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.

Citrus sudden death (CSD) is a new disease that has killed approximately 1 million orange trees in Brazil.Here we report the identification of a new virus associated with the disease. RNAs isolated from CSD-affected and nonaffected trees were used to construct cDNA libraries. A set of viral sequences present exclusively in libraries of CSD-affected trees was used to obtain the complete genome sequence of the new virus. Phylogenetic analysis revealed that this virus is a new member of the genus Marafivirus. Antibodies raised against the putative viral coat proteins allowed detection of viral antigens of expected sizes in affected plants. Electron microscopy of purified virus confirmed the presence of typical isometric Marafivirus particles. The screening of 773 affected and nonaffected citrus trees for the presence of the virus showed a 99.7% correlation between disease symptoms and the presence of the virus. We also detected the virus in aphids feeding on affected trees. These results suggest that this virus is likely to be the causative agent of CSD. The virus was named Citrus sudden death-associated virus.

The Citrus leprosis disease (CiL) is associated to a virus (CiLV) transmitted by Brevipalpus spp. mites (Acari: Tenuipalpidae). CiL is endemic in Brazil and its recently spreading to Central America represents a threat to citrus industry in the USA. Electron microscopy images show two forms of CiLV: a rare nuclear form, characterized by rod-shaped naked particle (CiLV-N) and a common cytoplasmic form (CiLV-C) associated with bacilliform-enveloped particle and cytoplasmic viroplasm. Due to this morphological feature, CiLV-C has been treated as Rhabdovirus-like. In this paper we present the complete nucleotide sequence and genomic organization of CiLV-C. It is a bipartite virus with sequence similarity to ssRNA positive plant virus. RNA1 encodes a putative replicase polyprotein and an ORF with no known function. RNA2 encodes 4 ORFs. pl5, p24 and p61 have no significant similarity to any known proteins and p32 encodes a protein with similarity to a viral movement protein. The CiLV-C sequences are associated with typical symptoms of CiL by RT-PCR. Phylogenetic analysis suggests that CiLV-C is probably a member of a new family of plant virus evolutionarily related to Tobamovirus.