Family: Kitaviridae

Genus: Blunervirus
 

Distinguishing features  

Viruses assigned to the genus Blunervirus form a well-supported monophyletic group based on a well-supported maximum likelihood tree inferred from their RNA-directed RNA polymerase (RdRP) amino acid sequences. Within the genus, distinct subclades can be distinguished, reflecting differences in genome organization and composition. Blunervirus genomes are split into four or five segments. These viruses cause non-systemic diseases, and members of at least two species are transmitted by eriophyid mites.  

Virion

Morphology 

Particles of the blunerviruses exhibit a range of shapes, varying by species. Tomato fruit blotch virus (ToFBV, Blunervirus solani) produces slender, short bacilliform particles measuring approximately 25 × 100 nm (Kitajima et al., 2022), whereas those of tea plant necrotic ring blotch virus (TPNRBV, Blunervirus camelliae) form spherical virus particles about 88 nm in diameter (Hao et al., 2018).  

Nucleic acid

Blunerviruses possess segmented, positive-sense single-stranded genomes. RNA1 and RNA2 are 5.7–6.0 kb and 3.5–4.1 kb, respectively. RNA3 is 2.5–2.8 kb, and RNA4 is 1.7–2.1 kb. A fifth segment, RNA5 (2,268 nt), has so far been detected only in apple blunervirus 1 (ApBV1, Blunervirus mali) (Zhang et al., 2024).  

Lipids

Particles of the blunervirus ToFBV are enveloped (Kitajima et al., 2022).  

Genome organization and replication

The genome of blunerviruses is segmented into four or five RNA molecules (Ramos-González et al., 2023). The fifth segment has been detected only in apple blunervirus 1 (ApBV1) (Zhang et al., 2024). Both RNA1 and RNA2 of blunerviruses encode a single large polypeptide, each containing helicase domains (Pfam01443). Generally, the RNA1-encoded protein also comprises a methyltransferase domain (Pfam01660) and an ovarian tumor (OTU)-like protease domain (cd22792), whereas the RNA2-encoded protein contains the RdRP2 domain (Pfam00978). RNA3 (2.5–2.8 kb) comprises four to five ORFs of variable size, including those encoding SP24 homologs (Pfam16504), and orphan ORFs or exhibiting limited similarity to sequences in public databases (Ciuffo et al., 2020, Reddy and Sidharthan 2024). Proteins predicted to be encoded by RNA3 ORFs usually display a signal peptide and numerous transmembrane domains. RNA4 usually carries a single ORF that encodes the putative movement protein (MP) containing the 3A/RNA2 domain (Pfam00803). ApBV1 RNA5 encodes a 68 kDa protein with a conserved helicase domain. RNA segments of several members have 3′-terminal polyadenylate tracts.  

Biology

Most blunerviruses have been detected in Asia and North America (Reddy and Sidharthan 2024). ToFBV has a wider distribution, having been reported in Europe, Africa, South America, and Australia (Ciuffo et al., 2020, Nakasu et al., 2022). Blunervirus infection typically remains localized, with limited systemic movement. In blunervirus infections in tea, blueberry, and tomato plants, certain viral genome segments have been detected in systemic, asymptomatic tissues, including roots (Hao et al., 2018, Ren et al., 2022, Luigi et al., 2024). The blunerviruses BNRBV and ToFBV are transmitted by the eriophyids Calacarus corymbosi and the tomato russet mite (Aculops lycopersici), respectively (Bertin et al., 2025, Tassi et al., 2025). BNRBV is neither transmitted by viruliferous Brevipalpus yothersi mites nor mechanically (Tassi et al., 2025). ToFBV can be propagated by grafting (Bertin et al., 2025); viral particles accumulate in the perinuclear space and the lumen of the endoplasmic reticulum of the epidermis, peri- and mesocarp cells of infected fruits, but no viroplasm-like inclusions have been observed either in the nuclei or in the cytoplasm of the infected cells (Kitajima et al., 2022). ToFBV particles have also been observed in infected tomato leaves.   

Species demarcation criteria

The demarcation criteria for species of the genus Blunervirus are based on (1) the extent of the serological relationship as determined by immunodiffusion and/or ELISA, (2) less than 75% aa sequence identity for the polyprotein containing the conserved domains of viral methyltransferase (VMT), RNA helicase (HEL), and RNA-directed RNA polymerase (RdRP), (3) natural host range and (4) vector species and transmission. Since the VMT, HEL, RdRP are typically encoded by two viral RNA segments, comparisons must be performed considering the concatenated products of these domains.  

Related, unclassified viruses

Virus names and abbreviations are not official ICTV designations.