This disease, caused by the fungus Didymascella thujina (Dur.) Maire, is the most damaging disease of western redcedar, Thuja plicata Donn, seedlings in British Columbia forest nurseries. Importance of the disease has increased in recent years with increasing production of 2-year-old, container-grown, western redcedar. Although the disease occurs on trees, including those around nurseries, throughout the range of redcedar in British Columbia, damage is particularly severe in coastal nurseries on container-grown seedlings (Figure 123). It is also severe on bareroot transplants (Figure 124), especially if they have acquired the disease as container-grown stock. In the Interior, very low levels of the disease, have been found on container-grown redcedar in wet belt nurseries and on bareroot transplants grown as a 1+0 container crop on the coast.
The disease occurs on several species of cedar including eastern white cedar, T. occidentalis L., Port Orford cedar, Chamaecyparis lawsoniana (A. Murr.) Par., and western redcedar. In local forest nurseries the disease is only of concern on the latter as it is the only host species grown for reforestation. On 1-year-old seedlings, symptoms first become evident from mid-summer through early fall on the lowermost foliage (Figure 125). Initial symptoms appear as one or more whitish, bleached areas on individual scale-leaves, usually on the upper surface. These spots coalesce, turn brown, and involve the entire leaf, each eventually becoming fruit bodies (apothecia).
Young fruit bodies appear as slightly swollen spots, usually on the upper leaf surface, later becoming red-to-olive brown against the dull background of the killed tissue. The entire awl-shaped leaf becomes brown, contrasting with the adjacent green, healthy foliage (Figure 126).
As they mature the fruit bodies , swell, breaking the epidermis around their margins, leaving each fruit body with a distinctive cover flap of host epidermis (Figure 127). When wet, the flap opens, exposing the golden-olive, spore-producing layer. Dry fruit bodies appear much darker and the covering flap is normally closed. Old fruit bodies may fall out or shrivel, leaving a cavity in their place. Death of fruit bodies and tissue may also occur on the awl-shaped needles on stems and on the stem itself. Indeed, stem infections may pose the greatest threat because when abundant, they can girdle a stem.
Disease that becomes established on 1-year-old nursery seedlings is of special concern. This special concern is due to their providing inoculum for subsequent disease spread if the crop is grown for a second year, or if it is later transplanted to bareroot beds. If left unchecked in the nursery, Keithia blight spreads both laterally among seedlings and upward on the shoots of affected seedlings. On the latter, it can eventually affect all but the most recent terminal and lateral growth. Severely affected seedlings are usually culled at lifting. Indeed, evidence suggests that even low levels of nursery acquired blight can result in further disease development and spread on seedlings outplanted to reforestation sites, especially wet sites. Survival of diseased seedlings may also be lower following outplanting to reforestation sites.
In the spring, when temperature and moisture conditions are favorable, inoculum is blown into nurseries on wind or rain. Such inoculum overwinters as quiescent or immature fruit bodies on diseased cedar seedlings, hedges, or ornamentals in or adjacent to the nursery. Except for Port Orford cedar, only species of Thuja are hosts. Although spores of the fungus commonly occur on cedar seed, their role in disease out-breaks is unknown, but it might be important as surveys to relate blight occurrence in nurseries to presence of diseased cedars in and around nurseries showed no clear relationships. Regardless of inoculum source, infection of 1-year-old nursery seedlings occurs in late spring through early summer during prolonged periods of high moisture and temperatures of 10-20' C, both of which favor spore liberation and germination, and infection. Free moisture on seedling foliage is a prerequisite for infection. hi mid-to-late summer, initial disease symptoms and fungus fruit bodies appear on the lowermost needles of affected seedlings. These fruit bodies provide spores for subsequent spread and development of the disease throughout the summer and fall. Disease that develops during the first growing season provides inoculum for subsequent blight development on 2-year-old seedlings or transplants.
All diseased cedar seedlings, hedge plants, and adjacent trees should be removed from the nursery site as they are important sources of inoculum. Since bits of needles and other debris may harbor the pathogen, all seeds should be thoroughly cleaned before sowing. Nurseries with Keithia blight histories, or those located in areas with blight nearby, should closely monitor the 1-year-old cedar crop for initial symptoms. If the disease is detected, a recommended fungicide control program, should be started to prevent blight spread on the crop during the current and subsequent years. To protect crops from inoculum outside the nursery, and from inoculum produced on the crop itself, fungicides should be applied following rainy, spring weather or following prolonged irrigation during; the growing season. Control of the disease during the first growing season is the key to preventing losses during the second year, and on bareroot transplants. Where practical, watering should be curtailed. If the crop is outdoors, it may be worthwhile to move it into a green-house to protect it from rain.
Spacing growing containers to decrease moisture levels by improving air circulation may help. All seedlings that are to be transplanted to bareroot beds should be either disease free or treated after transplanting to control the disease. Diseased trees should be removed from around bareroot fields where cedar are to be grown. Blight-free nurseries should be especially cautious of accepting diseased crops from other nurseries, or any cedar crops from nurseries with a history of disease. Although not always practical, the ultimate solution to the problem is to grow cedar in Okanagan Valley nurseries without nearby sources of inoculum and where weather conditions are not conducive for 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 (editors). INRA Editions, Paris, pp.27-44.
Sinclair, W.A., H.H. Lyon, and W.T. Johnson. 1987. Diseases of trees and shrubs. Comstock Publishing Assoc., a division of Comell Univ. Press, Ithaca, N. Y. and London.
|
Principal, locally grown hosts |
Host age and season when damage appears |
|
Nursery type and location |
|
|
|
|---|---|---|---|---|---|---|
|
Bareroot |
Container |
|||||
|
Age |
Season |
Coastal |
Interior |
Coastal |
Interior |
|
|
Western redcedar |
1+0 |
Late spring through fall |
? |
? |
Yes |
No |
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Figure 123. Severe Keithia blight on 1+0, container-grown, western redcedar.
Figure 124. Severe Keithia blight on western redcedar bareroot transplants. noth the seedlings had been top pruned.
Figure 125. keithia blight on lower foliage of 1+0, container-grown, western redcedar.
Figure 126. Keithia blight symptoms. Note the contrasting colour of diseased (brown) and healthy, (green) foliage.
Figure 127. Spore producing bodies of Kethia blight. Note the covering flaps (at arrows) of host epidermis.
Figure 128. Life history of Keithia blight.