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Citrus Canker In-Depth

I. Economic Hosts

II. Pathogens

III. Disease

IV. Symptoms and Signs:

V. Isolation:

VI. Identification:

Colonies of X. citri and X. campestris pv. aurantifolii have not been distinguished on the basis of cultural characteristics from X. campestris pv. campestris strains. Colonies on agar plates are circular, convex, semi-translucent and yellow, and the margins are entire and standard determinative tests are used to identify strains of the genus [13,14]. Crude methanol extracts (ten minutes at 65 C) of cells exhibit a major absorption peak between 443 and 446 nm [7], which is diagnostic of the xanthomonadin pigment and not found in other yellow bacteria. All strains of X. citri form a highly clonal group and all strains of X. campestris pv. aurantifolii form a different highly clonal group; the groups may be identified and distinguished from all other xanthomonads by characteristic restriction fragment length polymorphism (RFLP) profiles [7]. A detailed protocol on this identification technique and its application to Xanthomonas has been published [5]. The type strain of the species X. citri (3213, ATCC 49118) and pathotype strains of X. campestris pv. aurantifolii (Pathotype B: XC69, ATCC 51301; Pathotype C: XC340, IAPAR 9694, ICMP 8434, IBSBF 417, ATCC 51302) and X. campestris pv. citrumelo (3048, ATCC 49120) are available through the American Type Culture Collection for comparative purposes.

The use of RFLP data alone to formulate a taxonomy and reinstate X. citri to species [7] has been criticized [21], but the reinstatement was not invalidated. Most microbial taxonomists agree that phylogeny should determine taxonomy and that "the phylogenetic definition of a species generally would include strains with approximately 70% or greater DNA-DNA relatedness" [22]. Since X. citri strains are only 30% similar to X. campestris 33913 (the type strain) [4], the DNA-DNA hybridization data are consistent with the RFLP data, and the reinstatement to species is consistent with the phylogenetic standard. The taxonomic status of X. campestris pv. aurantifolii strains is unresolved. These strains are only 37-40% related to X. campestris 33913, but are also only 62-63% related to X. citri strains [4], form a distinct RFLP group [7], and differ serologically from X. citri strains [1]. The expedient of lumping these relatively distinct groups together as pathovars of X. citri may not be taxonomically sound.

Identical symptoms induced by two taxonomically distinct groups of strains is indicative of a common pathogenicity factor. Gene pthA is essential for X. citri to elicit cankers on citrus, and pthA confers this ability to various X. campestris strains (for example, pathovars alfalfae and citrumelo) [19,20]. Functionally homologous genes (pthB and pthC) have also been identified and cloned from X. campestris pv. aurantifolii pathotype B and pathotype C, respectively [Gabriel, unpublished data]. Both pthB and pthC are essential for X. campestris pv. aurantifolii pathotypes B and C, respectively, to cause cankers on citrus, and pthB and pthC confer this ability to various X. campestris strains [Gabriel, unpublished data]. All three genes are therefore functionally interchangeable, and these genes may have been transferred horizontally on plasmids between X. citri and X. campestris pv. aurantifolii strains. Apparently homologous genes are found in all canker-causing strains, and have not been found in non-canker inducing strains isolated from citrus, such as X. campestris pv. citrumelo. Therefore a single common gene appears to be diagnostic (and responsible) for a Xanthomonas strain's ability to induce cankers on citrus.

Blot Test
DNA probes, polymerase chain reaction (PCR) primers and antibodies against the protein products of these genes may provide rapid and certain diagnostic tests for xanthomonads capable of causing cankers on citrus.

Genes pthA, pthB and pthC are all members of an avirulence / pathogenicity gene family widely distributed in the genus Xanthomonas [3,20]. Avirulence genes determine race specificity and can determine host range [8]. Genes pthA, pthB and pthC, when transferred into X. citri, X. campestris pv. aurantifolii or X. campestris pv. citrumelo, confer ability to elicit hyperplasia (cell divisions or cankers) on all citrus species in the normal host range of the recipient strain.

X. campestris pv. aurantifolii
Host Common Name X. citri Pathotype B Pathotype C

C. sinensis Sweet Orange ++ + HR
C. paradisi Grapefruit ++ + HR
C. limon Lemon + ++ HR
C. aurantifolii Mexican Lime ++ ++C ++

+ indicates virulence; ++ indicates strong virulence; HR indicates a hypersensitive response; C indicates a distinctive chlorotic canker phenotype.

Pathotype B of X. campestris pv. aurantifolii causes "false" citrus canker or cancrosis B, while Pathotype C causes Mexican lime cancrosis or cancrosis C.

VII. Pathogenicity

VIII. Storage of Organism

IX. Reported Host Range

X. Geographical Range and Spread

XI. Suggested Taxonomic Keys

XII. References

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Copyright © October 2001 Integrated Plant Genetics, Inc. -- All Rights Reserved

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