POPULATION-DYNAMICS OF STRAINS OF XANTHOMONAS-CAMPESTRIS DIFFERING IN AGGRESSIVENESS ON SWINGLE CITRUMELO AND GRAPEFRUIT

The aggressiveness of strains of Xanthomonas campestris causing citrus canker (X. c. citri) and citrus bacterial spot (X. c. citrumelo) on Swingle citrumelo and Duncan grapefruit was assessed by comparing lesion expansion and population development for these strains in greenhouse, growth chamber, and field experiments, using different inoculation techniques and sampling methods. When leaves were pinprick inoculated and resultant lesions sampled over time, there was a positive relationship between internal populations (detected upon macerating lesions) and external populations (detected by swabbing the surface of moist lesions) and between each population and lesion diameter for the different pathovars and aggressiveness types of X. c. citrumelo. Correlations among internal and external populations and lesion diameter were higher in the field than under dew-forming conditions in the growth chamber. A leaf-infiltration method revealed few differences in internal populations among pathovars and strains. Strain X host interactions based on the populations and expansion of lesions were apparent for the different aggressiveness types of X. c. citrumelo in the field. The highly aggressive strain of X. c. citrumelo on Swingle citrumelo most consistently produced the highest bacterial populations and largest lesions. In the field, internal populations were indicative of external populations and therefore might be predictive of the ability of a strain of X. campestris to spread on a given host.