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Two genetically diverse groups of strains were identified among cultures of Xunthomonas campestris pv. vesicatoria isolated from plants with bacterial spot of pepper and tomato. Group A strains do not pit pectate gels or hydrolyze starch, whereas group B strains are strongly positive for these reactions. Group A strains cause a hypersensitive reaction in plants of tomato breeding line Hawaii 7998, but group B strains do not. Other differences between the two groups of strains were discovered in tests for utilization of carbon compounds, serology, fatty acid profiles, silver-stained protein bands on sodium dodecyl sulfate-polyacrylamide gel electrophoresis gels, and DNA restriction enzyme digestion profiles. The levels of DNA homology between strains belonging to the same group were more than 74%, but the levels of DNA homology between strains belonging to different groups were less than 46%. The two groups of strains have different genetic backgrounds, but cause essentially the same disease of tomato and pepper.The bacterial spot disease of tomato and pepper is caused by the bacterium Xanthomonas campestris pv. vesicatoria. Necrotic spots on leaves, stems, peduncles, and fruits are distinctive symptoms of the disease (5, 17). Lesions may enlarge or coalesce and cause leaves to become chlorotic and eventually die. When prevalent, lesions on fruits result in loss of marketability. Bacterial spot occurs in all regions of the world where tomatoes and peppers are grown (17).A large collection of strains of the bacterial spot pathogen of tomato and pepper was accumulated for study (4), and two characteristics, pitting of pectate gels and hydrolysis of starch, were determined for each strain as aids for identifying possible opportunistic xanthomonads in the collection (14). Strains that were pectolytic and amylolytic, characteristics expected for opportunistic xanthomonads, were discovered, but these strains were pathogenic for tomato and/or pepper. One of the pectolytic and amylolytic strains was from Brazil and was designated a new race of X. campestris pv. vesicatoria on the basis of its ability to cause disease in the resistant tomato breeding line Hawaii 7998 (29). Of 522 strains tested (4), 104 were pectolytic and amylolytic. Most of these 104 strains were from Argentina, but similar strains were received from Australia, Brazil, New Zealand, and Spain. Only 1 of 331 strains from the United States was pectolytic and amylolytic, and no such strain was among the 28 strains from Taiwan tested. The pectolytic and amylolytic strains were not evenly distributed in all regions of the world where bacterial spot of tomato and pepper occurs, but seemed to be most prevalent in countries in the southern hemisphere.The two groups of strains, which are placed in X. campestris pv. vesicatoria because of their host specificity, were further characterized to determine the extent of genotypic

Xanthomonas campestris strains that cause disease in citrus were compared by restriction endonuclease analysis of DNA fragments separated by pulsed-field gel electrophoresis and by DNA reassociation. Strains of X. campestris pv. citrumelo, which cause citrus bacterial spot, were, on average, 88% related to each other by DNA reassociation, although these strains exhibited diverse restriction digest patterns. In contrast, strains of X. campestris pv. citri groups A and B, which cause canker A and canker B, respectively, had relatively homogeneous restriction digest patterns. The groups of strains causing these three different citrus diseases were examined by DNA reassociation and were found to be from 55 to 63% related to one another. Several pathovars ofX. campestris, previously shown to cause weakly aggressive symptoms on citrus, ranged from 83 to 90% similar to X. campestris pv. citrumelo by DNA reassociation. The type strain ofX. campestris pv. campestris ranged from 30 to 40% similar in DNA reassociation experiments to strains of X. campestis pv. citrumelo and X. campestris pv. citri groups A and B. Whereas DNA reassociation quantified the difference between relatively unrelated groups of bacterial strains, restriction endonuclease analysis distinguished between closely related strains.