Cedar leaf blight
- French disease name: Brûlure des aiguilles du thuya
- Other disease names: Keithia leaf blight, needle blight of western red cedar.
- Pathogen name: Didymascella thujina (E.J. Durand) Maire
- Kingdom: Fungi
- Phylum: Ascomycota
- Class: Leotiomycetes
- Order: Helotiales
- Family: Cenangiaceae
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Partial list of synonyms:
- Keithia thujina E.J. Durand
General information and importance
Cedar leaf blight is a common foliar disease of western redcedar and eastern white-cedar (Thuja plicata and T. occidentalis). It causes an unsightly appearance and low levels of damage via foliar discoloration in mature trees. The disease can cause severe damage to seedlings in nurseries where dense growth and high humidity provide optimal conditions for fungal spore production, spread, and disease intensification.
Distribution and hosts
Didymascella thujina is endemic to North America. It is present throughout the natural range of its hosts (western redcedar and eastern white-cedar), and where hosts have been planted as ornamentals. It has been reported in Alberta, British Columbia, Ontario, New Brunswick, Newfoundland and Labrador, Prince Edward Island, Quebec, and the Northwest Territories. It also has been introduced to Europe on imported Thuja and has caused serious damage to nursery stock.
Tree parts affected
Foliar scales are infected and are easily detected by their bleached pale brown colour scattered amongst healthy green scales. This pattern distinguishes cedar leaf blight from winter burn, which causes an even foliar discoloration, and has no dark spots. Damage from leaf miners and mites can be distinguished by the presence of webbing and frass (chewed plant matter and feces).
Symptoms and signs
Symptoms first appear as small, pale spots on the upper leaf scale surface. The entire scale eventually turns light brown. Apothecia develop on the previous year’s growth, usually one per scale, but sometimes up to three per scale. They are oval to round, up to 1 millimetre in diameter, and turn black at maturity when dry. They appear lighter olive-brown when moist. As they develop and swell inside the scale, they raise up a flap of the cuticle and epidermis, and the asci on the apothecial surface are exposed so that the spores can be released. Old apothecia often fall out of the dead scale, leaving a dark pit. Asci are club-shaped and 100 x 20 micrometres in size. They have a terminal pore that does not stain blue in iodine (each contains two ascospores). Ascospores are two-celled (the upper cell is smaller than the lower), ellipsoid, and have a thick, sticky wall. They are brown, have spiny projections when mature, and are 22 to 25 x 15 to 16 micrometres in size.
Disease cycle
Completion of the disease cycle takes two growing seasons, with new infections from ascospores occurring in the first growing season, and apothecial maturation in the second. In the Pacific Northwest, ascospores are forcibly released from apothecia from April to June, and again at lower levels from late September to early November. They are not released during dry weather or if temperatures drop below 5°C. Ascospores become windborne, and when they land on foliage, their sticky surface adheres so tightly that they cannot be removed without being destroyed. Their thick cell walls protect them from desiccation until free water on the leaf surface (from rain, condensation, or irrigation) is present. Ascospores germinate to form an infection peg, which directly penetrates the leaf epidermis. The fungus colonizes the mesophyll of the leaf scale. Infections are limited to single scales. Apothecia release spores after repeated wet and dry periods for up to 2 weeks. If prolonged moisture is present on the foliage, the ascospores will be released continuously until the apothecium is spent.
Damage
Western redcedar is the most susceptible host wherever it is grown. Nursery seedlings and young saplings sustain the greatest damage and have the highest mortality. Heaviest levels of disease occur when humidity is high and vegetation is dense. Mortality rates of up to 97% in nursery seedlings have been reported. Reduction of shoot biomass on trees under 4 years old in reforestation plantations can be as high as 50%, delaying establishment of regenerating stands. A heavy cover of competing vegetation in plantations also increases humidity and, therefore, disease incidence. In mature natural stands, foliar blight is most obvious on lower branches. Growth reduction may occur, but mortality is rare.
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 (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.
Fungicides can be employed to control cedar leaf blight in nurseries when heavy infestations warrant control, but they must be applied early and consistently for the duration of the growing season until the seedlings are 2 years old.
Cultural practices also reduce disease levels. Removal of infected mature cedar trees near nurseries reduces local inoculum. Circulation of air through nursery crops reduces the levels of moisture on the foliage, especially during the summer, as does protection of seedlings from rain.
There is evidence of disease resistance in west coast populations of western redcedar. British Columbian coastal, low-elevation populations exhibited the most resistance. British Columbian high-elevation and Californian populations showed the least. Trees originating from milder and wetter ecosystems show more resistance than those from other ecosystems planted at the same site.
Pesticides registered for use against Didymascella thujina 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 the 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
Foliage of western redcedar infected with the fungus Didymascella thujina.
Bob Erickson
Apothecia (fruiting bodies) of Didymascella thujina on western redcedar foliage.
John Hopkins
Selected references
CABI (Centre for Agriculture and Bioscience International). 2021. Didymascella thujina (original text by H.H. Kope), In Invasive species compendium. CAB International. Wallingford, United Kingdom. www.cabi.org/isc
Gray, L.K.; Russell, J.H.; Yanchuk, A.D.; Hawkins, B.J. 2013. Predicting the risk of cedar leaf blight (Didymascella thujina) in British Columbia under future climate change. Forest Meteorology 180:152–163. https://doi.org/10.1016/j.agrformet.2013.04.023
Kope, H.H.; Trotter, D.; Sutherland, J.R. 1993. Keithia leaf blight of western red cedar. Forestry Canada, Pacific Forestry Centre. Victoria, British Columbia. R&D update, September.
Kope, H.H. 2000. Fungi Canadenses No. 343. Didymascella thujina. Canadian Journal of Plant Pathology 22(4): 407–409. https://doi.org/10.1080/07060660009500460
Kope, H.H.; Ekramoddoullah, A.K.M.; Sutherland, J.R. 1998. Analysis of proteins of disease-free and Didymascella thujina-infected leaves of western redcedar (Thuja plicata). Plant Disease 82(2): 210–212. https://doi.org/10.1094/PDIS.1998.82.2.210
Porter, W.A. 1957. Biological studies on western red cedar blight caused by Keithia thujina Durand. (Unpublished report). Government of Canada, Department of Northern Affairs and National Resources, Forestry Branch. Victoria, British Columbia. 31 p.