Citrus canker affects all aboveground plant parts, with initial symptoms on leaves appearing as tiny, slightly raised blister-like lesions. With time, foliar lesions turn grey, then tan/brown, a water soaked margin appears and the entire lesion is usually surrounded by a chlorotic margin. The centre of the lesion becomes characteristically raised, and spongy or corky. Lesions arising from stomatal infection are typically visible from both sides of the leaf (unlike these of scab – Elsinoe spp.). A shot-hole effect can also eventuate, after the centre of the lesion becomes crater-like and falls out. As the disease becomes more severe on foliage, defoliation generally occurs. A similar symptom to the above is observed on twigs and fruit, however chlorotic margins are not found on twig lesions. Severe infections can result in fruit abscission, and twig dieback due to girdling. Twigs not to be killed by girdling by the pathogen can harbour lesions for many years, with the old lesions tending to be raised corky patches in the otherwise smooth bark. Lesions can be associated with physical injuries to the host, in particular those caused by Asian citrus leaf miner (Phyllocnistis citrella Stainton) (Schubert et al. 2001).
The bacterium is an aerobic, gram-negative rod measuring 0·5–0·75 × 1·5–2·0 µm. It is motile by one polar flagellum. Carbohydrates are metabolised by oxidation. Growth on 2% sucrose-peptone agar is yellow and copiously mucoid. Gelatin, casein and starch are hydrolysed. Nitrite is not formed from nitrate, and aesculin is hydrolysed (Hayward and Waterston 1964).
A number of strains of the pathogen have been described, based on factors such as virulence, host range, and molecular markers. The strains are referred to as A (Asiatic/Oriental canker), B (cancrosis B), C (Mexican lime cancrosis), D (citrus bacteriosis) and E (citrus bacterial spot). The B and C strains are currently classified as X. axonopodis pv. aurantifolii. The D strain is controversial and E-strain is now classified as X. axonopodis pv. citrumelo. Schubert et al. (2001) provides a comprehensive explanation of each of the strains, and provides the relevant references for further reading.
When a typical citrus canker lesion is cut in through the centre, placed on a glass slide and covered with a drop of water, bacterial streaming can be observed under a light microscope. This indicates that a bacterium is associated with the lesion. It is then necessary to determine if the bacterium is Xanthomonas axonopodis pv. citri. A detailed summation of identification techniques can be found in the European and Mediterranian Plant Protection Organisation (2005).
Note: Additional synonyms are listed by Hayward & Waterston (1964), the Crop Protection Compendium (2005) and the European and Mediterranian Plant Protection Organisation (2005).