Xylella fastidiosa is the causal agent of Citrus variegated chlorosis disease
Since 1987 a new and serious disease, called citrus variegated chlorosis of sweet orange (Citrus sinensis) has been reported in Brazil. It spread rapidly in nurseries and commercial groves in Sao Paulo and Minas Gerais States. The symptoms are characterized by leaf patterns similar to zinc deficiency on new sprouts, conspicuous chlorotic variegation on the upper side of more developed leaves, and the undersides of variegated leaves contain small, light-brown, somewhat raised lesions which correspond to chlorotic areas on the upper leaf surface. These areas may become dark-brown or necrotic, punctiform or elongated, and occur in clusters or lines. Affected fruits are small and hard. Previous studies have led to the conclusion that the causal agent of this disease was structurally and morphologically similar to Xylella fastidiosa (EPPO A1 quarantine pest ; EU Annex I/A1) (1992, Chagas et al.). In EU Annex II/A1, the disease also appears separately as 'citrus variegated chlorosis'.
In recent studies, a xylem-limited bacterium serologically related to strains of Xylella fastidiosa (EPPO A1 quarantine pest) has been isolated from citrus trees in Brazil showing symptoms of citrus variegated chlorosis. After inoculation of this bacterium, typical symptoms of citrus variegated chlorosis were observed. The authors were able to reisolate the bacterium from petioles of these symptomatic and artificially inoculated plants, and its identity was confirmed by membrane entrapment immunofluorescence and Western blotting with the antiserum prepared against the original strain. The bacterium reisolated from symptomatic plant tissue was culturally, morphologically and serologically indistinguishable from the strain used to inoculate the plants (completing Koch's postulates and confirming the results obtained by Chang et al., 1993).
Other diseases of citrus have also been tentatively related to X. fastidiosa: 'pecosita' and 'citrus blight'. The first one occurs in Argentina. Though it is similar to citrus variegated chlorosis, it does not seem to be a limiting factor of citrus production. Concerning citrus blight, extracts from affected trees present a characteristic set of proteins which have not been found in extracts of trees affected by citrus variegated chlorosis. However, characteristic symptoms of 'citrus blight' have been obtained by inoculation of Citrus jambhiri cuttings with certain strains of X. fastidiosa (1988, EPPO RS 498/09) but Koch's postulates could not be completed.
The authors concluded that X. fastidiosa is the causal agent of citrus variegated chlorosis which can be considered as a strain of X. fastidiosa. Preliminary studies have demonstrated that this strain shows serological relationships with other strains of X. fastidiosa causing diseases of grapevine (Pierce's disease), almond (almond leaf scorch), ragweed (Ambrosia spp. - ragweed stunt disease) and oak (oak leaf scorch).
Sources
Chagas, C.M.; Rossetti, V.; Beretta, M.J.G. (1992) Electron microscopy studies of a xylem-limited bacterium in sweet orange affected with citrus variegated chlorosis disease in Brazil.
Journal of Phytopathology, 134, 306-312.
Hartung, J.S.; Beretta, J.; Brlansky, R.H.; Spisso, J.; Lee, R.F. (1994) Citrus variegated chlorosis bacterium: axenic culture, pathogenicity, and serological relationships with other stains of Xylella fastidiosa.
Phytopathology, 84 (6), 591-597.
Chang, C.J., Garnier, M.; Zreik, L.; Rossetti, V.; Bov‚, J.M. (1993) Culture and serological detection of the xylem-limited bacterium causing citrus variegated chlorosis and its identification as a strain of Xylella fastidiosa.
Current Microbiology, 27, 137-142.