ANALYSIS OF FOCI OF ASIATIC CITRUS CANKER IN A FLORIDA CITRUS ORCHARD

In October 1990, the occurrence of Asiatic citrus canker in an orchard in south Florida was apparently related to spread of Xanthomonas campestris pv. citri from dooryard trees 230 m away on an adjacent property. The establishment of apparent initial foci of disease in the orchard coincided temporally with a major rainstorm with high winds during mid-August 1989. Infection of the dooryard trees on the adjacent property was related by regulatory officials to an introduction of inoculum from an outbreak of Asiatic citrus canker on the west coast of Florida in 1986. Restriction endonuclease digest patterns of DNA taken from the pathogens during the 1986 and 1990 outbreaks were identical. There were three extensive and several minor areas of diseased trees in the orchard. The three most extensive areas of disease each had trees near the center of the cluster with stem lesions that predated all other foliar lesions in the cluster. From isopath maps of these areas a main focus of diseased trees was found, surrounded by what appeared to be secondary foci. A greater within-row than across-row aggregation for each area was detected by ordinary runs analyses. A predominant direction of disease spread among the areas of diseased trees was not found in analysis of disease gradients. A slightly stronger association of diseased trees within than across rows was found in spatial lag autocorrelation analyses, but noncontiguous groups of diseased trees also occurred that coincided with secondary foci at oblique angles to the oldest diseased trees. If natural spread within the orchard did occur, it may have been confounded by mechanical spread of X. c. citri caused by orchard management practices, such as pesticide applications,