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Variation in Aggressiveness of Xanthomonas campestris pv. citrumelo Associated with Citrus Bacterial Spot in Florida Citrus Nurseries. J. H. Graham, Associate professor, University of Florida, IFAS, Citrus Research and Education Center, 700 Experiment Station Road, Lake Alfred 33850; T. R. Gottwald, Research plant pathologist, U.S. Department of Agriculture, ARS, Orlando, FL 32803. Phytopathology 80:190-196. Accepted for publication 17 August 1989. Copyright 1990 The American Phytopathological Society.. DOI: 10.1094/Phyto-80-190.

Reactions on wound-inoculated detached leaves of Swingle citrumelo and Duncan grapefruit were used to characterize strains of Xanthomonas campestris pv. citrumelo associated with citrus bacterial spot (CBS) in Florida citrus nurseries and to distinguish these strains from X. c. citri, the cause of Asiatic citrus canker. Strains of X. c. citrumelo varied in aggressiveness based on the extent and persistence of water-soaking and the development of necrosis. Aggressiveness on detached leaves was correlated with that on wound-inoculated leaves in the greenhouse and field. Reactions on detached leaves developed rapidly and could be evaluated after 7 days, whereas 30 days were required for the development of lesions on attached leaves. In vitro inoculations distinguished the flat-spreading lesions of CBS from the erumpent, calluslike reaction produced by X. c. citri. In four nurseries, the incidence, severity, and spatial distribution of CBS was related to strain aggressiveness. Only the most aggressive strains were associated with natural spread, whereas less aggressive strains were evidently spread mechanically by nursery operations. In one nursery, where strains varied from weakly to moderately aggressive, aggressiveness differed among separate disease foci. Strains from 25 unrelated nursery infestations were evaluated, and the most aggressive strains occurred in only four nurseries. More than 75% of the nursery outbreaks were associated with Swingle citrumelo. This variety was more susceptible than Duncan grapefruit to the aggressive strain of X. c. citrumelo and less susceptible to X. c. citri in attached leaf tests. There were significant interactions of strains of X. c. citrumelo of different aggressiveness with the two citrus cultivars.

Additional keywords: disease eradication.