Didymascella thujina (E. J. Durand) Maire
(= Keithia thujina E. J. Durand)
Ascomycotina, Rhytismatales, Hypodermataceae
Hosts: Didymascella thujina is found only on western redcedar in B.C. Elsewhere in North America it is also reported on northern white-cedar and Oriental Arborvitae.
Distribution: This fungus is widely distributed throughout the range of western redcedar in B.C.
Identification: The disease appears on trees of all sizes, but most commonly on young seedlings and the lower branches of older trees. Symptoms first appear in the spring on 1-year old foliage as bleached, tan-brown areas on individual scale-leaves. The infected brown leaves are usually conspicuous against the healthy green foliage (Fig. 50a). Fruiting bodies (apothecia) become readily visible by June as olive-brown (maturing to black) spots on the upper surface of infected leaves (Fig. 50b). Usually one, but up to three apothecia form on each leaf. Twigs with heavily infected leaves are generally shed in the autumn. On diseased leaves that remain, apothecia often shrivel and drop out, leaving dark pits.
Microscopic Characteristics: Ascomata mainly on upper surface of needles, subepidermal, roughly circular in outline, olive-brown, up to 1 mm in diameter; hymenium not covered by fungal tissue, exposed by rupture of overlying epidermis. Ascomata may fall out completely when spent leaving a pit in the leaf. Asci clavate, 2-spored, pore not blue in iodine (J-), 100 x 20 µm. Ascospores ellipsoid, thick-walled, 1-septate near the upper end, walls pitted, brown, 22-25 x 15-16 µm, with a gelatinous sheath. Paraphyses filiform, branched, thickened at the tips.
Damage: Young seedlings and saplings sustain the most damage where stem or branch death may occur. Disease on trees older than 4-5 years can retard growth. Disease levels are highest in dense stands where humidity levels are high. Such conditions can occur in forest nurseries where the disease can be a serious problem.
Remarks: Cedar leaf blight (also known as Keithia blight) can be confused with normal foliage colour changes that occur on western redcedar in the autumn. However, seasonal colour changes affect the entire plant in contrast to the scattered symptoms of the disease.
Kope, H. H. and J. R. Sutherland. 1994. Keithia blight; review of the disease, and research on container grown western redcedar in British Columbia, Canada. In Diseases and Insects in Forest Nurseries. R. Perrin and J. R. Sutherland, eds. INRA Editions, Paris. pp. 27-44.
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Figure 50a: Western redcedar infected with Didymascella thujina.
Figure 50b: Apothecia of Didymascella thujina on western redcedar scale-leaves.