, such as drought and water stress, extreme heat and prolonged strong sunlight, and compacted water-impermeable soils. Climate projections forecast an increase in hotter, drier summers in western Canada; thus, sooty bark disease will likely become more common and widespread in British Columbia’s urban and natural forests.
Foresters and millworkers repeatedly exposed to a high concentration of spores may be at risk of developing respiratory conditions, such as hypersensitivity pneumonitis (‘maple-bark disease’). However, members of the public who only occasionally come into contact with the spores of sooty bark disease through their recreational activities are considered low risk for developing respiratory issues.
Prevention and management
In the endemic host range of C. corticale, control measures are not necessary. However, measures should be taken in its newly introduced environment in British Columbia, where the pathogen causes disease among maples. Control is not possible for trees that have already become infected. Management of healthy maple stands is recommended to prevent the spread of sooty bark disease. In forest stands, urban parks, or managed woodlots, infected trees should be removed as soon as it is practical to do so. The removal of infected trees can lower inoculum load, mitigate public safety risks, and promote regeneration. Removal should occur in the winter or during humid weather to limit spore dispersal. In natural maple stands, high-value host trees may be conserved as wildlife trees if infected branches are pruned, and the trunk is free of cankers.
In urban and peri-urban areas where maples are growing in compacted soil, the soils should be tilled to promote water retention and soil aeration. Soil compaction can also be limited around maples grown in parks by installing setbacks, grass strips, or garden beds to reduce foot traffic in proximity to the trees’ roots.
During periods of prolonged drought, individual high-value trees can be watered regularly to ensure that the roots receive sufficient moisture to maintain tree vigour.
Photos
A sycamore maple heavily damaged by Cryptostroma corticale, the causal agent of sooty bark disease.
Nicolas Feau
Light grey fungal stromata of Cryptostroma corticale under the bark of a sycamore maple trunk.
Joey Tanney
Dark grey fungal stromata of Cryptostroma corticale under the bark of a sycamore maple trunk.
Joey Tanney
Microscopic view of 5 micrometres × 4 micrometres conidia of Cryptostroma corticale.
Selected references
Braun, M.; Klingelhöfer, D.; Groneberg, D.A. 2021. Sooty bark disease of maples: the risk for hypersensitivity pneumonitis by fungal spores not only for woodman. Journal of Occupational Medicine and Toxicology 16: 2. https://doi.org/10.1186/s12995-021-00292-5
Brooks, R.; Hulbert, J.M.; Omdal, D.; Elliott, M.; Chastagner, G.A. 2022. Sooty bark disease diagnostic guide. Washington State University Extension. 5p.
Dickenson, S.; Wheeler, B.E.J. 1981. Effects of temperature, and water stress in sycamore, on growth of Cryptostroma corticale. Transactions of the British Mycological Society 76(2): 181–185. https://doi.org/10.1016/S0007-1536(81)80136-2
Gregory, P.H.; Waller, S. 1951. Cryptostroma corticale and sooty bark disease of sycamore (Acer pseudoplatanus). Transactions of the British Mycological Society 34(4): 579–597. https://doi.org/10.1016/S0007-1536(51)80043-3
Hulbert, J.M.; Dewitz, F. 2023. Maple health watch research: iNaturalist and sample collection field manual. Washington State University, Puyallup Research and Extension Center. Pyuallup, Washington. 20 p.
Kelnarová, I.; Černý, K.; Zahradnik, D.; Koukol, O. 2017. Widespread latent infection of Cryptostroma corticale in asymptomatic Acer pseudoplatanus as a risk for urban plantations. Forest Pathology 47(4): e12344. https://doi.org/10.1111/efp.12344
Koukol, O.; Kelnarová, I.; Černý, K. 2015. Recent observations of sooty bark disease of sycamore maple in Prague (Czech Republic) and the phylogenetic placement of Cryptostroma corticale. Forest Pathology 45(1): 21–27. https://doi.org/10.1111/efp.12129
Ogris, N.; Brglez, A.; Piškur, B. 2021. Drought stress can induce the pathogenicity of Cryptostroma corticale, the causal agent of sooty bark disease of sycamore maple. Forests 12(3): 377. https://doi.org/10.3390/f12030377
Schlößer; R.; Steffen, B.; Langer, G.J.; Langer, E.J. 2023. Fungi associated with woody tissues of Acer pseudoplatanus in forest stands with different health status concerning sooty bark disease (Cryptostroma corticale). Mycological Progress 22: 13. https://doi.org/10.1007/s11557-022-01861-6
Strouts, R.G.; Winter, T.G. 1994. Diagnosis of ill-health in trees. Department of the Environment, Transport and the Regions, Her Majesty’s Stationary Office, Colegate. Norwich, England. 332 p.