Mirafiori lettuce big-vein virus  (MLBVV)

- classification : Ophiovirus
- other name: "Big-vein"

The big vein virus lettuce Mirafiori lettuce big vein virus (MLBVV) (formerly Mirafiori lettuce virus MiLV) was reported for the first time in California on lettuce. Since then, its presence has been recognized in some twenty countries. Identified in most European countries, it would be responsible for losses currently estimated at 40 million euros per year. The disease manifests itself in temperate and Mediterranean regions, or even warm in areas where significant temperature contrasts can occur. It is also rife in Japan, Australia and New Zealand. The Big-vein is harmful in cultivation under cover and in NFT production; it is also common in the open field. The most serious attacks occur on soils where lettuce and chicory are frequently grown. In the case of monocultures of lettuce, the disease tends to worsen from year to year; thus attacks affecting 50 to 70% of plants have been reported in some plots. The damage usually manifests itself in late fall and winter.

MLBVV is the causative agent of "Big-vein", one of the most serious diseases of lettuce. It is also rampant on curly chicory and escarole. This condition is all the more serious because there is no satisfactory method of combating it.

The etiology of the Big Vein has remained a mystery for decades. Although this disease was described in 1934 in California, the causative agent ( Big-vein agent or BVA ) remained unidentified for a long time. Between 1983 and 1987, the Lettuce big vein virus ( LBVV ) (later named Lettuce big-vein associated virus - LBVaV ) was officially considered to be responsible for this disease and a typical member of the Varicosavirus family. In the absence of verification of Koch's postulate, doubts remained as to his implication. Also, other avenues were pursued, in particular that of a Tobamovirus. Studies carried out in Italy showed that a new virus called at the time Mirafiori lettuce virus (MiLV) was associated with this disease in the absence of LBVV. Note that Koch's postulate and virus transmission by the fungus Olpidium virulentus (now called Pleotrachelus virulentus Sahtiy.) Have only been demonstrated for MLBVV. Recent Japanese work confirms these results.

MLBVV particles have a singular morphology, like that of other viruses of the genus Ophiovirus, with filaments 3 nm in diameter more or less folded, forming masses of two different sizes and of indeterminate outline. Under electron microscopy, they are very difficult to observe in raw plant extracts (figure 1).

Figure 1