Spatial Pattern Analysis of Epidemics of Citrus Bacterial Spot in Florida Citrus Nurseries

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Abstract
The spatial pattern of citrus bacterial spot caused by Xanthomonas campestris pv citrumelo was studied in four nurseries in central and southwest Florida. When discrete frequency distributions were fitted to disease incidence data from each nursery, results were often inconclusive. Indices of dispersion suggested aggregation of disease in all nurseries. Ordinary runs analysis indicated within-row and across-row aggregation in all nurseries. Spatial lag correlation analyses further suggested a high degree of within-row autocorrelation of diseased levels in all nurseries, corresponding to apparent mechanical spread of the pathogen down nursery rows. The analysis also indicated some significant across-row autocorrelation of disease level, probably associated with natural spread across rows or perhaps mechanical spread across rows by handling of infected plants. In one nursery, high disease incidence on rootstock plants was directly related to high disease incidence on new scion shoots in the same area. Two nurseries had discrete areas of high disease incidence that were interpreted as apparent disease foci. Disease gradients in one of these nurseries were shallowest within rows to the north, corresponding to mechanical pruning in that direction. Diseases gradients from the apparent focus in a second nursery, where little maintenance were performed, were shallowest diagonally across north-south oriented rows, indicating that the predominance of natural spread emanated from a focus both to the northeast and southeast directions. The presence or absence of natural disease spread versus mechanical spread alone was directly related to the aggressiveness of the bacterial strains, presence or absence of copper bactericides, and cultural management.

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