Background
Plants, like all other living things, are subject to diseases caused by other organisms. Citrus canker is a globally distributed, highly contagiousbacterial diseaseof citrus species such as sweet orange, lime, lemon, grapefruit, or clementine among others.
Citrus canker is one of the most devastating diseases of citrus with enormous impact on the citrus production industry. Florida, the number one citrus producing state in the U.S., has been experiencing an epidemic of citrus canker since 1995. Approximately ¾ of a million acres are planted to citrus (mostly oranges) with a crop value of $9 billion per year.
A. Canker on immature grapefruitB. Canker and citrus leaf miner damage
A.
B.
Citrus canker is caused by different strains of the gram negative bacteriumXanthomonas axonipodis. Spread by wind, rain and touch, these bacteria enter plants through stomates in leaves, lenticels in green wood, and wounds, such as caused by the citrus leaf miner insect or wind damage. Younger plant parts are more susceptible than older. Symptoms include spots (lesions) out of which bacteria ooze in moist conditions. Cork-like pustules with indented centers form during dry periods. Severe canker can cause premature fruit drop, defolioation and dieback of branches (Timmeret al., 2005).
Control of the epidemic in Florida is regulated at national and state levels (DOACS, 2006). Control methods, some of which have been very controversial, have included eradication of trees within a 1900 foot zone (Gottwald et al, 2001,Gottwald, 2002); quarantine on transportation of fresh fruit and trees within Florida and to citrus producing states; and cleaning and decontamination (Timmer et al, 2005). In 2006, eradication was eliminated as a goal by the USDA and current efforts focus on management of citrus canker.
At present the naming of the species of Xanthomonas is in flux. The most common pathogen is X. axonipodis (synonym X. citri). A form found in South America is X. aurantifolia. Strains of pathogens are usually named according to the kinds of plant hosts they can infect. The X. axonipodispathogenic to citrus is calledX. axonipodispv. citri (pathogen variety or pathovar = pv). X. axonipodispv. ficiis the pathovar pathogenic to figs.
The ability of a pathogen to cause disease depends on interactions among gene products in both the host and the pathogen. The following information is summarized from Brunings and Gabriel, 2003. Pathogens have genes that initiate disease (pathogenicity genes, pth) or condition the level of disease (avirulence genes, avr). Hosts may have one or more genes for resistance (R genes), or none at all.
If a plant has an R gene that confers resistance to a pathogen,the plant R gene responds to the presence of proteins from the pth or avr genes. A hypersensitive response (HR) is initiated which causes programmed cell death in the vicinity of the pathogenand effectively prevents the pathogen from causing disease. If a plant does not have the appropriate R genes for a given pathogen (i.e., it is a susceptible host), then the pth or avr gene products can cause symptoms to develop.
In the case of citrus canker, there is a family of pth avr genes in Xanthomonas citrithe most common of which is called pthA. In a susceptible host citrus plant, such as sweet orange, pthAwill cause disease symptoms. The protein produced by a different pth gene (for example pthC) does not initiate disease in the sweet orange, instead the host responds with HR.