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Grapefruit (Citrus paradisi Macf.) expressing symptoms of Asiatic Citrus Canker
caused by Xanthomonas axonopodis pv. citri

Photograph courtesy Tim R. Gottwald
USDA, ARS, U.S. Horticultural Research Laboratory
2120 Camden Road, Orlando, FL 32803

Background: For the third time this century, citrus canker caused by the bacterium, Xanthomonas axonopodis pv. citri, (Xac), was again found in Florida. The current outbreak presently consists of two distinct areas. The first is in Manatee Co., in west central Florida, and the second in urban Miami, Dade Co. In the Manatee Co. area, the outbreak was first discovered in June 1997 and infections were mostly in commercial citrus. The age of the oldest lesions found indicated the disease had been in the area for about 1 to 1.5 years. This outbreak has largely been suppressed by destruction of several hundred acres of infected commercial citrus plantations. In contrast, the Miami outbreak consisted of approximately 14 square miles of infected residential properties when first discovered in September 1995, which has expanded to over 146 square miles as of May 1998. The oldest lesions in the Miami area indicated that the disease had existed in that area for about 2 to 3 years prior to discovery. The bacteria ooze onto the lesion surface when wetted by rain, dew, etc. Severe tropical weather patterns have affected Miami in the past several years including hurricanes, tropical storms, tornadoes, and numerous rainstorms associated with high winds. These have spread the infestation and greatly exacerbated the epidemic. Genomic analyses of bacterial isolates from each area have demonstrated significant differences between these isolates that suggest that the bacteria in the two areas were the result of two independent introductions. The USDA, Animal and Plant Health Introduction Service (APHIS) in collaboration with the Florida Department of Agriculture and Consumer Services, Division of Plant Industry has formed a joint state/federal eradication campaign to eliminate the disease. Over 12 million dollars per year and over 500 personnel are presently dedicated to this program. USDA, ARS continues to support citrus canker eradication efforts with epidemiological research activities.

Top panel: Three immature fruits expressing symptoms of citrus canker infection. Lesions are erumpent and corky in appearance. The appearance of the lesions is directly related to duration of infection. From left to right, lesion ages are estimated at, 6-mo., 3-mo., and 1-mo.- old, respectively.

Bottom panel: The introduction of another citrus pest, the Asian citrus leaf miner, Phyllocnistis citrella, is an exacerbating factor to canker inoculum production. For many citrus species and cultivars, some field resistance to Xac infection exists. However, when the leaf miner forms feeding galleries within the leaf just below the epidermis, mesophyll tissues are exposed which express a high susceptibility to infection (right). When these galleries become contaminated with Xac, numerous infections can occur resulting in tremendous inoculum production (left).

APS publication number: IW00001

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