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Citrus canker infection is evident on this fruit. Photograph courtesy USDA/APHIS. Photographer R. Anson Eaglin.

Species Name: 
Xanthomonas axonopodis pv. citri

Common Name:
Citrus Canker

Synonymy:
Xanthomonas axonopodis Hasse, 1915
Xanthomonas campestris pv. citri Dye
Xanthomonas citri ex Hasse

Species Description:
Citrus canker is a disease of cultivated citrus plants caused by nonindigenous bacterial pathogen Xanthomonas axonopodis pv. citri ('Xac').

Genus Xanthomonas consists of gram-negative, rod-shaped, polarly-flagellated bacteria whose members commonly occur as serious plant pathogens. Colonies are typically yellow in color due to the presence of a particular carotenoid pigment identified through relatively simple screening procedures and generally indicative of the genus (Starr and Stephens 1964).

The pathogen expresses itself in infected citrus plants as brown, raised lesions on leaves and fruits with oily, water soaked and necrotic margins and a surrounding yellow halo. Affected areas of leaves may fall out in time, leaving holes. Microscopic examination of tissue sliced from the edges of active lesions on infected plants reveals streaming masses of rod-shaped bacteria (DACS 2002, ISSG).

Potentially Misidentified Species:
There are multiple strains or pathotypes of the Xanthomonas spp. citrus canker bacterium. The most devastating of these is X. axonopodis pv. citri pathotype A, also known as Asiatic citrus canker. Also present in Florida are pathotypes B and C of X. aurantifolia pv. aurantifolia and at least two pathotypes of the congeneric bacterium X. campestris, responsible for bacterial leaf spot disease in citrus (Integrated Plant Genetics Inc. undated, ISSG). An outbreak of bacterial leaf spot disease in Florida in 1984 was previously misidentified as a citrus canker outbreak (ISSG).

Recent molecular studies have suggested that X. aurantifolii pv. citri should be reinstated to species status as X. citri.

Florida citrus trees are now additionally under threat from citrus greening disease (also called huanglongbing, HLB, or yellow dragon disease), also a bacterial disease, that is believed to be capable of worse damage than canker. The responsible pathogen (unnamed as of 2007) is also not native to the U.S., and is considered by some to be the most serious worldwide citrus disease. Citrus greening disease was first reported from Florida in 2005. Disease symptoms are dissimilar to those of citrus canker, and include yellowing of shoots, leaf mottling or discoloration, and the production of lopsided poor-quality fruit.

II.  HABITAT AND DISTRIBUTION 

Regional Occurrence:
Citrus canker is likely native to southeast Asia and India, but has now spread worldwide into to warm, moist, citrus-growing coastal regions. The pathogen is now also established in Australia, Japan, the Middle East, Africa, Papua New Guinea and elsewhere in the Pacific, and the Americas (Gotwald et al. 2002, Kumar et al. 2004).

In Florida, citrus canker had been reported from 24 counties as of January 2006. Recent northward spread of citrus canker into central Florida is believed to be a consequence of the hurricanes of 2004 and 2005 (Gaskalla 2006).

IRL Distribution:
As of January 2006, citrus canker had been reported in the IRL region from Brevard, Indian River, St. Lucie, Martin, and Palm Beach counties (Gaskalla 2006). Most reports of citrus canker in the IRL region north of Martin County did not occur until after the 2004 hurricanes.

III. LIFE HISTORY AND POPULATION BIOLOGY

Age, Size, Lifespan:
X. axonopodis pv. citri is a microscopic bacterium.

Abundance:
Citrus canker was known to occur in 24 Florida counties as of January 2006.

Reproduction:
Reproduction in genus Xanthomonas is similar to that of other bacteria, occurring through asexual binary fission in which each dividing clonal daughter cell receives an identical copy of the parental genome (Campbell et al. 1999).

IV.  PHYSICAL TOLERANCES

Temperature:
Xanthomonas axonopodis pv.citri thrives in subtropical regions of the world where citrus grows. It's invasive range , like the range of it's various host species, is temperature limited, although the first reported incidence of the pathogen in Florida in 1910 was reported in the northern part of the state from near the Florida-Georgia border.

Hydrology:
Citrus canker thrives in warm regions experiencing high humidity and heavy rainfall. Gottwald et al. (2002) note the exposed bacterium on contaminated surfaces perishes when the surfaces dry out, suggesting a humid environment is important for the organism.

V.  COMMUNITY ECOLOGY

Trophic Mode:
Xanthomonas axonopodis pv.citri is a heterotrophic bacterium that derives nutrition from the tissues of the plants it infects.