1USDA, Agricultural Research Service, Orlando, FL 32803, USA; 2Citrus Research and Education Center, University of Florida, Lake Alfred, FL, 33850, USA

Background and objectives

Asiatic citrus canker (ACC) caused by Xanthomonas axonopodis pv. citri (Xac) was discovered in residential citrus in Dade County, Florida, USA, in September 1995. When first detected, the infected area was believed to encompass approximately 33.7 km2 of urban properties south-west of Miami Airport. An extensive eradication campaign was established by the Florida Department of Agriculture and Consumer Services Division of Plant Industry in collaboration with USDA Animal and Plant Health Inspection Service. Despite eradication efforts, which have resulted in the removal or cutting back of over 85,000 infected and exposed dooryard citrus trees, the infected area has increased to 349.6 km2 as of January 1998 and the surrounding quarantine area is presently 935 km2. Concurrently, citrus canker was discovered in commercial citrus in Manatee County on the west coast of Florida in June 1997. A similar eradication effort is currently in progress there and has resulted in removal of c. 202 commercial hectares (500 acres or c. 50,000 trees) and around 400 dooryard trees. A complicating factor has been the 1993 introduction of the Asian leafminer, Phyllocnistis citrella, which quickly spread across Florida and whose feeding activities expose leaf mesophyll tissues to infection. The objectives were to attempt to determine the foci of infection and causes of such rapid spread of ACC.

Materials and methods

ACC disease gradients were examined and, where possible, traced back to older focal areas of infection. Infected areas were mapped and spatial analysis performed to examine the distribution of the disease. The ages of the earliest infections and subsequent new infections were estimated. Weather data were examined for both the Miami and Manatee areas and major meteorological events related to the presumed infection cycles.

Results and conclusions
In urban Miami, disease gradients emanated from a putative focus area 2.4 km south-east of Miami International Airport. The oldest stem lesions in the focal area documented the existence of citrus canker around 2-3 years prior to discovery of the epidemic. Therefore, meteorological events were examined from 1992 onward. The Miami area is considered subtropical with about 152-165 cm of rain annually distributed over 85-95 rainy days per year, with 70-80 thunderstorms per year. Since 1992, at least 30 of these storms have occurred annually and are associated with wind speeds in excess of 29 km/h - the speed required to drive bacteria-laden rainwater into foliar stomata. Thus, intermittent but frequent opportunities have occurred for inoculum production and localized spread.

In Manatee County, no disease gradients or focal areas were identified. The age of the oldest lesions dated the infection approximately 18-24 months prior to discovery, i.e., the middle to end of 1995. In contrast to Miami, the Manatee epidemic is a carry-over from a previous outbreak in that area which was believed to have been eradicated in 1992. Genetic analyses of bacterial isolates from the previous and present outbreaks support this conclusion.

The presumed infection cycles and age of infections in newly-infested areas corresponded well with five major meteorological events: hurricanes Gordon (November 1993) and Erin (August 1995), tropical storms Jerry (August 1995) and Josephine (October 1996), and a tornado crossing the urban Miami area (January 1996). In Miami, these events contributed to spread of Xac to the north-east and south-west of the focus. In Manatee, the spread was similar within individual groves but more omnidirectional between groves. Local rainstorms and inadvertant human transport resulted in local, short distance spread of Xac in both areas, whereas, individual rain storms with winds contributed to long-distance dispersal from the original focus in Miami.