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Didymascella thujina
(cedar leaf blight)

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Datasheet

Didymascella thujina (cedar leaf blight)

Summary

  • Last modified
  • 27 September 2018
  • Datasheet Type(s)
  • Invasive Species
  • Pest
  • Preferred Scientific Name
  • Didymascella thujina
  • Preferred Common Name
  • cedar leaf blight
  • Taxonomic Tree
  • Domain: Eukaryota
  •   Kingdom: Fungi
  •     Phylum: Ascomycota
  •       Subphylum: Pezizomycotina
  •         Class: Dothideomycetes
  • There are no pictures available for this datasheet

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    Compendia
    CAB International
    Wallingford
    Oxfordshire
    OX10 8DE
    UK
    compend@cabi.org
  • Distribution map More information

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Pictures

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PictureTitleCaptionCopyright
Close-up of dead leaflets, mature apothecia and an epidermal scale covering an apothecium of a D. thujina infected Thuja plicata branch.
TitleSymptoms
CaptionClose-up of dead leaflets, mature apothecia and an epidermal scale covering an apothecium of a D. thujina infected Thuja plicata branch.
CopyrightHarry H. Kope
Close-up of dead leaflets, mature apothecia and an epidermal scale covering an apothecium of a D. thujina infected Thuja plicata branch.
SymptomsClose-up of dead leaflets, mature apothecia and an epidermal scale covering an apothecium of a D. thujina infected Thuja plicata branch.Harry H. Kope
Close-up of a grey-coloured leaflet with dark cavities where the apothecia have either shrivelled up or fallen out, leaving a hole through the leaflet of a D. thujina-infected Thuja plicata branch.
TitleSymptoms
CaptionClose-up of a grey-coloured leaflet with dark cavities where the apothecia have either shrivelled up or fallen out, leaving a hole through the leaflet of a D. thujina-infected Thuja plicata branch.
CopyrightHarry H. Kope
Close-up of a grey-coloured leaflet with dark cavities where the apothecia have either shrivelled up or fallen out, leaving a hole through the leaflet of a D. thujina-infected Thuja plicata branch.
SymptomsClose-up of a grey-coloured leaflet with dark cavities where the apothecia have either shrivelled up or fallen out, leaving a hole through the leaflet of a D. thujina-infected Thuja plicata branch.Harry H. Kope
Apothecium of D. thujina colonized by pycnidia of Phoma sp.
TitleApothecium
CaptionApothecium of D. thujina colonized by pycnidia of Phoma sp.
CopyrightHarry H. Kope
Apothecium of D. thujina colonized by pycnidia of Phoma sp.
ApotheciumApothecium of D. thujina colonized by pycnidia of Phoma sp.Harry H. Kope
D. thujina-infected branch from a 10-year-old Thuja plicata tree. Dead leaflets are visibly scattered among the green healthy uninfected leaflets.
TitleSymptoms
CaptionD. thujina-infected branch from a 10-year-old Thuja plicata tree. Dead leaflets are visibly scattered among the green healthy uninfected leaflets.
CopyrightHarry H. Kope
D. thujina-infected branch from a 10-year-old Thuja plicata tree. Dead leaflets are visibly scattered among the green healthy uninfected leaflets.
SymptomsD. thujina-infected branch from a 10-year-old Thuja plicata tree. Dead leaflets are visibly scattered among the green healthy uninfected leaflets.Harry H. Kope
D. thujina-infected branches of a 20-year-old Thuja plicata tree in a forest on Vancouver Island, British Columbia, Canada.
TitleSymptoms
CaptionD. thujina-infected branches of a 20-year-old Thuja plicata tree in a forest on Vancouver Island, British Columbia, Canada.
CopyrightHarry H. Kope
D. thujina-infected branches of a 20-year-old Thuja plicata tree in a forest on Vancouver Island, British Columbia, Canada.
SymptomsD. thujina-infected branches of a 20-year-old Thuja plicata tree in a forest on Vancouver Island, British Columbia, Canada.Harry H. Kope
D. thujina-infected 1-year-old Thuja plicata seedlings grown in containers at a reforestation nursery on Vancouver Island, British Columbia, Canada.
TitleInfected seedlings
CaptionD. thujina-infected 1-year-old Thuja plicata seedlings grown in containers at a reforestation nursery on Vancouver Island, British Columbia, Canada.
CopyrightHarry H. Kope
D. thujina-infected 1-year-old Thuja plicata seedlings grown in containers at a reforestation nursery on Vancouver Island, British Columbia, Canada.
Infected seedlingsD. thujina-infected 1-year-old Thuja plicata seedlings grown in containers at a reforestation nursery on Vancouver Island, British Columbia, Canada.Harry H. Kope
A cross-section of an apothecium of D. thujina.
TitleApothecium
CaptionA cross-section of an apothecium of D. thujina.
CopyrightHarry H. Kope
A cross-section of an apothecium of D. thujina.
ApotheciumA cross-section of an apothecium of D. thujina.Harry H. Kope
Asci of D. thujina containing two ascospores.
TitleAsci
CaptionAsci of D. thujina containing two ascospores.
CopyrightHarry H. Kope
Asci of D. thujina containing two ascospores.
AsciAsci of D. thujina containing two ascospores.Harry H. Kope
Scanning electron micrograph (SEM) of a germinated ascospore of D. thujina. Note the sticky mucilaginous sheath at the base of the ascospore, which cements the ascospore to the leaf surface (note scale bar; original magnification x 3300).
TitleGerminated ascospore
CaptionScanning electron micrograph (SEM) of a germinated ascospore of D. thujina. Note the sticky mucilaginous sheath at the base of the ascospore, which cements the ascospore to the leaf surface (note scale bar; original magnification x 3300).
CopyrightHarry H. Kope
Scanning electron micrograph (SEM) of a germinated ascospore of D. thujina. Note the sticky mucilaginous sheath at the base of the ascospore, which cements the ascospore to the leaf surface (note scale bar; original magnification x 3300).
Germinated ascosporeScanning electron micrograph (SEM) of a germinated ascospore of D. thujina. Note the sticky mucilaginous sheath at the base of the ascospore, which cements the ascospore to the leaf surface (note scale bar; original magnification x 3300).Harry H. Kope

Identity

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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

EPPO code

  • DIDSTH (Didymascella thujina)

Taxonomic Tree

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  • 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 Nomenclature

Top 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.

Description

Top 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.

Distribution

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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 Table

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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.

Continent/Country/RegionDistributionLast ReportedOriginFirst ReportedInvasiveReferenceNotes

North America

CanadaPresentPresent based on regional distribution.
-British ColumbiaWidespreadNative Invasive Kope, 2000
-New BrunswickWidespreadNative Invasive Kope, 2000
-Newfoundland and LabradorWidespreadNative Invasive Kope, 2000
-OntarioWidespreadNative Invasive Kope, 2000
-Prince Edward IslandWidespreadNative Invasive Kope, 2000
-QuebecWidespreadNative Invasive Kope, 2000
USAPresentPresent based on regional distribution.
-AlaskaWidespreadNative Invasive Anon., 1985
-CaliforniaWidespreadNative Invasive Shaw, 1973
-IdahoWidespreadNative Invasive Shaw, 1973
-MichiganWidespreadNative Invasive Anon, 1960
-New YorkWidespreadNative Invasive Anon, 1960
-OregonWidespreadNative Invasive Shaw, 1973
-TexasWidespreadNative Invasive Anon, 1960
-VermontPresentNative Invasive Anon, 1960
-VirginiaPresentNative Invasive Anon, 1960
-WashingtonWidespreadNative Invasive Shaw, 1973
-WisconsinPresentNative Invasive Anon, 1960

Europe

AustriaPresentIntroduced Invasive Pardatscher, 1977
BelgiumWidespreadIntroduced Invasive Boudru, 1945
Czech RepublicPresentIntroduced Invasive Prihoda, 1986
DenmarkWidespreadIntroduced Invasive Ferdinandsen et al., 1938; Buchwald, 1936
FranceRestricted distributionIntroduced Invasive Anon., 1953
GermanyRestricted distributionIntroduced Invasive Burmeister, 1966
IrelandWidespreadIntroduced Invasive Pethybridge, 1919; Forbes, 1920; Forbes, 1921
ItalyWidespreadIntroduced Invasive Vegni and Ferro, 1964
LithuaniaPresentJuronis et al., 2006
NetherlandsWidespreadIntroduced Invasive van Poetern, 1931
NorwayWidespreadIntroduced Invasive Jorstad and Roll-Hasen, 1943
PolandPresentIntroduced Invasive Benben, 1969
SlovakiaPresentIntroduced Invasive Holubcik, 1978
SpainPresentIntroduced Invasive Fernandez-Magan, 1974
SwitzerlandPresentIntroduced Invasive Peace, 1955
UKRestricted distributionIntroduced Invasive Loder, 1919; Miles, 1922; Boyce, 1927; Alcock, 1928; Laing, 1929

History of Introduction and Spread

Top 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 Introduction

Top 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.

Habitat

Top 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 List

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CategorySub-CategoryHabitatPresenceStatus
Terrestrial
Terrestrial – ManagedCultivated / 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-naturalNatural 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)
Littoral
Coastal areas Present, no further details Harmful (pest or invasive)

Hosts/Species Affected

Top 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.

Growth Stages

Top of page Seedling stage, Vegetative growing stage

Symptoms

Top 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/Signs

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SignLife StagesType
Leaves / abnormal colours
Leaves / abnormal colours
Leaves / necrotic areas
Leaves / necrotic areas
Whole plant / seedling blight
Whole plant / seedling blight

Biology and Ecology

Top 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.

Disease Cycle

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 Dispersal

Top 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 Aspects

Top 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 Trade

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Plant parts liable to carry the pest in trade/transportPest stagesBorne internallyBorne externallyVisibility 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 Summary

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CategoryImpact
Animal/plant collections None
Animal/plant collections None
Animal/plant products None
Animal/plant products None
Biodiversity (generally) None
Biodiversity (generally) None
Crop production None
Crop production None
Environment (generally) None
Environment (generally) None
Fisheries / aquaculture None
Fisheries / aquaculture None
Forestry production Negative
Forestry production Negative
Human health None
Human health None
Livestock production None
Livestock production None
Native fauna None
Native fauna None
Native flora None
Native flora None
Rare/protected species None
Rare/protected species None
Tourism None
Tourism None
Trade/international relations None
Trade/international relations None
Transport/travel None
Transport/travel None

Impact

Top 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 Impact

Top 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 Impact

Top of page There are no known environmental impacts due to D. thujina.

Diagnosis

Top 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 Inspection

Top 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/Conditions

Top 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 Control

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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.

Chemical Control

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.

Cultural Control

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.

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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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