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Significance

History: Citrus canker probably originated in Southeast Asia, which is the ancestral home of citrus, but the disease continues to increase its geographic range in spite of the heightened regulations imposed by many countries to prevent introduction. Citrus canker presently occurs in over thirty countries in Asia, the Pacific and Indian Ocean islands, South America, and the U.S.

Citrus canker was first described after it was discovered in the U.S. in the Gulf States in 1915. However, canker lesions were noted on herbarium specimens collected in India as early as 1827. The Gulf States outbreak, which included 7 southern states, resulted from a shipment of infected trifoliate orange nursery stock from Japan. The disease also appeared earlier this century in South Africa and Australia. However, it was reportedly eliminated in these countries as well as the Gulf States through nursery and orchard inspections, quarantines, and the on-site burning of infected trees. Subsequent epidemics have occurred in Australia, Argentina, Uruguay, Brazil, Oman, Saudi Arabia, and Reunion Island. In some locations, eradication efforts have been attempted and failed. In others, active eradication campaigns continue ( Australia, Brazil, Florida).

Citrus canker was found in Florida in 13 locations from 1985 to 1992. Through extensive inspection and tree removal, eradication was believed to have been achieved. However, the disease reemerged in commercial plantations in Manatee County, Florida in June 1997 were eradication efforts had previously taken place. 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.

A new and extensive outbreak was discovered in urban Miami, Florida in 1995. The original Miami outbreak consisted of approximately 14 square miles of infected residential properties when first discovered in September 1995, but had expanded to over 202 square miles as of December 1998 (Figure 34). 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. 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 along with human movement have spread the infestation and greatly exacerbated the epidemic. At the time of this writing (September 2005), nearly all Florida counties within the commercial citrus range have had one or more outbreaks and about 10% of the commercial acreage has been removed in attempts to eradicate the disease (Figure 35).

Figure 34

Figure 35

Genomic analyses of bacterial isolates from the Miami and Manatee Co. areas have demonstrated significant differences between these isolates that suggest that the bacteria in the two areas were the result of two independent introductions. In July 1998, another outbreak was discovered in commercial citrus in Southwest Florida. Genomic analysis indicated that a bacterial isolate from this outbreak was identical to that found in Miami and therefore likely resulted from transport of inoculum or infected plant material from the Miami area.

Impact: Citrus canker is an extremely costly disease. Worldwide millions of dollars are spent annually on prevention, quarantines, eradication programs, and disease control. Undoubtedly, the most serious consequence of citrus canker infestations is the impact on commerce resulting from restrictions to interstate and international transport and sale of fruit originating from infested areas. The USDA, Animal and Plant Health Inspection Service 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. An average of over 50 million dollars per year and over 600 personnel are presently dedicated to this program. In Florida alone, costs of running an eradication program from 1995 through 2005 plus compensation to commercial growers and homeowners for residential citrus destroyed is approaching $1 billion dollars.

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