A new potexvirus associated with strawberry mild yellow edge disease

A physical map of cDNA clones prepared from dsRNA associated with the MY-18 source of strawberry mild yellow edge (SMYE) was constructed and 854 nucleotides adjacent to the 3' poly(A) tail were sequenced. The larger open reading frame product of Mr 25714 showed considerable amino acid homology to the coat protein cistrons of six potexviruses and two carlaviruses. A second product of Mr 11216 encoded completely within the coat protein cistron, but in a different frame, has similarities to two potexvirus polypeptides. The Mr 25714 ORF was fused to the Protein A gene in an expression vector and the fusion protein was purified by affinity chromatography and used to immunize a rabbit. The resulting polyclonal antiserum reacted strongly in immunoelectron microscopical tests with filamentous particles resembling those of potexviruses. Such particles were detected in the following SMYE sources: D-74 from Germany, two from the United Kingdom in Fragaria vesca 'Alpine' indicator plants and Oregon MY-18 in Rubus rosifolius. Among 27 potexvirus antisera tested for serological reactions none yielded strong decoration. Examination of ultrathin sections of R. rosifolius and F. vesca tissue infected with SMYE revealed aggregates of filamentous particles in phloem parenchyma cells. dsRNA from nine sources of SMYE collected from around the world reacted with the cDNA clones of this potexvirus in Northern hybridizations. It is concluded that the potexvirus is hitherto undescribed and the name strawberry mild yellow edge-associated potexvirus is proposed.