Bean leafroll virus is widespread in subterranean clover (Trifolium subterraneum L.) seed crops and can be persistently transmitted by bluegreen aphid (Acyrthosiphon kondoi Shinji)

In the mid 2000s subterranean clover (Trifolium subterraneum) seed producers in South Australia reported symptoms of a red-leaf disease in fields with reduced seed yields. The red-leaf symptoms resembled those caused by several clover-infecting viruses. A set of molecular diagnostic tools were developed for the following viruses which are known to infect subterranean clover: Alfalfa mosaic virus; Bean leafroll virus (BLRV); Beet western yellows virus; Bean yellow mosaic virus; Cucumber mosaic virus; Pea seed-borne mosaic virus; Soybean dwarf virus and Subterranean clover stunt virus. Surveys of subterranean clover seed production fields in 2008 in the south-east of South Australia and western Victoria identified Bean leafroll virus, Alfalfa mosaic virus and Cucumber mosaic virus as present, with BLRV the most widespread. Surveys of pasture seed production fields and pasture evaluation trials in 2009 confirmed that BLRV was widespread. This result will allow seed producers to determine whether control measures directed against BLRV will overcome their seed losses. Bluegreen aphid (Acyrthosiphon kondoi) was implicated as a potential vector of BLRV because it was observed to be colonising lucerne plants adjacent to subterranean clover seed production paddocks with BLRV, and in a glasshouse trial it transmitted BLRV from an infected lucerne plant to subterranean clover in a persistent manner.

Nucleotide sequence shows that Bean leafroll virus has a Luteovirus-like genome organization

The complete nucleotide sequence of the Bean leafroll virus (BLRV) genomic RNA and the termini of its smallest subgenomic RNAs were determined to better understand its mechanisms of gene expression and replication and its phylogenetic position within the Luteoviridae. The number and placement of open reading frames (ORFs) within the BLRV genome was Luteovirus-like. The nucleotide and predicted amino acid sequences of BLRV were most similar to those of Soybean dwarf virus (SbDV). Phylogenetic analyses employing the neighbour-joining method and sister-scanning analysis indicated that the BLRV nonstructural proteins were closely related to those of Barley yellow dwarf virus-PAV (BYDV-PAV), a Luteovirus. The region surrounding the frameshift at the junction between ORFs 1 and 2 also contained sequences very similar to those of BYDV-PAV and a Dianthovirus, Red clover necrotic mosaic virus. Similar analyses showed that the structural proteins were most similar to those of the Polerovirus genus. The 3′-noncoding regions downstream of ORF5 contained sequences similar to translational control elements identified in the BYDV-PAV genome. These data suggest that BLRV, like SbDV, is derived either through selection from a common ancestor with BYDV-PAV or that BLRV is the product of two recombination events between luteovirus-like and polerovirus-like ancestors where the 5′ 2900 nt and 3′ 700 nt of the BLRV genome are from a Luteovirus and the intervening sequences are derived from a Polerovirus.

Screening and selection of faba bean (Vicia faba L.) germplasm resistant to Bean leafroll virus

Three hundred and fifty-eight faba bean accessions, originating from 28 countries in 5 continents, were evaluated for their reaction to a Syrian isolate (SV64-95) of Bean leafroll virus (BLRV, family Luteoviridae). Selection for resistance was based on: (i) visual evaluation for symptom expression (disease severity); (ii) virus concentration in the plant tissues, based on the intensity of reaction in the tissue-blot immunoassay; and (iii) seed yield. A large variability in resistance to BLRV was found both between and within faba bean accessions. Progenies of single plants, selected for resistance under inbreeding conditions, were evaluated for another 2-year period. Repeated reselection of inoculated plants, in the absence of outcrossing, significantly increased the proportion of resistant plants. The most resistant selections were made from populations originating from Afghanistan, China, Colombia, Pakistan, Russia, Sudan, Switzerland, Tunisia, Turkey, and Yemen.