Didymascella thujina (cedar leaf blight)
- Taxonomic Tree
- Notes on Taxonomy and Nomenclature
- Distribution Table
- History of Introduction and Spread
- Risk of Introduction
- Habitat List
- Hosts/Species Affected
- Host Plants and Other Plants Affected
- Growth Stages
- List of Symptoms/Signs
- Biology and Ecology
- Means of Movement and Dispersal
- Seedborne Aspects
- Plant Trade
- Impact Summary
- Economic Impact
- Environmental Impact
- Detection and Inspection
- Similarities to Other Species/Conditions
- Prevention and Control
- Distribution Maps
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PicturesTop of page
IdentityTop of page
Preferred Scientific Name
- Didymascella thujina (E.J. Durand) Maire 1927
Preferred Common Name
- cedar leaf blight
Other Scientific Names
- Keithia thujina E.J. Durand 1913
International Common Names
- English: Keithia blight; Keithia leaf blight; needle blight: western red cedar
- French: brunissure des aiguilles du Thuya; rouille des aiguilles du Thuya
Local Common Names
- Germany: Keithia-Krankheit: Lebensbaum; Laubbraeune: Lebensbaum; Schuppenbraeune: Lebensbaum; Thujablattbräune
- DIDSTH (Didymascella thujina)
Taxonomic TreeTop of page
- Domain: Eukaryota
- Kingdom: Fungi
- Phylum: Ascomycota
- Subphylum: Pezizomycotina
- Class: Dothideomycetes
- Subclass: Dothideomycetidae
- Order: Capnodiales
- Family: Mycosphaerellaceae
- Genus: Didymascella
- Species: Didymascella thujina
Notes on Taxonomy and NomenclatureTop of page Common names for this disease include cedar leaf blight, Keithia blight and Keithia leaf blight. The name Keithia was derived from the first generic designation for the fungus and has remained in use for many years. Saccardo erected the genus Keithia in 1892, and the genus Didymascella was published in 1903 (Maire et al., 1901; Marie and Saccardo, 1903). Maire (1905) rejected the generic designation of Didymascella when he followed a suggestion that it was synonymous with Keithia. In 1927, Maire again took up Didymascella when it was reported that Keithia was previously used for a genus of flowering plants. The name Keithia thujina was used in the literature until 1960 when Pawsey corrected the name to Didymascella thujina.
DescriptionTop of page No anamorphic stage is known. The apothecioid is olive-coloured, epiphyllous and hypophyllous, circular to elliptical in outline, 0.6-1.5 x 0.4-0.8 mm, usually one per leaflet, sometimes 2-5 and then coalescing to give a larger fruiting surface. Subhymenial tissues, 40-50 µm deep, composed of hyaline textura angularis while brown hyphae are conspicuous within the mesophyll cells of the leaf. Paraphyses hyaline, filiform, some tips clavate or slightly swollen, smooth and septate, 87.5-100.0 x 1.5-3.0 µm. Asci clavate, unitunicate, with non-amyloid apical tip, 82.5-102.5 x 15-20 µm; forcible discharge of ascospores results in prominent tear across the ascus tip. Ascospores, two per ascus, uniseriate, hyaline, with a single transverse septum closer to one end of the ascospore, resulting in two very unequal-sized cells, ellipsoid to ovoid, 15-20 x 14-18 µm. Ascospore shape and ornamentation are essentially the same for North American and European collections but size varies slightly, averaging 15-25 x 14-18 µm for North American (Durand, 1913; Porter, 1957; Kope, 2000) and 15-22 x 11.5-17.5 µm for European collections (Burmeister, 1966; Søegaard, 1969). Ascospores have been described as having a gelatinous covering, which aids in adhesion and prevents desiccation (Pawsey, 1957; Phillips and Burdekin, 1982), although no such covering had been noted in early taxonomic descriptions (Durand, 1909; Pantidou and Korf, 1954). Kope (2000) provided photographic evidence that discharged ascospores are enveloped in a sticky mucilaginous sheath, which runs off and cements the ascospore to a surface. Ascospore ornamentation, when viewed under a compound light microscope, appears verrucose. Ascospores without the mucilaginous sheath, when viewed with a scanning electron microscope, have a capitate or strongly irregularly projecting surface ornamentation. The exposed ascospores are olive-brown and are slightly flattened on the end of the larger of the two cells of the ascospore.
DistributionTop of page
D. thujina is native to North America where it was first found in 1908 on mature trees of eastern white cedar (Thuja occidentalis) in Wisconsin, USA (Durand, 1913). Durand felt that the disease only affected the host when environmental conditions favoured epidemics. From 1912 to 1915 the disease was reported as seriously affecting western red cedar (Thuja plicata) in the western US state of Idaho (Weir, 1916) where it was widely distributed throughout the range of T. plicata, including the adjoining areas of central Oregon, western Montana and southern British Columbia, Canada. Porter (1957) studied the occurrence and distribution of the disease in western red cedar forests in British Columbia. More recently, the disease been found on western red cedar throughout its coastal and interior ranges in British Columbia (Foster and Wallis, 1969; Wood, 1986; Finck et al., 1989) and Alaska (Anon., 1985). Carew (1988) noted Keithia blight on eastern white cedar in Newfoundland, Canada. Keithia blight has been found on 2-year-old, container-grown, western red cedar (Dennis and Sutherland, 1989) and western red cedar transplants in coastal nurseries in British Columbia and on western red cedar in bare-root nurseries in California (Frankel, 1990).
Distribution TableTop of page
The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.Last updated: 10 Jan 2020
|Continent/Country/Region||Distribution||Last Reported||Origin||First Reported||Invasive||Reference||Notes|
|Belgium||Present, Widespread||Introduced||Invasive||Boudru (1945)|
|Denmark||Present, Widespread||Introduced||Invasive||Buchwald (1936); CABI (Undated)|
|France||Present, Localized||Introduced||Invasive||Anon (1953)|
|Germany||Present, Localized||Introduced||Invasive||Burmeister (1966)|
|Ireland||Present, Widespread||Introduced||Invasive||Pethybridge (1919); Forbes (1920); Forbes (1921)|
|Italy||Present, Widespread||Introduced||Invasive||Vegni and Ferro (1964)|
|Lithuania||Present||Juronis et al. (2006)|
|Netherlands||Present, Widespread||Introduced||Invasive||CABI (Undated)||Original citation: van Poetern, 1931|
|Norway||Present, Widespread||Introduced||Invasive||Jorstad and Roll-Hasen (1943)|
|Spain||Present||Introduced||Invasive||CABI (Undated)||Original citation: Fernandez-Magan, 1974|
|United Kingdom||Present, Localized||Introduced||Invasive||Loder (1919); Miles (1922); BOYCE (1927); Alcock (1928); LAING (1929)|
|Canada||Present||CABI (Undated a)||Present based on regional distribution.|
|-British Columbia||Present, Widespread||Native||Invasive||Kope (2000)|
|-New Brunswick||Present, Widespread||Native||Invasive||Kope (2000)|
|-Newfoundland and Labrador||Present, Widespread||Native||Invasive||Kope (2000)|
|-Ontario||Present, Widespread||Native||Invasive||Kope (2000)|
|-Prince Edward Island||Present, Widespread||Native||Invasive||Kope (2000)|
|-Quebec||Present, Widespread||Native||Invasive||Kope (2000)|
|United States||Present||CABI (Undated a)||Present based on regional distribution.|
|-Alaska||Present, Widespread||Native||Invasive||CABI (Undated)||Original citation: Anon. (1985)|
|-California||Present, Widespread||Native||Invasive||Shaw (1973)|
|-Idaho||Present, Widespread||Native||Invasive||Shaw (1973)|
|-Michigan||Present, Widespread||Native||Invasive||Anon (1960)|
|-New York||Present, Widespread||Native||Invasive||Anon (1960)|
|-Oregon||Present, Widespread||Native||Invasive||Shaw (1973)|
|-Texas||Present, Widespread||Native||Invasive||Anon (1960)|
|-Washington||Present, Widespread||Native||Invasive||Shaw (1973)|
History of Introduction and SpreadTop of page Cedar leaf blight is endemic to North America with its introduction to Europe probably being on western red cedar (Thuja plicata) imported from North America (Peace, 1955). The first European records were from Ireland (Pethybridge, 1919; Forbes 1920, 1921) where the disease destroyed nursery-grown T. plicata. Soon after its discovery in Ireland the disease was found to be well established in England (Loder, 1919; Miles 1922; Boyce, 1927) and Scotland (Alcock, 1928; Laing, 1929, Wilson, 1937). Later, it was reported from Holland (Van Poetern, 1931) and Denmark (Buchwald, 1936; Ferdinandsen and Jorgensen, 1938) where it was so serious that nurseries curtailed or stopped growing cedar.
Risk of IntroductionTop of page There is a risk of cedar leaf blight becoming established wherever Thuja species are introduced and grown, as is evidenced by its occurrence on Thuja species introduced into Europe. Thuja has been introduced to other countries, i.e., India, Australia, Chile and the Ukraine (Minter, 1997) but at present D. thujina has not been reported as a disease.
HabitatTop of page Cedar leaf blight is endemic to the coastal and interior ranges of western red cedar of the Pacific Northwest of North America and to the range of eastern white cedar in eastern North America. It also occurs on western red cedar and eastern white cedar transplanted to Europe. Keithia leaf blight is currently the most important disease in reforestation nursery-grown western red cedar seedlings.
Habitat ListTop of page
|Terrestrial – Managed||Cultivated / agricultural land||Present, no further details||Harmful (pest or invasive)|
|Protected agriculture (e.g. glasshouse production)||Present, no further details||Harmful (pest or invasive)|
|Managed forests, plantations and orchards||Present, no further details||Harmful (pest or invasive)|
|Managed grasslands (grazing systems)||Present, no further details||Harmful (pest or invasive)|
|Disturbed areas||Present, no further details||Harmful (pest or invasive)|
|Rail / roadsides||Present, no further details||Harmful (pest or invasive)|
|Urban / peri-urban areas||Present, no further details||Harmful (pest or invasive)|
|Terrestrial ‑ Natural / Semi-natural||Natural forests||Present, no further details||Harmful (pest or invasive)|
|Natural grasslands||Present, no further details||Harmful (pest or invasive)|
|Riverbanks||Present, no further details||Harmful (pest or invasive)|
|Wetlands||Present, no further details||Harmful (pest or invasive)|
|Cold lands / tundra||Present, no further details||Harmful (pest or invasive)|
|Coastal areas||Present, no further details||Harmful (pest or invasive)|
Hosts/Species AffectedTop of page D. thujina occurs on eastern white cedar (Thuja occidentalis) in eastern North America and on western red cedar (Thuja plicata) and T. plicata var. atrovirens on the west coast of North America (Sinclair et al., 1987). It also occurs on some varieties of ornamental Thuja (Burmeister, 1966). D. thujina was once reported occurring on Port Orford cedar (Chamaecyparis lawsoniana) seedlings in a nursery in Washington state, USA (Boyce, 1961). In Germany, Burmeister (1966) reported that D. thujina probably occurred on C. lawsoniana, but he was unable to confirm this.
Host Plants and Other Plants AffectedTop of page
Growth StagesTop of page Seedling stage, Vegetative growing stage
SymptomsTop of page Symptoms on mature foliage first appear as several small, cream-coloured spots on the upper surface of individual cedar leaves. These spots later develop into lesions, which coalesce. The entire awl-shaped leaf becomes brown, contrasting with adjacent green, healthy leaves. Infected leaflets are scattered over a branch infecting the previous years' growth. In cross section, the apothecium is initially covered by the leaf cuticle and an epidermal layer, one-cell thick, which is golden brown to brown in colour. The apothecium is raised slightly above the surrounding leaf tissue. When the epidermal covering breaks three quarters around the circumference of the apothecium, it lifts up as a scale, exposing the hymenial layer. As the exposed apothecium matures it changes colour from golden brown to dark brown. When moist, the cushion-like hymenial layer of the apothecium is golden-olive. When dry, apothecia appear much darker and the epidermal scale often folds back over the apothecium. Affected leaflets often remain on the tree, weathering to a grey colour with dark cavities, where spent apothecia have either shrivelled or fallen out.
List of Symptoms/SignsTop of page
|Leaves / abnormal colours|
|Leaves / abnormal colours|
|Leaves / necrotic areas|
|Leaves / necrotic areas|
|Whole plant / seedling blight|
|Whole plant / seedling blight|
Biology and EcologyTop of page Most observations of cedar leaf blight damage are on western red cedar (Thuja plicata), which is the most susceptible host (Boyce, 1927; Peace, 1962; Foster and Wallis, 1969; Wood, 1986). The disease is favoured by prolonged foliage wetness and restricted air movement, conditions that occur in nurseries where seedlings are grown at high density and reforestation sites where seedlings are over-grown by ground cover plants (Lainer, 1964; Kope and Sutherland, 1994; Kope et al., 1996). Individual T. plicata trees vary in susceptibility to the disease and, although all age classes are affected, seedlings and young trees suffer most (Rankin, 1927; Boyce, 1961; Phillips and Burdekin, 1982; Sinclair et al., 1987; Kope and Dennis, 1992). Damage is usually minimal on 1-year-old seedlings, where it is confined to juvenile needles on the lower stem, whereas severe damage can occur on 2- to 4-year-old seedlings where the disease progresses upwards among the branches. Burdekin (1970) suggested that several lesions on branches or stems might result in girdling injuries.
Pathogen-produced toxins may have a role in the killing of host tissue (Anon., 1967). A physiological change in T. plicata appears to account for an increase in blight resistance when trees are 4 or 5 years old (Soegaard, 1969). The consequence of cedar leaf blight infection of the foliage of mature cedar trees (>50 year of age) is unknown; mortality is rare and loss of incremental growth may be the chief result. However, cedar leaf blight may be a pioneering fungal pathogen that induces stress on the host tree allowing a succession of further disease-causing organisms.
The disease requires two growing seasons before the symptoms become apparent (Phillips and Burdekin, 1982; Sutherland et al., 1995). In natural stands of its host, infection occurs during one growing season and apothecial formation occurs during the next growing season. When apothecial formation occurs in the first year, such as 1-year-old seedlings grown in a reforestation nursery, infection and subsequent disease expression is often restricted to the juvenile needles on the stem and, in some cases, to the lower branches (Kope and Sutherland, 1994). It is rare for apothecial formation to occur in the same growing season as infection.
New infections are initiated by ascospores and the disease is spread only by ascospores. These spores are released during two periods in a growing season in the Pacific Northwest of North America, in the spring from April to the end of June, and in the autumn from late September to early November (Kope and Sutherland, 1994). More ascospores are released during the spring season release period than during the autumn season release period, thus an ascospore release cycle can be predicted throughout a growing season. In Denmark, Søegaard (1969) found two periods of ascospore discharge: one in early June and another from late September to early October. In England, UK, ascospores are most abundant from June to July and from September to October (Phillips, 1967; Burdekin, 1968).
Ascospore germination forms an infection peg, which penetrates the host mesophyll cells mechanically rather than via stomata (Porter, 1957; Pawsey, 1960; Søegaard, 1969). Although the mycelium spreads throughout the mesophyll, it does not penetrate the vascular bundles and its growth is limited to cells of an individual infected leaflet (Pawsey, 1960). At temperatures above 15°C, the affixed ascospores can germinate within 12 hours and penetrate the host leaflet. The germination rate drops sharply below 15°C, but affixed ascospores can remain quiescent. Fungal growth within the leaflet requires the accumulation of approximately 1200 growing degree days above 5°C in a glasshouse environment (Kope and Trotter, 1998b). Ordinarily, in natural stands of its host, the disease cycle occurs over two growing seasons with an accumulation of degree days during the first growing season, overwintering, then a further accumulation of degree days to approximately 1480 (Kope and Trotter, 1998b) before apothecia form.
When the apothecia are fully mature and become wet, they swell and break through the leaf epidermis. The swelling pushes up the fruiting body and exposes it to the circulating air. As the apothecia dries in the circulating air, the ascospores are forcibly released. At temperatures between 10 and 20°C, ascospores can be released for up to 6 hours; at temperatures below 10°C, very few ascospores are released over a shorter period. Almost no ascospore release occurs below 5°C. Ascospore release stops when the epidermal scale folds over and covers the fruiting body until the next episode of wetting. A repeating cycle of wetting and drying of apothecia (i.e., irrigation) at temperatures between 10 and 20°C will cause the release of all viable ascospores during 5 days. Should intervals between wettings be prolonged, ascospores within apothecia can remain viable for up to 14 days. Ascospore release will recur once the apothecia are rewetted. However, apothecia and ascospores do not last indefinitely and during a prolonged dry period they will be colonized by other microorganisms such as Phoma sp., thus destroying the ascospores and apothecia. If the apothecia are continually wet, ascospore release can continue until exhausted, a period of 2 days. At cooler temperatures the ascospores and fruiting bodies remain viable but will be colonized by other fungi. Ascospores are not released until the temperature reaches 5°C.
Means of Movement and DispersalTop of page Ascospores of D. thujina are forcibly released up to 5 mm from the apothecium (Burmeister, 1966) and can be caught in the ambient airflow and carried considerably greater distances. Ascospores are thick-walled and have a sticky gelatinous covering, which prevents desiccation and aids the ascospore in adhering to its host. Once stuck to the host leaflet, the ascospore cannot be removed without being destroyed.
Seedborne AspectsTop of page As cedar leaf blight occurred on seedlings in some nurseries with no obvious external sources of inoculum, Pethybridge (1919) suggested that the fungus might be seedborne. Alcock (1928) felt that the pathogen could be present on cones or debris with seeds. However, there was no proof in either report that the disease is transmitted from one tree to another by this means. Kope and Sutherland (1994) observed ascospores on cedar seed, but the viability of the attached ascospores was not determined. It was assumed the strong adhesion of ascospores to surfaces via their sticky coating makes subsequent lateral transfer of the ascospores unlikely.
Plant TradeTop of page
|Plant parts liable to carry the pest in trade/transport||Pest stages||Borne internally||Borne externally||Visibility of pest or symptoms|
|Leaves||fruiting bodies||Yes||Yes||Pest or symptoms usually invisible|
|Seedlings/Micropropagated plants||fruiting bodies; spores||Yes||Yes||Pest or symptoms usually invisible|
|True seeds (inc. grain)||spores||Yes||Pest or symptoms not visible to the naked eye but usually visible under light microscope|
Impact SummaryTop of page
|Fisheries / aquaculture||None|
|Fisheries / aquaculture||None|
ImpactTop of page In North America seedling mortality rates of up to 97% have been recorded resulting in large economic losses to growers. Seedling mortality of nursery-grown seedlings also occurs when they are planted into reforestation areas. Seedling mortality greatly reduces the number of plants suitable for planting. Kope and Trotter (1998b) have shown that the growth and yield of younger <4 years of age) western red cedar trees can be affected by cedar leaf blight. A decrease of 30% in stem diameter, 50% in shoot biomass and 35% in root biomass has been recorded for blight-affected trees compared to unblighted trees in reforestation sites. This decrease in tree growth and yield can contribute to additional years of tree maintenance in reforestation sites, before the trees can compete with other plants and the site is considered adequately reforested.
Economic ImpactTop of page In North America seedling mortality rates of up to 97% have been recorded resulting in large economic losses to growers. Seedling mortality of nursery-grown seedlings also occurs when they are planted into reforestation areas. Seedling mortality greatly reduces the number of plants suitable for planting. Kope and Trotter (1998b) have shown that the growth and yield of younger <4 years of age) western red cedar trees can be affected by cedar leaf blight. A decrease of 30% in stem diameter, 50% in shoot biomass and 35% in root biomass has been recorded for blight-affected trees compared to unblighted trees in reforestation sites. This decrease in tree growth and yield can contribute to additional years of tree maintenance in reforestation sites, before the trees can compete with other plants and the site is considered adequately reforested.
Environmental ImpactTop of page There are no known environmental impacts due to D. thujina.
DiagnosisTop of page An electrophoresis technique employed by Kope et al. (1998) analyzed the proteins of disease-free and D. thujina-infected leaves of Thuja plicata and revealed differences in several protein bands, suggesting that distinct proteins of D. thujina origin can be identified by SDS-PAGE.
Detection and InspectionTop of page Detection of cedar leaf blight is via visual inspection. The characteristic signs and symptoms of the disease include individual infected brown leaflets scattered among the contrasting adjacent healthy green foliage. As the disease develops, apothecia within the brown leaflets become conspicuous as golden-olive to dark-brown, circular pustules.
Similarities to Other Species/ConditionsTop of page Winter bronzing (Phillips and Burdekin, 1982) or leaf browning (Moore and Green, 1976) can appear similar to a severe attack of cedar leaf blight. These conditions can be strikingly evident on cedar seedlings grown in reforestation nurseries in the winter. They can be differentiated from cedar leaf blight by the bronze coloration occurring on all leaflets on a branch rather than the browning of individual leaflets on a branch, with many unaffected adjacent leaflets remaining green. Furthermore, bronzing is a winter phenomenon, whereas cedar leaf blight occurs during the spring, summer and autumn.
Prevention and ControlTop of page
Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.Introduction
The same conditions that make nursery production of seedlings successful - high densities, favourable temperatures and moisture, and intensive crop management techniques - are also ideal for the Keithia leaf blight fungus. Healthy trees result when pest management is integrated with other aspects of nursery culture throughout the crop cycle.
Numerous studies have been conducted using fungicides to control Keithia blight on nursery seedlings. As early as 1916, Weir found that a soap-Bordeaux mixture provided protection when applied at 10-day, or shorter, intervals. In the UK, several fungicides were tested against Keithia blight. Inconclusive results were obtained with a lime sulphur mixture (Peace, 1955), probably because the material was short lived. Pawsey (1962b), reporting on trials conducted over 10 years, found that a high degree of resistance to various fungicides developed but some protection was provided by copper-based fungicides. Burdekin (1969) found that the fungicide cycloheximide and its derivatives were effective against blight. Although there was some phytotoxicity with cycloheximide, there was none with oxime, semicarbazone and other acetate derivatives (Pawsey, 1964; Phillips, 1965, 1966). Further experiments (Pawsey, 1964; Phillips, 1965, 1966) demonstrated that if fungicides were applied once per month in March, April and June, all concentrations of cycloheximide derivatives reduced blight severity. Additional applications in the summer and autumn continued to reduce blight levels, but the cycloheximide derivatives were not systemic and had no effect on blight reduction the following spring. Fungicides that were ineffective against the disease included zineb and maneb (Pawsey, 1964), captan (Burdekin, 1968), fentin hydroxide, diodine acetate, diathion, streptomycin and an organomercurial fungicide (Phillips, 1967). In follow-up studies, Burdekin and Phillips (1971) found that one application of cycloheximide in March, another in April, and sometimes again in June, gave the best disease control, probably because spore germination was inhibited, which limited the build up and spread of Keithia blight. In France, Boudier (1983) tested several fungicides against Keithia blight. Two application schedules were tested, i.e., one where the fungicides were applied every 3 weeks from June to October and another in which they were applied every 3 weeks from September to October. At both application schedules he found that the systemic triadimenol was the most effective in protecting seedlings into the next growing season whereas fixed copper was ineffective. The efficacy of fixed copper, benomyl and triadimefon was determined against blight on 1-year-old western red cedar in a Californian bareroot nursery (Frankel, 1990). Keithia blight was least severe on seedlings sprayed monthly from March through November with the systemic triadimefon. A subsequent trial demonstrated that three triadimefon applications in April, May and June, or in April, June and August controlled blight (Frankel, 1991, 1992).
More recently, Kope and Trotter (1998a) tested two fungicides for the control of cedar leaf blight in nurseries. Mancozeb is a foliar spray that must directly contact the target fungus to be effective. Its protective value is short-lived, thus the seedlings must be sprayed bi-weekly, at the recommended rate, for the entire growing season (12-14 applications). Propiconazole is a systemic fungicide that is absorbed through the leaves and translocated throughout the seedling. Propiconazole can be used to protect 1+0 or older container-grown or bare-root western red cedar seedlings. Applied at the recommended rate and at a frequency of once every 4 weeks to a maximum of six times within a growing season, propiconazole is effective in the control of Keithia leaf blight of western red cedar. If control of cedar leaf blight is to be achieved through fungicides alone, they must be applied early and throughout the growing season of 1-year-old seedlings and the full growing season of 2-year-old seedlings. The effect of the fungicide is to damage ascospores that are on the leaflet surface and drift into the growing areas during a spring release period and the newly forming fruiting bodies and their ascospores forming in the autumn on the foliage.
Various modifications of cultural practices have been tried for the control of cedar leaf blight. In the UK, western red cedar seedlings were grown in nurseries isolated from known sources of inoculum, but the results were inconclusive (Peace, 1958). Another suggestion was to grow Thuja in isolated nurseries (Pawsey, 1962a). Thuja would be raised from seed at isolated nurseries with a periodic clearance of all cedar stock before resowing. This was partly successful, but some outbreaks of Keithia blight occurred (Burdekin and Phillips, 1971). Growing bare-root nursery or transplant cedar at reduced densities (seedlings per unit area), e.g., to decrease humidity within the canopy, has provided some measure of blight control (Keatinge, 1948; Evans, 1950; Penistan, 1966). Growing cedar as a mixture with other species, both in seed and transplant beds, has also given some control of the disease (Peace, 1955).
Resistance to Keithia blight is known in Thuja. Søegaard (1954, 1956) inoculated both cuttings from a mature western red cedar tree and seedlings from the same tree, and showed that the cuttings were more resistant than the seedlings. Søegaard (1956, 1969) also determined that resistance is the result of a recessive gene in T. plicata. Crossing Thuja plicata with a Japanese species, Thuja standishii, where resistance was dominant, resulted in a 1:1 ratio of resistant to susceptible offspring. Søegaard suggested that cuttings could be produced from such hybrids.
On the basis of results over four growing seasons, Burmeister (1966) listed the disease ratings of several varieties of T. plicata, T. occidentalis and T. standishii. Two varieties of T. plicata, 'Fastigata' and 'Auerovariegata' were highly susceptible, whereas T. standishii was highly resistant. Porter (1957), in attempting to explain the variation in blight severity in British Columbia, Canada, collected T. plicata cuttings and inoculated some in the field and others in the laboratory. No resistance was found, regardless of host origin, inoculation site or technique. On the basis of these results, Porter (1957) concluded that the microclimate is mainly responsible for differences in blight severity.
Older infected cedar stock or infected cedar trees surrounding a nursery can provide inoculum and the ascospores can travel through the air. Airborne ascospores can be kept out of cedar-rearing greenhouses by keeping a roof and side walls in place until after the spring ascospore release period (end of June, on Vancouver Island and the lower mainland of North America). This practice directly prevents ascospores from coming in contact with the cedar crop.
Through the summer months of July, August and September, when climatic conditions are warmer and drier, circulating dry air through the crop will decrease the time that foliage remains wet. As the crop matures, foliage density increases and moisture is retained on leaf surfaces, especially the lower branches. Free water is required for the germination and infection of leaves by the Keithia leaf blight fungus. Foliage wetness can be decreased by separating the growing containers to allow more efficient drying out between waterings.
From October through to the end of November, air should be allowed to circulate through the crop, and where possible, seedlings should be protected from the rain. If infection has occurred, fruiting bodies can form and disease spread will be encouraged by the presence of moisture on the infected foliage.
Seedlings held over for growth for another year into the spring are particularly susceptible to Keithia leaf blight infection and expression. Such seedlings must be protected throughout the previous growing season, or disease expression will occur early in the spring. The first signs of the disease are often subtle and overlooked in the early spring and the disease can easily spread. By applying fungicides to the crop in the first growing season and continuing through the second growing season, a disease-free crop of 2+0 western red cedar can be produced.
ReferencesTop of page
Adams JF, 1918. Keithia on Chamaecyparis thyoides. Torreya, 18:157-160.
Anon, 1960. United States Department of Agriculture. Agricultural Handbook, Number 165. Washington, DC, USA: USDA.
Anon., 1953. Notes phytosanitaires. Evolution des maladies des plantes en 1951. Annales de L’institut national de recherche agronomique, serie C (Annales des Épiphyties), 511-515.
Anon., 1985. Insects and diseases of Alaskan forests. Alaska Region Report Number 181. USDA Forest Service.
Boudier B, 1983. Didymascella thujina principal ennemi du Thuya dans l’ouest de la France. Phytoma - Defense des cultures, novembre, 51-56.
Boyce JS, 1927. Observations on forest pathology in Great Britain and Denmark. Phytopathology, 17:1-18.
Buchwald NF, 1936. En ny svampesygdom i Danmark. Didymascella thujina paa Thuja plicata. Dansk Skovforenings Tidsskrift, 21:51-59.
Burdekin DA, 1968. Needle blight of Western Red Cedar caused by Didymascella thujina. Report on Forestry Research. Forestry Commission, London.
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ContributorsTop of page
18/05/04 Original text by:
Harry H Kope, Resource Practices Branch, British Columbia Ministry of Forests, Lands and Natural Resource Operations, Victoria, BC, Canada
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