Phenotypic Characteristics
X. citri is a Gram-negative, straight, rod-shaped bacterium measuring 1.5-2.0 x 0.5-0.75 µm. It is motile by means of a single, polar flagellum. It shares many physiological and biochemical properties with other members of the genus Xanthomonas. It is chemoorganotrophic and obligately aerobic with the oxidative metabolism of glucose. Colonies are formed on nutrient agar plates containing glucose and are creamy-yellow with copious slime. The yellow pigment is xanthomonadin. Catalase is positive, but Kovacs' oxidase is negative or weak;nitrate reduction is negative. Asparagine is not used as a sole source of carbon and nitrogen simultaneously;various carbohydrates and organic acids are used as a sole source of carbon. Hydrolysis of starch, casein, Tween 80 and aesculin is positive. Gelatine and pectate gel are liquefied. Growth requires methionine or cysteine and is inhibited by 0.02% triphenyltetrazolium chloride. Biovars may be distinguished by utilization of mannitol. For further information on the bacteriological properties of X. citri, see Goto (1992).
Strains of groups B, C and D have many properties in common with group A, the differences being detected by the utilization of only a few carbohydrates (Goto et al., 1980).
Molecular Characterization
Features of citrus-attacking xanthomonads including X. citri and the genus Xanthomonas as a whole, have been characterized at the molecular level for the development of quick and accurate methods for reclassification and identification. The procedures include DNA-DNA hybridization (Vauterin et al., 1995), genomic fingerprinting (Lazo et al., 1987), fatty acid profiling (Yang et al., 1993), SDS-PAGE (Vauterin et al., 1991) and isoenzyme profiles (Kubicek et al., 1989) and monoclonal antibodies (Alverez et al., 1991).
Bacteriophages
Phage-typing is applicable to X. citri with greater reliability than any other plant pathogenic bacterium investigated so far. Many strains of X. citri are lysogenic (Okabe, 1961). Two virulent phages, Cp1 and Cp2, can infect 98% of the strains isolated in Japan (Wakimoto 1967). Similar results were also obtained in Taiwan (Wu et al., 1993). The filamentous temperate phages and their molecular traits have been studied in detail (Kuo et al., 1994;Wu et al., 1996). Phage Cp3 is specific to the canker B strains (Goto et al., 1980). No phages specific to canker C and D strains have been isolated.
Methods of detecting X. citri from natural habitats include leaf-infiltration, bacteriophage, fluorescent antibody and ELISA (Goto, 1992). The polymerase chain reaction and dot blot immunobinding assay (DIA) were developed for rapid, sensitive, and specific detection of the pathogen. The detectable limits were reported to be around 30 c.f.u./ml for the former and 1000 c.f.u./ml for the latter (Hartung et al., 1993, 1996;Wang et al., 1997;Miyoshi et al., 1998).
Related invasive species
- Xanthomonas citri
Related Farm Practice
- Host plants
- Utilization
- Development
- Groups
- Hosts
- Light
- Identification
X. citri is a bacterial pathogen that causes citrus canker - a disease which results in heavy economic losses to the citrus industry worldwide either in terms of damage to trees (particularly reduced fruit production), reduced access to export markets, or the costs of its prevention and control. Lesions appear on leaves, twigs and fruit which cause defoliation, premature fruit abscission and blemished fruit, and can eventually kill the tree. It is introduced to new areas through the movement of infected citrus fruits and seedlings, and inadvertent re-introduction is highly likely despite the quarantine restrictions that are in place in many countries. Locally, X. citri is rapidly disseminated by rainwater running over the surfaces of lesions and splashing onto uninfected shoots;spread is therefore greatest under conditions of hight temperature, heavy rainfall and strong winds. Some areas of the world have eradicated citrus canker, others have on-going eradication programmes, however, this pathogen remains a threat to all citrus-growing regions.
Canker lesions begin as light yellow, raised, spongy eruptions on the surface of leaves, twigs and fruits. The lesions continuously enlarge from pin-point size over several months and can be of many different sizes based on the age of the lesion. As the lesions enlarge, the spongy eruptions begin to collapse, and brown depressions appear in their central portion, forming a crater-like appearance. The edges of the lesions remain raised above the surface of host tissue and the area around the raised portion of the lesion may have a greasy appearance. The lesions become surrounded by characteristic yellow halos. Canker lesions retain the erupted and spongy appearance under dry conditions, such as in a greenhouse;whereas they quickly enlarge and turn to flat lesions with a water-soaked appearance with frequent rain. Canker lesions vary in maximum size from 5 to 10 mm, depending on the susceptibility of the host plant. The symptoms are similar on leaves, fruit and stems.
Canker lesions are histologically characterized by the development of a large number of hypertrophic cells and a small number of hyperplastic cells. At an early stage of infection, the cells increase in size and the nuclei and nucleoids stain more easily;there is also an increase in the amount of cytoplasm synchronized with rapid enlargement. However, these hypertrophied cells do not divide;cell division is only detected in the peripheral areas of lesions adjacent to healthy tissue.
The lesions of canker B, C and D are similar in appearance and histology to those of canker A (Goto, 1992).
Reddy and Naidu (1986) reported canker lesions on roots;however, this has not been confirmed.
The Citrus species listed in the table of hosts, and the following hybrids, are natural hosts of X. citri, with varying degrees of susceptibility to X. citri. In addition to host plant, susceptibility is also affected by the plant part affected, whether leaves, fruits or twigs. Reddy and Naidu (1986) reported canker lesions on roots but this has not been confirmed.
Hybrids:
C. aurantiifolia x Microcitrus australasica (Faustrime), C. limon x M. australasica (Faustrimon), C. madurensis x M. australasica (Faustrimedin), C. sinensis x Poncirus trifoliata (Citrange), C. paradisi x P. trifoliata (Citrumelo) (Schoulties et al., 1987), C. aurantifolium x P. trifoliata (Citradia), C. nobilis x P. trifoliata (Citrandin), C. unshiu x P. trifoliata (Citrunshu), Citrange x P. trifoliata (Cicitrangle), C. adurensis x Citrange (Citrangedin), C. deliciosa x Citrange (Citrangarin), C. unshiu x Citrange (Citranguma), Fortunella margarita x Citrange (Citrangequat), F. japonica x C. aurantiifolia (Limequat), C. maxima x C. aurantiifolia (Limelo), C. madurensis x C. aurantiifolia (Bigaraldin), C. maxima x C. sinensis (Orangelo), F. margarita x C. sinensis (Orangequat), C. nobilis (Clementine) x C. maxima (Clemelo), C. nobilis (King of Siam) x C. maxima (Siamelo), C. unshiu x C. maxima (Satsumelo), C. deliciosa x C. maxima (Tangelo), C. nobilis (King of Siam) x C. sinensis (Siamor), C. deliciosa x C. madurensis (Calarin), C. unshiu x C. madurensis (Calashu). C. aurantiifolia x F. marginata is immune (Reddy, 1997).
Other than Citrus species and their hybrids, most plants, except P. trifoliata, are not sufficiently susceptible to X. citri under natural conditions to warrant attention as hosts of the bacterium. Although the potential of these plants as natural hosts seems to be negligible, further investigation is necessary because no confirmative host surveys have been undertaken since the 1920s. Species names within the genus Citrus also merit some attention due to their inconsistent use by authors.
Plants other than Citrus spp.:
Unless otherwise stated, the following plants refer to Peltier and Frederich (1920, 1924) who defined susceptibility on the basis of artificial inoculation in the greenhouse (G) and/or in the field (F): Aeglopsis chevalieri (G), Atalantia ceylonica (G), Atalantia citrioides (G), Atalantia disticha (G) (Lee, 1918), Chalcas exotica (G), Casimiroa edulis (G, F), Chaetospermum glutinosum (G, F), Clausena lansium (G), Citropsis schweinfurthii (G), Eremocitrus glauca (G, F), Evodia latifolia (G), Evodia ridleyei (G), Feronia limonia [ Limonia acidissima ] (G), Feroniella lucida (G, F), Feroniella crassifolia (G), Fortunella hindsii (G, F), Fortunella japonica (G, F), Fortunella margarita (G, F), Hesperethusa crenulata (G, F), Lansium domesticum (G), Melicope triphylla (G), Microcitrus australasica (G, F), Microcitrus australasica var. sanguinea (G, F), Microcitrus australis (G, F), Microcitrus garrowayi (G, F), Paramignya monophylla (G), Paramignya longipedunculata (G) (Lee, 1918), Poncirus trifoliata (G, F), Xanthoxylum clava-herculis [ Zanthoxylum clava-herculis ] (G, F), Xanthoxylum fagara [ Zanthoxylum fagara ] (G, F) (Jehle, 1917). Atalantia ceylanica, A. monophylla, Microcitrus australis, Feronia limonia and Severinia buxifolia are immune (Reddy, 1997). In India, goat weed (Ageratum conyzoides) is reported to be a host (Pabitra et al., 1997) but confirmation is needed.
The following plants have also been reported as susceptible to X. citri, however, the original descriptions were either not confirmed (U) or contradict those of other authors (C): Aegle malmelos (C), Balsamocitrus paniculata (U), Feroniella obligata (U), Matthiola incana var. annua (U) and Toddalia asiatica (C).
Of the primary hosts listed, yuzu is highly resistant (Goto, 1992) and calamondins, Cleopatra mandarin and Sunki mandarin are immune (Reddy, 1997). Both Fortunella japonica and F. margarita are highly resistant (Goto, 1992).