Didymascella thujina

Didymascella thujina is an ascomycete from the family of Hemiphacidiaceae, which causes Cedar leaf blight (or Keithia disease, after the original genus name) in Northern Whitecedar (Thuja occidentalis) and Western Redcedar (Thuja plicata). A similar species (Didymascella juniperus) is known to be a disease causing agent in the Juniper tree.^[1] Its range extends from Canada and the USA to central and northern Europe, and is a source of seedling loss in natural forests and ornamental hedging throughout these areas.^[2]

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Cedar leaf blight spreads extensively by airborne ascospores in humid conditions^[2] and can affect large areas (up to several kilometres), though rarely lethal on hosts older than 4 or 5 years of age. Because of this, Cedar leaf blight is of greater concern in nurseries, where conditions are ideal for fungal growth and spread.^[3]

Symptoms appear as white spots that occur on the upper side of leaves, usually in spring, on one year-old seedlings. Over the course of the summer, leaves will turn reddish-brown as apothecia become apparent, leading to the potential misdiagnosis as this colour is similar to that of natural leaf death in cedars. However, the leaves will eventually develop black spots as the fungus matures to its reproductive state. As fall approaches, ascospores are released,^[3] and leaves that are gravely damaged will fall off.^[4] Spores will overwinter on uninfected green leaves, and begin their life cycle the following spring.^[3]

Control of the disease can be carried out with cycloheximide and has been done so with a high degree of effectiveness in the UK, particularly in nurseries.^[5] Resistance has also been developed to Cedar leaf blight by breeding Western Redcedar with Japanese Thuja (Thuja standishii),^[6] although for ecological reasons, this is often not feasible for reforestation in natural settings.

References

1. ^ Durand, Elias J. The Genus Keithia. 1913. Mycologia. Mycological Society of America. Vol. 5, No. 1.
2. ^ ^{a b} Minter, D. W. Didymascella thujina. 1997. Descriptions of Fungi and Bacteria. CABI Bioscience. Surrey. No. 134, p. 1334
3. ^ ^{a b c} Cedar Needle Blight. 2007. Natural Resources Canada. http://imfc.cfl.scf.rncan.gc.ca/maladie-disease-eng.asp?geID=1000002
4. ^ Ebata, Tim. Cedar Leaf Blight. Ministry of Forests and Ranges. 2002. http://www.for.gov.bc.ca/hfp/publications/00198/cedar_leaf_blight.htm
5. ^ Minore, Don. Western Redcedar-- A Literature Review. 1983. Department of Agriculture Forest Service. http://www.fs.fed.us/pnw/pubs/pnw_gtr150.pdf
6. ^ Soegaard, Bent. Resistance studies in Thuja. A. Investigations of resistance to attack by Didymascella thujina (Dur.) Maire in Thuja plicata D. Don and its hybrids with Thuja standishii (Gord.) Carr. B. 1969. Det Forstlige Forsogsvaesen. Denmark. Vol. 31 No. 3 p. 279-398.