Virus diseases that have emerged in the past two decades limit the production of important vegetable crops in tropical, subtropical, and temperate regions worldwide, and many of the causal viruses are transmitted by whiteflies (order Hemiptera, family Aleyrodidae). Most of these whitefly-transmitted viruses are begomoviruses (family Geminiviridae), although whiteflies are also vectors of criniviruses, ipomoviruses, torradoviruses, and some carlaviruses. Factors driving the emergence and establishment of whitefly-transmitted diseases include genetic changes in the virus through mutation and recombination, changes in the vector populations coupled with polyphagy of the main vector, Bemisia tabaci, and long distance traffic of plant material or vector insects due to trade of vegetables and ornamental plants. The role of humans in increasing the emergence of virus diseases is obvious, and the effect that climate change may have in the future is unclear.

Viruses of the genus Begomovirus (family Geminiviridae) are emergent pathogens of crops throughout the tropical and subtropical regions of the world. By virtue of having a small DNA genome that is easily cloned, and due to the recent innovations in cloning and low-cost sequencing, there has been a dramatic increase in the number of available begomovirus genome sequences. Even so, most of the available sequences have been obtained from cultivated plants and are likely a small and phylogenetically unrepresentative sample of begomovirus diversity, a factor constraining taxonomic decisions such as the establishment of operationally useful species demarcation criteria. In addition, problems in assigning new viruses to established species have highlighted shortcomings in the previously recommended mechanism of species demarcation. Based on the analysis of 3,123 full-length begomovirus genome (or DNA-A component) sequences available in public databases as of December 2012, a set of revised guidelines for the classification and nomenclature of begomoviruses are proposed. The guidelines primarily consider a) genus-level biological characteristics and b) results obtained using a standardized classification tool, Sequence Demarcation Tool, which performs pairwise sequence alignments and identity calculations. These guidelines are consistent with the recently published recommendations for the genera Mastrevirus and Curtovirus of the family Geminiviridae. Genome-wide pairwise identities of 91 % and 94 % are proposed as the demarcation threshold for begomoviruses belonging to different species and strains, respectively. Procedures and guidelines are outlined for resolving conflicts that may arise when assigning species and strains to categories wherever the pairwise identity falls on or very near the demarcation threshold value.

The family Geminiviridae includes viruses with mono- or bipartite single-stranded, circular DNA genomes of 2.5–5.2 kb. They cause economically important diseases in most tropical and subtropical regions of the world. Geminiviruses infect dicot and monocot plants and are transmitted by insect vectors. DNA satellites are associated with some geminiviruses. This is a summary of the International Committee on Taxonomy of Viruses (ICTV) Report on the family Geminiviridae which is available at ictv.global/report/geminiviridae.

Most plant viruses require a biological vector to spread from plant to plant in nature. Among biological vectors for plant viruses, hemipteroid insects are the most common, including phloem-feeding aphids, whiteflies, mealybugs, planthoppers, and leafhoppers. A majority of the emerging diseases challenging agriculture worldwide are insect borne, with those transmitted by whiteflies (Hemiptera: Aleyrodidae) topping the list. Most damaging whitefly-transmitted viruses include begomoviruses (Geminiviridae), criniviruses (Closteroviridae), and torradoviruses (Secoviridae). Among the whitefly vectors, Bemisia tabaci, now recognized as a complex of cryptic species, is the most harmful in terms of virus transmission. Here, we review the available information on the differential transmission efficiency of begomoviruses and other whitefly-borne viruses by different species of whiteflies, including the cryptic species of the B. tabaci complex. In addition, we summarize the factors affecting transmission of viruses by whiteflies and point out some future research prospects.

In September 2012, a novel disease syndrome was observed in zucchini (Cucurbita pepo L.) crops in Murcia Province (southeastern Spain). Symptoms included curling, vein swelling, and severe mosaic in young leaves, short internodes, and fruit skin roughness, resembling begomovirus infection. Similar symptoms were observed in May 2013 in Almería Province (southern Spain). DNA was isolated from 8 and 7 symptomatic leaf samples collected in Murcia and Almería, respectively, and analyzed by PCR with primers GemCP-V-5′ and GemCP-C-3′ designed to detect begomoviruses by amplifying the core of coat protein gene (CP) (3). DNA fragments of the expected size (~600 bp) were amplified supporting a begomovirus infection. The DNA sequences obtained from four samples were identical. BLAST analysis showed the highest nucleotide identity (98%) with partial CP gene sequences from isolates of Tomato leaf curl New Delhi virus (ToLCNDV) infecting cucumber in India (GenBank Accession No. KC846817). ToLCNDV, a bipartite begomovirus first reported from tomato, also infects other solanaceous and cucurbitaceous crops in India and neighboring countries (1). DNA from two samples from Murcia and three from Almería was used for rolling-circle amplification using ϕ29 DNA polymerase (TempliPhi kit, GE Healthcare, Little Chalfont, UK) and digested with a set of restriction endonucleases. All five samples yielded amplification products with identical restriction patterns. Two samples from Murcia (MU-8.1 and MU-11.1) and one from Almería (AL-661) were selected to clone the putative DNA-A and DNA-B begomovirus genome components by using single BamHI or NcoI sites. Inserts of two clones from each sample, one corresponding to DNA-A and one to DNA-B, were completely sequenced. The cloned genomes exhibited the typical organization of Old World bipartite begomoviruses (1). Sequences were aligned with begomovirus sequences available in databases using MUSCLE and pairwise identity scores were calculated with SDT (species demarcation tool [4]). DNA-A sequences obtained from Murcia (2,738 nt, KF749224 and KF749225) and Almería (2,738 nt, KF749223) shared >99% nucleotide identity, with the highest nucleotide identity (91.3 to 91.5%) with that of an Indian ToLCNDV isolate from chilli (HM007120). DNA-B sequences (2,684 nt, KF749226, KF749227, and KF749228) shared >99% nucleotide identity, and showed the highest nucleotide identity (83.1 to 83.3%) with that of a Pakistani ToLCNDV isolate from Solanum nigrum (AJ620188). Nucleotide sequence identity of DNA-A with the most closely related begomoviruses was above the 91% threshold for species demarcation (2), thus confirming that the begomoviruses found infecting zucchini in Spain are isolates of ToLCNDV. In fall 2013, the disease was widespread in zucchini both in Murcia and Almería, and ToLCNDV has also been found infecting melon and cucumber crops. To our knowledge, this is the first report of a bipartite begomovirus in Spain and Europe. References: (1) J. K. Brown et al. Page 351 in: Virus Taxonomy. Ninth Report of the ICTV. A. M. Q. King et al., eds. Elsevier/Academic Press, London, 2012. (2) ICTV Geminiviridae Study Group. New species and revised taxonomy proposal for the genus Begomovirus (Geminiviridae). ICTV. Retrieved from http://talk.ictvonline.org/files/proposals/ taxonomy_proposals_plant1/m/plant04/4720.aspx , 10 October 2013. (3) H. Lecoq and C. Desbiez. Adv. Virus Res. 84:67, 2012. (4) B. Muhire et al. Arch. Virol. 158:1411, 2013.