In 2003, leaf samples from faba bean plants (Vicia faba L.) showing slight growth reductions and yellowing symptoms were collected in a field near Hebenshausen, Hesse, Germany. Some of these samples did not react in triple-antibody sandwich enzyme-linked immunosorbent assay (TAS-ELISA) with species-specific monoclonal antibodies (Mabs) to either Bean leaf roll virus or Turnip yellows virus, but did react with a broad-spectrum Mab (B-2-5G4) used to detect viruses in the genera Polerovirus and Luteovirus (family Luteoviridae) (1). Since this indicated the occurrence of a hitherto unrecognized polero- or luteovirus in faba bean in Germany, attempts were made to obtain nucleotide sequence information on two of the unknown faba bean isolates using a pair of degenerate primers (S2 [5′-ATCACITTCGGGCCGWSTCTATCAGA-3′] and AS3 [5′-CACGCGTCIACCTATTTIGGRTTITG-3′] [I = inosine]) derived from conserved domains in the capsid protein (CP) genes of several polero- and luteoviruses. Following reverse transcription (RT)-PCR amplification and cloning, the CP gene sequences of two genetically distinct isolates of Soybean dwarf virus (SbDV), a species of the genus Luteovirus, were obtained. To our knowledge, SbDV has not been reported from Germany or Europe but only from Africa, Australia, Japan, and the United States. In the two latter countries, at least two SbDV strain groups, SbDV-Y (for yellowing) and SbDV-D (for dwarfing), are distinguished on the basis of differences in symptomatology, host range, and molecular properties (2--4). On the basis of CP aa sequences, the two faba bean isolates from Hebenshausen differed by 8%, with one (FB1) most similar (>96% identity) to SbDV-D isolates and the other (FB2) closely related (>96%) to SbDV-Y isolates. Similar to observations in Japan (3) and the United States (2), we were able to detect SbDV in numerous samples from red clover (Trifolium pratense) and white clover (T. repens) in Braunschweig using SbDV antibodies (Agdia, Elkhart, IN) in DAS-ELISA. This was confirmed by RT-PCR amplification of CP gene sequences using SbDV-specific primers (SbDVs: 5′-GTCTACCTAAAAATTTCAAAGAATCTG-3′; SbDVas: 5′-CGGACCCGGTTCTCCGTCTA-3′). CP sequence analysis of two SbDV-positive clover samples revealed the presence of a SbDV-D isolate in red clover. However, a white clover plant contained an unusual SbDV isolate that possessed a unique CP, sharing aa sequence identities of approximately 92% with the two faba bean isolates from Germany and only 88.5 to 90.5% with other SbDV isolates. Attempts at aphid transmission of SbDV isolates from clover to faba bean were only successful for the combination Acyrthosiphon pisum and the white clover isolate. No faba bean seedlings became infected when the aphid species Aulacorthum solani and Aphis craccivora were given acquisition access feedings of 48 to 72 h on SbDV-infected white and red clover plants. The sequences determined in this study were deposited in GenBank (Accession Nos. EF466131--EF466134).
References: (1) A. D. Abraham et al. Phytopathology 96:437, 2006. (2) V. D. Damsteegt et al. Phytopathology 89:374, 1999. (3) T. Tamada and M. Kojima. No. 179 in: Descriptions of Plant Viruses. Assoc. Appl. Biol. Kew, England, 1977. (4) H. Terauchi et al. Arch. Virol. 146:1885, 2001.
