[8] Didymascella thujina was found occurring on Port Orford cedar (Chamaecyparis lawsoniana) seedlings in a nursery in Washington, US.
[11][12] The name Keithia thujina was used by Elias Judah Durand in 1908 for the first record of the disease in the United States on eastern white cedar.
[35][36] Cedar leaf blight continued to spread to Norway,[37] Belgium,[38] France,[39] Switzerland,[24] Italy,[40] Germany,[8] Poland,[41] Spain,[42] Austria,[43] Slovakia,[44] and the Czech Republic.
Thuja has been introduced to other countries, including India, Australia, Chile and Ukraine,[46] but at present D. thujina has not been reported as a disease.
[7][9][51][52][53] A physiological change in western red cedar appears to account for an increase in blight resistance for certain individual trees when they reach 4 or 5 years of age.
[54] The use of degree days to establish biological events for Didymascella thujina, a foliar fungal leaf blight of Thuja plicata seedlings.
However, cedar leaf blight may be a pioneering fungal pathogen that induces some stress on the host tree allowing a further succession of biotic or abiotic problems.
Germinating ascospores cause very early symptoms on mature foliage that appear as several small cream-coloured spots on the upper surface of individual cedar leaflets.
Mycelial growth occurs within individual leaflets and once sufficient growing degree days have accumulated under suitable environmental conditions, apothecia are formed.
When dry, apothecia appear much darker and the epidermal scale often folds back over the apothecium, and no further ascospores are released.
Affected leaflets often remain on the tree, weathering to a grey colour with dark cavities where spent apothecia have either shrivelled or fallen out.
[57] 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 cedar leaf blight fungus.
In Britain, western red cedar seedlings were grown in nurseries isolated from known sources of inoculum, but the results were inconclusive.
[65] Growing bareroot nursery or transplant cedar at reduced densities (seedlings per unit area), e.g., to decrease within canopy humidity, has provided some measure of blight control.
Through the summer months (in the northern hemisphere) 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.
From October through to the end of November (in the northern hemisphere), air should be allowed to circulate through the crop, and where possible, seedlings should be protected from the rain.
Held-over seedlings must be protected throughout the previous growing season, or disease expression will occur early in the spring.
[76] Porter, in attempting to explain the variation in blight severity in British Columbia, collected T plicata cuttings and inoculated some in the field and others in the laboratory.
[75] In North America western red cedar seedling mortality rates of up to 97% have been recorded, resulting in large economic losses to growers.
To avoid significant mortality and growth reduction of planted western red cedar, it is recommended that reforestation efforts deploy adapted or disease resistant western red cedar seedlots in high risk environments common to coastal North America.