Characterization of Ti Ringspot-Associated Virus, a Novel Emaravirus Associated with an Emerging Ringspot Disease of Cordyline fruticosa.

Ti ringspot is an emerging foliar disease of the ti plant (Cordyline fruticosa) in Hawaii that is quickly spreading throughout the islands. Symptoms include small chlorotic ringspots on leaves that often coalesce to form larger lesions. Although several virus species have been discovered in symptomatic plants, none have been associated with these symptoms. Here, we report and characterize a novel virus closely associated with ti ringspot symptoms in Hawaii. The presence of double membrane bodies approximately 85 nm in diameter in symptomatic cells and sequence analyses of five genomic RNA segments obtained by high-throughput sequencing indicate that this virus is most closely related to members of the plant virus genus Emaravirus. Phylogenetic and sequence homology analyses place this virus on a distinct clade within the Emaravirus genus along with High Plains wheat mosaic emaravirus, blue palo verde broom virus, and Raspberry leaf blotch emaravirus. Sequence identity values with taxonomically relevant proteins indicate that this represents a new virus species, which we are tentatively naming ti ringspot-associated virus (TiRSaV). TiRSaV-specific reverse transcription PCR assays detected the virus in several experimental herbaceous host species following mechanical inoculation. TiRSaV was also detected in eriophyid mites collected from symptomatic ti plants, which may represent a putative arthropod vector of the virus.

Differentiation and distribution of cordyline viruses 1-4 in Hawaiian ti plants (Cordyline fruticosa L.).

Common green ti plants (Cordyline fruticosa L.) in Hawaii can be infected by four recently characterized closteroviruses that are tentative members of the proposed genus Velarivirus. In this study, a reverse-transcription polymerase chain reaction (RT-PCR) assay developed to detect and distinguish Cordyline virus 1 (CoV-1), CoV-2, CoV-3, and CoV-4 was used to determine: (i) the distribution of these viruses in Hawaii; and (ii) if they are involved in the etiology of ti ringspot disease. One hundred and thirty-seven common green ti plants with and without ti ringspot symptoms were sampled from 43 sites on five of the Hawaiian Islands and underwent the RT-PCR assay. Eleven ornamental ti varieties were also sampled and assayed. Based on this survey, it appears none of the CoVs are involved in the etiology of ti ringspot. The observation of a non-uniform geographic distribution of the CoVs in common green ti, combined with the presence of CoVs in seed-derived ornamental varieties, suggests active vector transmission. Eight herbarium specimens collected between 1903 and 2003 from plants on the island of Oahu also underwent the RT-PCR assay. Amplifiable RNA was isolated from accessions collected in 1985 or later, however only the 2003 accession was found to harbor CoVs.

Molecular characterization of closteroviruses infecting Cordyline fruticosa L. in Hawaii.

In Hawaii, common green ti plants (Cordyline fruticosa L.) have been shown to harbor Cordyline virus 1 (CoV-1) which, along with Little cherry virus 1 (LChV-1), and Grapevine leafroll-associated virus 7 (GLRaV-7), form a distinct clade within the family Closteroviridae. Preliminary work has indicated that, aside from CoV-1, three additional closteroviruses may infect common green ti plants in Hawaii. In this study, pyrosequencing was used to characterize the genomes of closteroviruses infecting a single common green ti plant. The sequence data confirmed the presence of CoV-1 as well as three additional closteroviruses. Although all four viruses had the same general genome organization, the sequence divergence between the RNA-dependent RNA polymerase, heat shock protein 70 homolog, and coat protein ranged from 22 to 61%, indicating these represent four distinct closterovirus species. The names CoV-2, CoV-3, and CoV-4 are proposed for the three new viruses. Phylogenetic analyses placed CoV-2, CoV-3, and CoV-4 in the same clade as CoV-1, LChV-1, and GLRaV-7.