Pathological, Restriction-Fragment Length Polymorphism, and Fatty Acid Profile Relationships BetweenXanthomonas campestrisfrom Citrus and Noncitrus Hosts

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Abstract
Pathogenic strains of Xanthomonas campestris from noncitrus hosts were screened for their ability to cause necrosis on wounded, detached leaves of the citrus cultivars Swingle citrumelo and Duncan grapefruit. Thirteen of 56 noncitrus strains produced reactions that were similar to those caused by X. c. pv. citrumelo strains isolated from several outbreaks of citrus bcterial spot in Florida nurseries. Noncitrus strains of the weakly to moderately aggressive type, including strains of X. c. pv. alfalfae, X. c. pv fici, X. c. pv. maculifoliigardeniae, and three strains from Strelitzia, elicited necrotic spots on spray-inoculated immature foilage of both Swingle citrumelo and Duncan grapefruit. When noncitrus strains were injection-infiltrated into Swingle citrumelo leaves, they mutiplied and reached populations as high as those attained by a weakly aggressive strain from citrus. Strains of X. c pv. campestris, X. c pv. phaseoli and X. c. pv malvacearum that did not elicit necrosis on detched leaves failed to multiply in leaves. The group of weakly to moderately aggressive strains from noncitrus hosts was compared with aggressive and less agressive strains from citrus by restriction-fragment length polymorphism analysis of genomic DNA and by cellular fatty-acid profiles. Most of the weakly to moderately aggressive strains of non-citrus origen could not be separated from the group of citrus strains by either analysis. Other X. campestris strains that did not grow in planta and give a disease reaction were less related to the citrus and the other noncitrus strains by these analysis. These findings raise doubts as to the role of less aggressive strains as primary pathogens of citrus and their inclusion within a separate pathovar, X. c. pv citrumelo.