Virus Diseases of Lettuce in Brazil

Several viruses have been reported to infect lettuce. The most important is Lettuce mosaic virus (LMV), a potyvirus found worldwide transmitted by seeds and aphids, in a non-persistent manner. LMV causes quite variable symptoms, including mosaic, dwarfing, failure to form proper heads, and sometimes necrotic reactions. Cultivars carrying the mo11 and mo12 genes have resistance to the common strains, although most (mo breaking seed transmitted) strains can overcome this resistance. At least three species of tospovirus, including Tomato spotted wilt virus (TSWV), Tomato chlorotic spot virus (TCSV) and Groundnut ringspot virus (GRSV) causes significant losses, especially during summer, in which high populations of thrips vectors can be found in the field. Tospoviruses causes systemic necrosis and plant death. During the cooler season (May to September), two viruses have been found associated with big-vein disease, Mirafiori lettuce big-vein virus (MLBVV) which belongs to the genus Ophiovirus and Lettuce big-vein associated virus (LBVaV; genus Varicosavirus). LBVaV and MLBVV are both transmitted by the soil-borne fungus Olpidium brassicae. Lettuce mottle virus (LeMoV, genus Sequivirus); Cucumber mosaic virus (CMV, genus Cucumovirus), and Bidens mosaic virus (BiMV, genus Potyvirus) are also found to cause mosaic symptoms on lettuce, although their incidence in lettuce fields is low throughout the year. Epidemiological aspects, variability of viruses, methods of control, genetic variability for lettuce resistance and breeding programs will be discussed. _____________________________________________________________________________________________________________