Taxonomy of the family Arenaviridae and the order Bunyavirales: update 2018.

In 2018, the family Arenaviridae was expanded by inclusion of 1 new genus and 5 novel species. At the same time, the recently established order Bunyavirales was expanded by 3 species. This article presents the updated taxonomy of the family Arenaviridae and the order Bunyavirales as now accepted by the International Committee on Taxonomy of Viruses (ICTV) and summarizes additional taxonomic proposals that may affect the order in the near future.

Safeguarding Fruit Crops in the Age of Agricultural Globalization.

The expansion of fruit production and markets into new geographic areas provides novel opportunities and challenges for the agricultural and marketing industries. Evidence that fruit consumption helps prevent nutrient deficiencies and reduces the risk of cardiovascular disease and cancer has assisted in the expansion of all aspects of the fruit industry. In today's competitive global market environment, producers need access to the best plant material available in terms of genetics and health if they are to maintain a competitive advantage in the market. An ever-increasing amount of plant material in the form of produce, nursery plants, and breeding stock moves vast distances, and this has resulted in an increased risk of pest and disease introductions into new areas. One of the primary concerns of the global fruit industry is a group of systemic pathogens for which there are no effective remedies once plants are infected. These pathogens and diseases require expensive management and control procedures at nurseries and by producers locally and nationally. Here, we review (i) the characteristics of some of these pathogens, (ii) the history and economic consequences of some notable disease epidemics caused by these pathogens, (iii) the changes in agricultural trade that have exacerbated the risk of pathogen introduction, (iv) the path to production of healthy plants through the U.S. National Clean Plant Network and state certification programs, (v) the economic value of clean stock to nurseries and fruit growers in the United States, and (vi) current efforts to develop and harmonize effective nursery certification programs within the United States as well as with global trading partners.

A member of a new genus in the Potyviridae infects Rubus.

Blackberry yellow vein disease causes devastating losses on blackberry in the south and southeastern United States. Blackberry yellow vein associated virus (BYVaV) was identified as the putative causal agent of the disease but the identification of latent infections of BYVaV led to the investigation of additional agents being involved in symptomatology. A potyvirus, designated as Blackberry virus Y (BVY), has been identified in plants with blackberry yellow vein disease symptoms also infected with BYVaV. BVY is the largest potyvirus sequenced to date and the first to encode an AlkB domain. The virus shows minimal sequence similarity with known members of the family and should be considered member of a novel genus in the Potyviridae. The relationship of BVY with Bramble yellow mosaic virus, the only other potyvirus known to infect Rubus was investigated. The presence of the BVY was verified in several blackberry plants, but it is not the causal agent of blackberry yellow vein disease since several symptomatic plants were not infected with the virus and BVY was also detected in asymptomatic plants.

Viral Interactions Lead to Decline of Blackberry Plants.

Blackberry yellow vein disease (BYVD) poses a new threat to the blackberry industry in the United States. Blackberry yellow vein associated virus (BYVaV) was originally thought to be the sole cause of this disease. However, BYVaV has been found in several asymptomatic blackberry cultivars. An unusual member of the family Potyviridae was identified recently from symptomatic plants and named Blackberry virus Y (BVY). BVY has been shown to spread in the field and cause BYVD when co-infected with BYVaV. Both viruses are asymptomatic in single infections but are readily detectable in asymptomatic plants by reverse transcription-polymerase chain reaction (RT-PCR). However, in mixed infections, the titer of BYVaV is repressed, sometimes to levels undetectable by RT-PCR, while the concentration of BVY is increased several fold. Electron microscopy revealed a variety of viral inclusions in symptomatic leaf samples, but none could be found in single infections with either BVY or BYVaV. Although BYVaV has been consistently associated with BYVD in different geographical regions, the detection of BVY has thus far been restricted to northwest Arkansas. It has been hypothesized that BYVaV is the synergistic determinant of BYVD that causes symptoms in different cultivars at various locations during co-infection with other viruses.

Incidence and Ecology of Blackberry yellow vein associated virus.

Symptoms of leaf vein yellowing and bush decline in blackberry were attributed to infection by a novel crinivirus named Blackberry yellow vein associated virus (BYVaV). The disease is an emerging threat to blackberry production because it can cause substantial yield loss. The objective of this study was to identify the source and means of spread of BYVaV. A survey of blackberry plants for BYVaV from wild, cultivated, and nursery stocks was conducted. Insect traps and healthy blackberry sentinel plants were placed among symptomatic plants in a production field throughout two growing seasons to monitor the occurrence of potential vectors and virus spread. Virus indicator plants were grafted with BYVaV-infected blackberry because this virus was latent in some blackberry cultivars, but indicator plants failed to express symptoms when infected with BYVaV. Reverse-transcription polymerase chain reaction detection revealed the occurrence of BYVaV in blackberry nurseries in the United States, in production fields in Arkansas, South Carolina, and North Carolina, and in wild blackberry populations in Arkansas. Whiteflies (Trialeurodes packardii and T. ruborum), potential vectors of BYVaV, were observed on sticky traps placed in blackberry fields and were found colonizing blackberry plants; however, transmission studies failed to produce whitefly-mediated transmission of BYVaV. Further understanding of the disease etiology is needed to devise viable management strategies for this disease.

A virus between families: nucleotide sequence and evolution of Strawberry latent ringspot virus.

Several clones of golden ginger mint (Mentha x gracilis, 'Variegata') were found infected with Strawberry latent ringspot virus (SLRSV). The virus was purified and cloned and the complete nucleotide sequence of a mint isolate was obtained. RNA 1 consists of 7,496 nucleotides excluding the poly-A tail and encodes a polyprotein with signature enzymatic motifs found in other picorna-like plant viruses. RNA 2 consists of 3,842 nucleotides excluding the poly-A tail, encoding a polyprotein that is processed to a putative movement protein and the two coat proteins of the virus. A satellite RNA of 1,117 nucleotides was associated with this isolate encoding for a putative protein of 31 kDa. Phylogenetic analysis revealed that SLRSV shares characteristics with members of the Cheravirus, Fabavirus, Comovirus and Sadwavirus genera indicative of the uniqueness of SLRSV. The close relationship of SLRSV with these genera led to the examination of aphid and beetle transmission of the virus with, however, negative results.

Nucleotide sequence of Blackberry yellow vein associated virus, a novel member of the Closteroviridae.

The complete nucleotide sequence of a novel member of the genus Crinivirus (family Closteroviridae), isolated from blackberry and tentatively named Blackberry yellow vein associated virus, was determined. The virus possesses a bipartite genome. RNA 1 is 7,801 nucleotides in length and papain-like protease, methyltransferase, RNA helicase and RNA-dependent RNA polymerase motifs have been identified in the proteins coded for by this molecule. The polymerase is probably expressed via a +1 ribosomal frameshift, a common feature among members of the Closteroviridae. RNA 2 is 7,917 nucleotides long and encodes nine open reading frames, similar in size and position to orthologous genes of other criniviruses with the exception of a second hydrophobic peptide found near the 5' terminus of the molecule. Phylogenetic analysis revealed a close relationship between Blackberry yellow vein associated virus and Beet pseudo yellows virus, another crinivirus that infects small fruit crops.

A novel emaravirus is associated with redbud yellow ringspot disease.

Yellow ringspot is the only virus-like disease reported in redbud (Cercis spp.) with symptoms including vein clearing, chlorotic ringspots and oak-leaf pattern. A putative new emaravirus was present in all trees displaying typical yellow ringspot symptoms and the name redbud yellow ringspot associated virus is proposed. The virus genome is composed of at least five RNA segments. Two coding regions were studied to determine isolate diversity with results pointing to a homogeneous virus population. Host range was evaluated using graft transmission and by testing species found in close proximity to infected trees. Mite transmission with Aculops cercidis, the predominant species found in redbud trees in the epicenter of the disease, was evaluated but was not found to be a vector of the virus. Based on this study and the accumulated knowledge on emaravirus evolution we propose that speciation is allopatric, with vectors being a major component of the process.

Localization of Transmissible and Nontransmissible Viruses in the Vector Nematode Xiphinema americanum.

ABSTRACT The inner lining of the food canal of nematodes that transmit plantinfecting viruses is regarded as the retention region of viruses. To characterize the location of transmissible and nontransmissible viruses in the vector nematode Xiphinema americanum, three nepoviruses, Tobacco ringspot virus (TRSV), Tomato ringspot virus(TomRSV), and Cherry leaf roll virus(CLRV), and one non-nematode-transmissible virus, Squash mosaic virus (SqMV), were evaluated for transmission efficiency and localization sites in the nematode. Transmission trials showed highest transmission efficiency for TomRSV (38% with 1 and 100% with 10 nematodes, respectively), intermediate efficiency for TRSV (27% with 1 and 65% with 10 nematodes, respectively), and no transmission for CLRV and SqMV. Electron microscopy and immunofluorescent labeling revealed that TRSV was primarily localized to the lining of the lumen of the stylet extension and the anterior esophagus, but only rarely in the triradiate lumen. Within a nematode population, particles of TRSV were no longer observed in these three regions 10 weeks after acquisition, and it is assumed that there was gradual and random loss of the virus from these areas. The percentage of nematodes that were labeled by virus-specific immunofluorescent labeling in a population of viruliferous nematodes decreased gradually after TRSV acquisition when the nematodes were placed on a nonhost of the virus, and the loss of immunofluorescent labeling paralleled the decrease in the ability of the nematode population to transmit the virus. TomRSV was localized only in the triradiate lumen based on thin-section electron microscopy. No virus-like particles were observed in any part of the food canal of nematodes that had fed on CLRV-infected plants. Virus-like particles that appeared to be partially degraded were observed only in the triradiate lumen of nematodes that had fed on SqMV-infected plants. These results clarified the status of localization of two nontransmissible viruses in X. americanum and presented evidence that two nematode-transmissible viruses, TRSV and TomRSV, are localized in different regions of the food canal of X. americanum.

Evidence that ribonuclease activity present in beetle regurgitant is found to stimulate virus resistance in plants.

Phaseolus vulgaris L. cv. 'Pinto' bean is a local lesion host for the plant pathogen Southern bean mosaic virus (SBMV) and its vector is the Mexican bean beetle, Epilachna varivestis Mulsant. The objective of this study was to determine if prior feeding by the beetle would affect 'Pinto' bean's resistance to SBMV and determine if ribonuclease (RNase), a major constituent of beetle regurgitant, mediated the plant's response to the virus. 'Pinto' bean plants fed upon by beetles had increased resistance to plant viruses compared to non-wounded or mechanically wounded and buffer-treated plants. Plants that were mechanically wounded and treated with RNase had increased resistance to plant viruses that was equal to plants fed upon by adult beetles. The induction of plant pathogen defenses could be a good adaptation for the plant in the presence of a beetle and pathogen threat. This evidence suggests that RNase activity in the beetle regurgitant could function as an insect-derived elicitor of plant resistance to viruses.

Cowpea mosaic virus-based chimaeras. Effects of inserted peptides on the phenotype, host range, and transmissibility of the modified viruses.

Expression of foreign peptides on the surface of cowpea mosaic virus particles leads to the creation of chimaeras with a variety of phenotypes and yields. Two factors were shown to be particularly significant in determining the properties of a given chimaera: the length of the inserted sequence and its isoelectric point. The deleterious effect of high isoelectric point on the ability of chimeras to produce a systemic infection occurs irrespective of the site of insertion of the peptide. Ultrastructural analysis of tissue infected with chimaeras with different phenotypes showed that all produced particles with a tendency to aggregate, irrespective of the size or isoelectric point of the insert. Host range and transmission studies revealed that the expression of a foreign peptide did not (1) alter the virus host range, (2) increase the rate of transmission by beetles or through seed, or (3) change the insect vector specificity. These findings have implications for both the utility and the biosafety of Cowpea mosaic virus-based chimaeras.