Bacterial Spot - Worldwide Distribution, Importance and Review

Bacterial spot disease on tomato is a major problem in tropical, subtropical, and temperate climates. Variation within this organism has been well documented. Two distinct groups, i.e., A and B, were identified in the early 1990s based on phenotypic and genotypic tests. These two groups were determined to be distributed widely throughout the world. The two groups were placed in Xanthomonas axonopodis pv. vesicatoria and X. vesicatoria, respectively in 1995. Following the identification of these two groups, two additional groups, C and D, have been identified that are pathogenic to pepper, tomato, or both hosts. The C group was originally isolated in 1991 and has more recently been found in many tomato production regions in the United States and in Mexico. Group D strains originally isolated in Costa Rica were determined to be closely related to X. gardneri, isolated by Sutic in Yugoslavia in 1957. The group D strains have been identified more recently in Michigan and in Brazil. Based on DNA homology experiments the C and D strains were determined to have less than 70% DNA similarity with strains from the other groups, with the type strain of X. axonopodis, and with the currently classified species within Xanthomonas. We have proposed that the C group strains be designated X. perforator sp. nov. and that the D group strains, which had less than 70% DNA similarity with any of the Xanthomonas species and which never had taxonomic status be named X. gardneri to reflect the specific epithet proposed by Sutic in 1957. Tomato races designated T1 (group A strains), T2 (group B strains) and T3 (group C strains) have been identified based on differential reactions with a group of tomato genotypes. Recently, two new races, T4 and T2, were identified in Florida. These races are group C strains with mutations in the avrXv3 and the avrXv3 and avrXv4, respectively. Doidge (1921) identified the pathogen causing bacterial spot disease on tomato (Lycopersicon esculentum Mill.) in South Africa and designated it Bacterium vesicatorium. Gardner and Kendrick (1921), also identified a bacterium causing a leaf spot of tomato in Indiana, USA and proposed naming it Bacterium exitiosum. However, they were aware of Doidge’s designation and deferred to B. vesicatorium because of an apparent similarity between the two organisms and Doidge’s prior publication. The only apparent difference between the strains was that the strains in Gardner and Kendrick’s study were strongly amylolytic while the strains isolated by Doidge were feebly or nonamylolytic. In the 1990s, Vauterin et al. (1990, 1995) and Stall et al. (1994) determined that two genetically and phenotypically distinct groups existed within X. campestris pv. vesicatoria; group A and group B. These groups were differentiated based on amylolytic and pectolytic activity along with pulsed-field gel electrophoresis. Bouzar et al. (1994a) compared over 150 strains isolated from tomato and pepper and determined that the A strains were uniformly negative for starch hydrolysis and pectolytic activity while the B strains consisted of strongly amylolytic and pectolytic strains (Table 1). The groups were also separated based on protein profiles in which A strains contained a unique 32 kDa protein and B strains contained a 25to 27-kDa protein (Bouzar et al., 1994a). Furthermore, the A group consisted of only tomato race 1 (T1) strains and the B group of 27 Proc. 1st IS on Tomato Diseases Eds. M.T. Momol, P. Ji and J.B. Jones Acta Hort 695, ISHS 2005 T2 strains. Bouzar et al. (1994b) were also able to distinguish the strains serologically using a panel of monoclonal antibodies (Table 1). Based on DNA-DNA hybridization tests (Vauterin et al., 1995; Stall et al., 1995), it was evident that group A and B strains were not related at the species level. Vauterin et al. (1995) proposed a reclassification of the xanthomonads and divided X. campestris pv. vesicatoria into two species, with the B strains being retained in X. vesicatoria and the A strains placed as a pathovar of X. axonopodis. Later two other genetically distinct xanthomonads were identified that were associated with tomato. The first was isolated in 1957 by Sutic (1957) who identified a bacterium on tomato in Yugoslavia and named it Pseudomonas gardneri. Dye (1966) compared Sutic’s strain with a number of xanthomonads using standardized tests and concluded that P. gardneri was synonymous with X. vesicatoria since both caused disease on tomato and could not be distinguished in the laboratory or on the plant. We have since identified the same organism in Costa Rica in the early 1990s based on REP-PCR (Bouzar et al., 1999) and more recently determined the strains to be members of the same species (Jones et al., 2000). This organism has since been identified in Brazil (Rademaker et al., 1997; Quezado-Duval et al., 2004) and appears to be tomato race 2 (QuezadoDuval et al., 2003; Jones et al., unpublished). The second unique group consisted of the T3 strains that were isolated in Florida in the United States in the early 1990s (Jones et al., 1995). Besides their response on the tomato differentials, these strains were different from the other strains based on serology and pulsed-field gel electrophoresis (Bouzar et al., 1996; Jones et al., 2000). Jones et al. (2000) characterized these two types of strains and determined that they were two additional groups, C and D based on characteristics listed in Table 1. Group C represented by T3 strains originally were determined to be most closely related to the A group based on DNA-DNA hybridization experiments and were thus placed within X. axonopodis pv. vesicatoria, whereas the D group represented by strains including the one originally identified by Sutic (1957) were genetically distinct from the other three groups of xanthomonads associated with tomato. The taxonomic position of the organism isolated by Sutic has been very unclear. In Bergey’s Manual of Systematic Bacteriology published in 1984 (Bradbury, 1984), the bacterium was referred to as Pseudomonas gardneri. Based on the work of Dye (1966) and more recently fatty acid analysis and carbon substrate utilization patterns (Bouzar et al., 1994b) this organism is clearly a xanthomonad. Furthermore, sequence analysis of the 16S rRNA confirmed that this bacterium is phylogenetically closely related to the other xanthomonads associated with tomato (Jones et al., 2000). In the most recent work we presented DNA homology data showing that four distinct Xanthomonas species cause bacterial diseases on pepper and tomato (Jones et al., 2005), recommended reclassification of X. axonopodis pv. vesicatoria in order to avoid confusion and properly designated the species as originally intended in 1921, and provided evidence for naming two new species that represent the C and D groups of xanthomonads associated with tomato. The Group C tomato/pepper pathogen was analyzed among Group A, B, C and D tomato/pepper pathogens and other strains representing selected species of xanthomonads. The Group C tomato/pepper pathogen was related at greater than 40% to Group A strains. Although Group A strains were related at species-level (≥ 70% (based on the criteria of Wayne et al. (1987)) DNA relatedness), they had less than 50% relatedness with the type strain of X. axonopodis pv. axonopodis, the Group C tomato/pepper pathogen, and other Xanthomonas species (data for the other latter Xanthomonas species not shown). This confirms earlier results indicating low DNA relatedness between group A strains and X. axonopodis using the fluorometric method (Jones et al., 2000). Group B and D tomato/pepper pathogens were less than 10% related to each other and to Group A and C pathogens (Table 2). The strains originally identified by Gardner and Kendrick (1921) in Indiana were probably cosmopolitan B strains. Given that A group strains are not pathovars within X. axonopodis, that the latter strains should be elevated to species status based on the DNADNA hybridization data, and that the cosmopolitan feebly amylolytic A strains in all