Occurrence of a New Pathotype of Lettuce mosaic virus on Lettuce in Brazil

Since 1970 lettuce mosaic has not been an important disease of lettuce in Brazil, due to the growing of cultivars that contain gene g, derived from cv. Gallega de Invierno, for tolerance. Recently, however, a widespread, serious outbreak of lettuce mosaic occurred in the State of Sao Paulo. Both lettuce cultivars that lack and those that contain gene g have been affected, suggesting the emergence of a new pathotype of Lettuce mosaic virus (LMV). Commercial lettuce fields were visited, and 20 samples were collected for virus identification by bioassay on differential hosts, serological tests, and electron microscopy. Of these, 12 were infected only by LMV, and 8 contained mixed infections of LMV and a possible new sequivirus, Lettuce mottle virus. Seedlings of susceptible cv. White Boston were inoculated with LMV from tolerant cultivars, and plants were allowed to flower. Seeds were collected and sown, and seedlings with mosaic symptoms were recovered. Twenty monolesional isolates, obtained by passing the virus three times through Chenopodium amaranticolor, were inoculated individually on lettuce differential cvs. Salinas and White Boston (susceptible) and cvs. Salinas 88, Vanguard 75, Ithaca, Malika, and Gallega de Invierno (tolerant) to previously described pathotypes of LMV (1). Considering the susceptibility of all test differentials, we concluded that a LMV pathotype IV exists that has overcome tolerance conferred by genes m and g and is responsible for the new outbreak of LMV in Brazil. This is the first report of LMV-IV in Latin America. Reference: (1) D. A. C. Pink et al. Plant Pathol. 41:5, 1992.