The “big vein” attacks

The thickening of the veins or “big vein” is a recent disease in Brazil, which this winter limited the production of lettuce, mainly the American type, in certain areas of the State of São Paulo. The leaves of lettuce plants present the symptom of generalized thickening of the veins, due to hyperplasia of the cells close to the veins, which increase their volume exaggeratedly. The size of the plant is reduced, the physiological maturity of the plant is delayed. Plants infected at the seedling stage may fail to develop and die. In iceberg lettuce cultivars, poor head formation often occurs, while in the smooth type, the thickening may be generalized or visible only in some older leaves of the plant, depending on the seedling's stage of infection.

The disease is apparently caused by the Mirafiori lettuce virus (MiLV), a possible member of the Ophiovirus genus. However, historically this disease had been attributed to another virus, Lettuce big vein virus (LBVV), of the Varicosavirus genus. MiLV particles are very difficult to visualize under an electron microscope, as they are thin, filamentous and tend to become circular. MiLV is frequently found in mixed infections with LBVV, however recent studies reveal that LBVV causes latent infection in lettuce, with MiLV alone being responsible for the symptom of vein thickening.

Both LBVV and MiLV are transmitted by soil fungus

, an obligate parasite that lives in the roots of various plants. The virus is transmitted by fungal spores infected with the virus (zoospores) that require a film of water to move and reach the lettuce roots. As there is a need for water, this disease is common in waterlogged and poorly drained soils. O. brassicae itself does not cause problems with lettuce as long as it is not infected with the virus. The fungus can remain viable for more than 25 years in the soil.

The disease typically occurs in temperate regions of the United States and Europe, but has recently been reported to occur in subtropical areas of Brazil. Symptoms of the disease are favored by mild temperatures around 16C°, disappearing at temperatures above 22C°. Thus, in Brazil, vein thickening is a typical winter disease, when daily temperatures vary from 18 to 22 C° and night temperatures from 10 to 16 C°, disappearing completely in the summer.

The fungus O. brassicae is difficult to control, mainly due to the ability of the oospores to remain infective for many years in the soil. If the disease has not been verified, seedlings containing good quality substrate, absent of the fungus infected by MiLV, should be used. The elimination of herbs such as Sonchus asper and S. oleraceus, hosts of O. brassicae, irrigation control, use of quality substrate, in addition to the use of tolerant lettuce cultivars, are options for managing the disease in the field.

However, vein thickening is not a disease that is restricted to the field. Seedlings grown in hydroponics can also present the disease, as long as they are in contact with a fungus infected by the MiLV virus. In this case, the fungus may be transmitted mainly by the water used in the system, from contaminated equipment and water tanks. In this case, we suggest general cleaning of the system before the next installation of seedlings.

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UNESP-Botucatu