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Disease symptoms Symptoms include water‐soaked areas surrounded by chlorosis turning into necrotic spots on all aerial parts of plants. On tomato fruits, small, water‐soaked, or slightly raised pale‐green spots with greenish‐white halos are formed, ultimately becoming dark brown and slightly sunken with a scabby or wart‐like surface. Host range Main and economically important hosts include different types of tomatoes and peppers. Alternative solanaceous and nonsolanaceous hosts include Datura spp., Hyoscyamus spp., Lycium spp., Nicotiana rustica, Physalis spp., Solanum spp., Amaranthus lividus, Emilia fosbergii, Euphorbia heterophylla, Nicandra physaloides, Physalis pubescens, Sida glomerata, and Solanum americanum. Taxonomic status of the pathogen Domain, Bacteria; phylum, Proteobacteria; class, Gammaproteobacteria; order, Xanthomonadales; family, Xanthomonadaceae; genus, Xanthomonas; species, X. euvesicatoria, X. hortorum, X. vesicatoria. Synonyms (nonpreferred scientific names) Bacterium exitiosum, Bacterium vesicatorium, Phytomonas exitiosa, Phytomonas vesicatoria, Pseudomonas exitiosa, Pseudomonas gardneri, Pseudomonas vesicatoria, Xanthomonas axonopodis pv. vesicatoria, Xanthomonas campestris pv. vesicatoria, Xanthomonas cynarae pv. gardneri, Xanthomonas gardneri, Xanthomonas perforans. Microbiological properties Colonies are gram‐negative, oxidase‐negative, and catalase‐positive and have oxidative metabolism. Pale‐yellow domed circular colonies of 1–2 mm in diameter grow on general culture media. Distribution The bacteria are widespread in Africa, Brazil, Canada and the USA, Australia, eastern Europe, and south‐east Asia. Occurrence in western Europe is restricted. Phytosanitary categorization A2 no. 157, EU Annex designation II/A2. EPPO codes XANTEU, XANTGA, XANTPF, XANTVE. In this review we provide a historical perspective as well as an updated overview on the aetiology, epidemiology, and management strategies of bacterial spot of tomato and pepper.

The genus Xanthomonas is a diverse and economically important group of bacterial phytopathogens, belonging to the gamma-subdivision of the Proteobacteria. Xanthomonas axonopodis pv. citri (Xac) causes citrus canker, which affects most commercial citrus cultivars, resulting in significant losses worldwide. Symptoms include canker lesions, leading to abscission of fruit and leaves and general tree decline. Xanthomonas campestris pv. campestris (Xcc) causes black rot, which affects crucifers such as Brassica and Arabidopsis. Symptoms include marginal leaf chlorosis and darkening of vascular tissue, accompanied by extensive wilting and necrosis. Xanthomonas campestris pv. campestris is grown commercially to produce the exopolysaccharide xanthan gum, which is used as a viscosifying and stabilizing agent in many industries. Here we report and compare the complete genome sequences of Xac and Xcc. Their distinct disease phenotypes and host ranges belie a high degree of similarity at the genomic level. More than 80% of genes are shared, and gene order is conserved along most of their respective chromosomes. We identified several groups of strain-specific genes, and on the basis of these groups we propose mechanisms that may explain the differing host specificities and pathogenic processes.

Four phenotypic xanthomonad groups have been identified that are pathogenic to pepper, tomato, or both hosts. These include groups A and C which are found in Xanthomonas axonopodis pv. vesicatoria, group B found in X. vesicatoria, and group D found in ‘X. gardneri’. We present DNA:DNA hybridization data in which X. axonopodis pv. vesicatoria group A and C strains have less than 70% DNA relatedness with each other, with the type strain of X. axonopodis, and with the currently classified species within Xanthomonas and, therefore, should be removed from this species and given species status. We present information that the A strains most closely resemble the strains originally isolated by Doidge in 1921. In an attempt to avoid confusion in nomenclature as stated in Principle 1 of the Bacteriological Code, we propose that the A strains of X. axonopodis pv. vesicatoria be renamed as X. euvesicatoria (ATCC11633 T = NCPPB2968 T = ICMP 109 T = ICMP 98 T). Use of the euvesicatoria epithet should be reserved for strains originally identified by Doidge, which she designated Bacterium vesicatorium (Ann. Appl. Biol. 7: 407–430, 1921) in the original description when she referred to those strains as being feebly amylolytic. The name X. perforans sp. nov. is proposed for the C group of strains previously designated as X. axonopodis pv. vesicatoria (ATCC BAA-983 T = NCPPB 4321 T). We also propose that ‘X. gardneri’, which has less than 70% DNA relatedness with any of the Xanthomonas species and which has never had taxonomic status, be named X. gardneri (ATCC 19865 T = NCPPB 881 T) to reflect the specific epithet proposed by Sutic [17] in 1957.

Abstract Historically, Xanthomonas species are primarily known for their pathogenicity against plants, but recently, there have been more findings of non-pathogenic xanthomonads. In the present study, we report isolates from healthy rice seeds that belong to a new species, Xanthomonas protegens, a protector of the rice plants against a serious pathogenic counterpart, i.e. X. oryzae pv. oryzae upon leaf clip co-inoculation. The new member species is non-pathogenic to rice and lacks a type III secretion system. The pangenome investigation revealed a large number of unique genes, including a novel lipopolysaccharide biosynthetic gene cluster, that might be important in its adaptation. The phylo-taxonogenomic analysis revealed that X. protegens is a taxonomic outlier species of X. sontii, a core, vertically transmitted rice seed endophyte with numerous probiotic properties. Interestingly, X. sontii is also reported as a keystone species of healthy rice seed microbiome. The findings and resources will help in the development of unique gene markers and evolutionary studies of X. sontii as a successful symbiont and X. oryzae as a serious pathogen. Here, we propose X. protegens sp. nov. as a novel species of the genus Xanthomonas with PPL118 = MTCC 13396 = CFBP 9164 = ICMP 25181 as the type strain. PPL117, PPL124, PPL125, and PPL126 are other strains of the species. The study reports a new non-pathogenic Xanthomonas species that can protect rice plants from pathogenic Xanthomonas species.

ABSTRACT Xanthomonas is a well-studied genus of bacterial plant pathogens whose members cause a variety of diseases in economically important crops worldwide. Genomic and functional studies of these phytopathogens have provided significant understanding of microbial-host interactions, bacterial virulence and host adaptation mechanisms including microbial ecology and epidemiology. In addition, several strains of Xanthomonas are important as producers of the extracellular polysaccharide, xanthan, used in the food and pharmaceutical industries. This polymer has also been implicated in several phases of the bacterial disease cycle. In this review, we summarise the current knowledge on the infection strategies and regulatory networks controlling virulence and adaptation mechanisms from Xanthomonas species and discuss the novel opportunities that this body of work has provided for disease control and plant health. Here, we discuss the current knowledge surrounding regulatory networks and systems that control virulence and adaption mechanisms in Xanthomonas species. Additionally, we detail how study of these pathogens has provided novel opportunities for disease control and plant health.

Several secreted and surface-associated conserved microbial molecules are recognized by the host to mount the defence response. One such evolutionarily well-conserved bacterial process is the production of cell–cell signalling molecules which regulate production of multiple virulence functions by a process known as quorum sensing. Here it is shown that a bacterial fatty acid cell–cell signalling molecule, DSF (diffusible signal factor), elicits innate immunity in plants. The DSF family of signalling molecules are highly conserved among many phytopathogenic bacteria belonging to the genus Xanthomonas as well as in opportunistic animal pathogens. Using Arabidopsis, Nicotiana benthamiana, and rice as model systems, it is shown that DSF induces a hypersensitivity reaction (HR)-like response, programmed cell death, the accumulation of autofluorescent compounds, hydrogen peroxide production, and the expression of the PATHOGENESIS-RELATED1 (PR-1) gene. Furthermore, production of the DSF signalling molecule in Pseudomonas syringae, a non-DSF-producing plant pathogen, induces the innate immune response in the N. benthamiana host plant and also affects pathogen growth. By pre- and co-inoculation of DSF, it was demonstrated that the DSF-induced plant defence reduces disease severity and pathogen growth in the host plant. In this study, it was further demonstrated that wild-type Xanthomonas campestris suppresses the DSF-induced innate immunity by secreting xanthan, the main component of extracellular polysaccharide. The results indicate that plants have evolved to recognize a widely conserved bacterial communication system and may have played a role in the co-evolution of host recognition of the pathogen and the communication machinery.

Bacterial plant-pathogenic Xanthomonas strains translocate transcription activator-like (TAL) effectors into plant cells to function as specific transcription factors. Only a few plant target genes of TAL effectors have been identified, so far. Three plant SWEET genes encoding putative sugar transporters are known to be induced by TAL effectors from rice-pathogenic Xanthomonas oryzae pv. oryzae (Xoo). We predict and validate that expression of OsSWEET14 is induced by a novel TAL effector, Tal5, from an African Xoo strain. Artificial TAL effectors (ArtTALs) were constructed to individually target 20 SWEET orthologs in rice. They were used as designer virulence factors to study which rice SWEET genes can support Xoo virulence. The Tal5 target box differs from those of the already known TAL effectors TalC, AvrXa7 and PthXo3, which also induce expression of OsSWEET14, suggesting evolutionary convergence on key targets. ArtTALs efficiently complemented an Xoo talC mutant, demonstrating that specific induction of OsSWEET14 is the key target of TalC. ArtTALs that specifically target individual members of the rice SWEET family revealed three known and two novel SWEET genes to support bacterial virulence. Our results demonstrate that five phylogenetically close SWEET proteins, which presumably act as sucrose transporters, can support Xoo virulence.

Banana Xanthomonas wilt is a newly emerging disease that is currently threatening the livelihoods of millions of farmers in East Africa. The causative agent is Xanthomonas campestris pathovar musacearum (Xcm), but previous work suggests that this pathogen is much more closely related to species Xanthomonas vasicola than to X. campestris. We have generated draft genome sequences for a banana-pathogenic strain of Xcm isolated in Uganda and for a very closely related strain of X. vasicola pathovar vasculorum, originally isolated from sugarcane, that is nonpathogenic on banana. The draft sequences revealed overlapping but distinct repertoires of candidate virulence effectors in the two strains. Both strains encode homologues of the Pseudomonas syringae effectors HopW, HopAF1 and RipT from Ralstonia solanacearum. The banana-pathogenic and non-banana-pathogenic strains also differed with respect to lipopolysaccharide synthesis and type-IV pili, and in at least several thousand single-nucleotide polymorphisms in the core conserved genome. We found evidence of horizontal transfer between X. vasicola and very distantly related bacteria, including members of other divisions of the Proteobacteria. The availability of these draft genomes will be an invaluable tool for further studies aimed at understanding and combating this important disease.

AvrHah1 [avirulence (avr) gene homologous to avrBs3 and hax2, no. 1] is a transcription activator-like (TAL) effector (TALE) in Xanthomonas gardneri that induces water-soaked disease lesions on fruits and leaves during bacterial spot of tomato. We observe that water from outside the leaf is drawn into the apoplast in X. gardneri-infected, but not X. gardneriΔavrHah1 (XgΔavrHah1)-infected, plants, conferring a dark, water-soaked appearance. The pull of water can facilitate entry of additional bacterial cells into the apoplast. Comparing the transcriptomes of tomato infected with X. gardneri vs. XgΔavrHah1 revealed the differential up-regulation of two basic helix–loop–helix (bHLH) transcription factors with predicted effector binding elements (EBEs) for AvrHah1. We mined our RNA-sequencing data for differentially up-regulated genes that could be direct targets of the bHLH transcription factors and therefore indirect targets of AvrHah1. We show that two pectin modification genes, a pectate lyase and pectinesterase, are targets of both bHLH transcription factors. Designer TALEs (dTALEs) for the bHLH transcription factors and the pectate lyase, but not for the pectinesterase, complement water soaking when delivered by XgΔavrHah1. By perturbing transcriptional networks and/or modifying the plant cell wall, AvrHah1 may promote water uptake to enhance tissue damage and eventual bacterial egression from the apoplast to the leaf surface. Understanding how disease symptoms develop may be a useful tool for improving the tolerance of crops from damaging disease lesions.

Current challenges in controlling phytopathogenic bacteria lie in widespread chemical resistance, biosafety concerns, and the scarcity of novel biomacromolecule targets. While transposable elements have emerged as critical drivers of genetic variability and virulence in plant pathogens, their potential as druggable targets remains unexplored. Here, we report the first discovery of ISXoo15 transposase in Xanthomonas oryzae pv. oryzae (Xoo) as the bactericidal receptor for J9, a pyrimidine‐substituted pleuromutilin derivative. In vitro assays demonstrate J9's superior anti‐Xoo activity, with an EC50 of 0.12 mg/L—significantly lower than commercial agents thiodiazole copper (86.39 mg/L) and zinc thiazole (26.15 mg/L). In vivo pot trials reveal enhanced curative and protective efficacy of J9 against rice bacterial leaf blight compared to these metal‐based controls. A photoaffinity probe, P‐J9, is synthesised and coupled with activity‐based protein profiling to unequivocally identify ISXoo15 transposase (encoded by PXO_03433) as J9's specific target. Reverse transcription‐quantitative PCR confirmed significant downregulation of PXO_03433 expression in J9‐treated Xoo. Physiological and virulence‐related functional analyses of a homologous recombination‐mediated PXO_03433‐knockout strain (ΔPXO_03433) showed markedly attenuated virulence and impaired pathogenicity. Conversely, PXO_03433‐complemented strain CΔPXO_03433 possessed substantial restoration of pathogenicity‐related traits. Proteomic profiling revealed significant downregulation of pathways associated with DNA repair, recombination and binding proteins in both J9‐treated and mutant strains. ISXoo15 transposase may serve as a key regulator in enabling the homeostasis of the DNA metabolic network in the bacteria. This study provides pioneering evidence for targeting bacterial transposases as a novel antibacterial strategy, establishing a foundation for effective management of phytopathogenic bacteria. The study identifies the previously unknown transposase ISXoo15 as an antiphytobacterial target. By constructing mutant strains, this enzyme's essential role in regulating bacterial pathogenicity and virulence is revealed.