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Citrus variegated chlorosis

Scientific Name
Xylella fastidiosa
Scientific Author
Wells et al.
Taxonomy
Gracilicutes

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Diagnostic Notes
Symptoms:

Citrus variegated chlorosis (CVC): Symptoms may occur in twigs, leaves and fruits. The plants do not usually die. Symptoms can manifest on trees from nursery age to over ten years old. Symptoms in older trees (15 years) are generally restricted to a few branches. Symptoms are more obvious on 3–6-year-old trees and mainly on sweet orange cultivars. The characteristic symptoms are small interveinal chlorotic yellow spots, resembling zinc deficiency, on the upper side of mature leaves, which correspond to slightly raised gummy lesions, light to dark brown on the underside of leaves. On a newly affected tree, symptoms generally appear in sectors, whereas old infections are more uniform throughout the canopy. Fruits are much smaller than normal, very firm and ripen earlier. The growth rate of affected trees is greatly reduced and twigs and branches may wilt. When chronic, the disease can cause stunting and dieback of twigs (Laranjeira et al. 2002, Crop Protection Compendium 2005, EPPO 2005).

Pierce’s disease of grapevine (PD): on Vitis spp, the typical symptom is leaf scorching in conjunction with areas of chlorosis, progressing into necrosis at the leaf margins. However, the disease is also commonly associated with defoliation, shoot dwarfing, dehydration of fruit clusters, cane stunting, irregular maturation of cork of infected canes (leading “green islands”), and eventual plant death (Krivanek et al. 2005).

Coffee leaf scorch (CLS): on coffee, symptoms appear on young flushes of field plants as large marginal and apical scorched areas on recently mature leaves. Symptoms begin with apical and marginal leaf scorch, reduction in internode length of new flush, small, pale green to yellow leaves, shoot dieback, and overall plant stunting. Fruit size and yield are generally reduced. Symptoms are more evident during the winter, especially during periods of water stress. In the spring, with rain, affected leaves drop and plant appearance improves as new shoots are produced. Actual plant death due to the disease may take several years to occur (Lima et al. 1998).

On other hosts, typical symptoms include abnormally shaped fruit, leaves with lesions and/or abnormal colour, pattern, or shape. Abscission or death of leaves can also occur. Stems can show both internal and external discoloration, as well as dieback and abnormal growth (Crop Protection Compendium 2005).

The bacteria:

Gram-negative, non-motile and non-flagellate rods (0.25—0.35 × 0.9—3.5 µm). Can sometimes form filamentous strands under some conditions, but usually they are single. It is an obligate aerobe, inhibited by 2.5% carbon dioxide. It is non-pigmented, non-halophilic, non-fermentative, Kovacs’ oxidase negative and catalase positive. Two colonies types occur: umbonate and rough with finely undulate margins, or convex, smooth and entire (Bradbury 1991). Further detail of the biochemical and physiological characters are found in Bradbury (1991) and Wells (1987).

Xylella fastidiosa is a fastidious bacterium, xylem-limited and is transmitted by leafhopper vectors (Hemiptera: Cicadellidae) and infected plant material (Purcell & Hopkins 1996). It is difficult to isolate and grow in axenic culture. It does not grow on most common bacterial media. The PW medium (Davis et al. 1981) has gained the widest acceptance and supports the growth of all known strains of X. fastidiosa. Almeida et al. (2004) has proposed a simpler solid medium for supporting the growth of PD strains of X. fastidiosa. It was the first nonviral plant pathogen to have its genome completely sequenced (Simpson et al. 2000, Moreira et al. 2004)

While all strains of Xylella have been classified into a single species, there definitely are different pathotypes, most of which have not been characterized and compared with each other (Hopkins 2002). Schaad et al. (2004) proposed three subspecies: X. fastidiosa subspecies piercei, multiplex and pauca.

Comprehensive details of the
More Information
Specimen Contact
Dr Jose R. Liberato
Author
Liberato JR, Queiroz-Voltan, RB, Matsuoka K, Laranjeira FF & Miles AK
Created
06/06/2006 02:15 AEST
Last Updated
27/07/2016 05:10 AEST
Citation
Liberato JR, Queiroz-Voltan, RB, Matsuoka K, Laranjeira FF & Miles AK (2006) Citrus variegated chlorosis (Xylella fastidiosa ) Updated on 7/27/2016 5:10:00 AM Available online: PaDIL - http://www.padil.gov.au.
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Free for use under the Creative Commons Attribution 3.0 Australia License

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