Analysis of the Spatial Distribution of Citrus Bacterial Spot in a Florida Citrus Nursery

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Abstract
The spatial distribution of citrus bacterial spot was analyzed in a central Florida citrus nursery. Detached-leaf bioassay was used to confirm the pathogenicity of bacteria recovered from leaf, stem, and fruit infections, and from leaf washings of symptomatic and asymptomatic leaf tissue. The presence of a bacterium of an undetermined pathovar of Xanthomonas campestris on plants at least 3 m beyond plants with visible symptoms was demonstrated by immunofluorescence microscopy and DNA-DNA hybridization probe analysis of leaf washings of asymptomatic tissue. A proposed infection focus of disease was based on visual assessment of disease incidence in a row of 11-yr-old trees to the south of the most heavily infected nursery bed. Isopathic contour mapping further predicted the highest disease incidence in the area of the proposed focus. Computer software was developed to examine the direction of disease spread from a point focus by comparing directional disease gradients. Using the predicted focus, predominant spread was predicted to be east by northeast. The prediction was consistent with pattern of windblown rain observed in the nursery during the previous month.