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Pathogen Biology

Causal Organism: The bacterium Xanthomonas axonopodis pv. citri is a rod-shaped, gram-negative, and has a single polar flagellum. Colonies on laboratory media are usually yellow due to ‘xanthomonadin’ pigment production (Figure 12). When glucose or other sugars are added to the culture medium, colonies become very mucoid due to the production of an exopolysaccaride slime. A semi-selective medium can be prepared by adding an antibiotic, kasugamycin, which inhibits many contaminants but not xanthomonads. The maximum and optimum temperature ranges for growth are to 39°C (95 to 102°F) and 28 to 30°C (82 to 86°F), respectively.

Figure 12

Pathogen Confirmation: Serology, bacteriophage typing, fatty acid profiles, PCR (polymerase chain reaction), and DNA analysis are useful for identification and classification of bacterial isolates into pathovars. However, when such techniques are unavailable, strains of X. axonopodis pv. citri can be distinguished from other pathovars by a panel of susceptible and resistant citrus hosts. Bioassays can also be run on detached-leaves or leaf-disks (Figure 13). On detached leaves, lesions will be apparent 2 to 4 days after inoculation and different isolates can be tested side by side on the same leaf for comparison.

Figure 13

Host Susceptibility: In general, in field plantations, grapefruit (Figure 14), Mexican limes (Figure 15), and trifoliate orange (Figure 16) are highly susceptible to canker; sour orange, lemon (Figure 17), and sweet orange (Figure 18) are moderately susceptible; and mandarins (Figure 19) are moderately resistant. Within orange cultivars, early maturing cultivars are more susceptible than mid season cultivars which are in turn more susceptible than late season cultivars. However, when plant tissues are disrupted by wounds or by the feeding galleries of the Asian leafminer, internal leaf tissues (mesophyll) are exposed. When this occurs, all cultivars and most citrus relatives that express some level of field resistance can become infected (Figure 20).

Figure 14	Figure 15
Figure 16	Figure 17
Figure 18	Figure 19
Figure 20

Pathogen Diversity and Distribution: There are distinct forms of citrus canker disease caused by various pathovars and variants of the bacterium, X. axonopodis. Because symptoms are generally similar, separation of these forms from each other is based on host range, cultural and physiological characteristics, bacteriophage sensitivity, serology, DNA-DNA homology, and by PCR (polymerase chain reaction) analysis of genomic DNA. The latter assays demonstrate that these forms are genetically unique.

The Asiatic form of canker (Canker A), caused by X. axonopodis pv. citri (syn. Xanthomonas citri, Xanthomonas campestris pv. citri) is by far the most widespread and severe form of the disease.

Cancrosis B, caused by X. axonopodis pv. aurantifolii is a disease of lemons in Argentina, Paraguay, and Uruguay, but Mexican lime, sour orange, and pummelo are also susceptible (Figure 21). Cancrosis B isolates can be differentiated serologically from the Canker A, but not from Cancrosis C.

Cancrosis C, also caused by X. axonopodis pv. aurantifolii, was isolated from Mexican lime in Brazil. The only other known host for this bacterium is sour orange.

Other forms of canker bacteria have at times been reported. For example, Canker D, sometimes called citrus bacteriosis, was reported on Mexican lime in Mexico in 1981 but its existence remains controversial. Isolates were discovered in Oman, Saudi Arabia, Iran, and India that produce Canker A-like lesions only on Mexican lime (termed A* strains), and appear to be distinct serologically from all other known forms. Another atypical form of Canker A bacterium has been described from Reunion and surrounding islands in the Indian Ocean which has high levels of resistance to a number of antibiotics. Recently, a variant of Canker A, known as Canker A^w was discovered on the south east coastal areas of Florida. The Canker A^w isolate is distinct via PCR primer reaction and has a host range apparently restricted to Mexican lime and Alemow (Citrus macrophylla). Other as yet undescribed variants probably exist.

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