Pathogen characteristics
Bacterial spot is caused by several species of gram-negative bacteria in the genus Xanthomonas. In culture, these bacteria produce yellow, mucoid colonies (Figure 12). A "mass" of bacteria can be observed oozing from a lesion by making a cross-sectional cut through a leaf lesion, placing the tissue in a droplet of water, placing a cover-slip over the sample, and examining it with a microscope (~200X) (Figure 13).

Figure 12	Figure 13

Pathogen classification and hosts
Bacterial spot was first observed on tomato in South Africa as early as 1914. For almost half a century, a single bacterial species, classified as Xanthomonas vesicatoria and later as X. campestris pv. vesicatoria, was considered the cause of bacterial spot of both pepper and tomato. In the early 1990s, it was shown that two distinct genetic groups (possibly species) existed within strains of pv. vesicatoria. In 1995, Vauterin et al. restructured the classification of the genus Xanthomonas by proposing species status for these groups: X. vesicatoria and X. axonopodis (syn. campestris) pv. vesicatoria. More recently, it has been shown that bacteria belonging to four distinct groups (previously designated A, B, C, and D) cause bacterial spot. Jones et al. (2004) proposed that these should have species status: X. euvesicatoria = X. campestris (axonopodis) pv. vesicatoria (group A), X. vesicatoria = X. vesicatoria (group B), X. perforans = group C strains, and X. gardneri = group D strains. Strains of groups A and B are most widely distributed. The vast majority of strains that infect pepper are in group A and possibly some in groups B and D. No pepper strains have been found in group C; however, strains from all four groups have been isolated from tomato. Some strains infect only pepper (designated pepper strains), some infect only tomato (designated tomato strains), and some can infect both pepper and tomato (designated pepper/tomato strains). The host range of the different strains can be determined by infiltrating a bacterial suspension into the leaf and observing the response. Within 18 to 36 hours, a resistant response is indicated by a rapid collapse of the infiltrated area (a hypersensitive response, HR) (Figure 14). In a susceptible plant, the infiltrated area develops a chlorotic, water-soaked appearance, but not until 3 to 5 days after infiltration.

Figure 14

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by The American Phytopathological Society