An Ophiovirus isolated from lettuce with big-vein symptoms

Big-vein is a widespread and damaging disease of lettuce, transmitted throu gh soil by the chytrid fungus Olpidium brassicae, and generally supposed to be caused by Lettuce big-vein virus (LBVV; genus Varicosavirus). This viru s is reported to have rigid rod-shaped particles, a divided double-stranded RNA genome, and one capsid protein of 48 kD, but has not been isolated or rigorously shown to cause the disease. We provide evidence that a totally d ifferent virus, here named Mirafiori lettuce virus (MiLV), is also very fre quently associated with lettuce showing big-vein symptoms. MiLV was mechani cally transmissible from lettuce to Chenopodium quinoa and to several other herbaceous test plants. The virus was partially purified, and an antiserum prepared, which did not react with LBVV particles in decoration tests. As reported for LBVV, MiLV was labile, soil-transmitted and had a single capsi d protein of 48 kD, but the particles morphologically resembled those of op hioviruses, and like these, MiLV had a genome of three RNA segments approxi mately 8.5, 1.9 and 1.7 kb in size. MiLV preparations reacted strongly in W estern blots and in ISEM with antiserum to Tulip mild mottle mosaic virus, an ophiovirus from Japan also apparently Olpidium-transmitted. They reacted weakly but clearly in Western blots with antiserum to Ranuculus white mott le virus, another ophiovirus. When lettuce seedlings were mechanically inoc ulated with crude or partially purified extracts from MiLV-infected test pl ants, many became systemically infected with MiLV and some developed bigvei n symptoms. Such plants did not react in ELISA using an LBVV antiserum or a n antiserum to tobacco stunt virus, and varicosavirus-like particles were n ever seen in them in the EM after negative staining. We conclude that MiLV is a hitherto undescribed virus assignable to the genus Ophiovirus. The cau se or causes of lettuce big-vein disease and the properties of LBVV may nee d to be re-evaluated in light of our results.