Presence of lettuce big-vein disease and associated viruses in a subtropical area of Brazil

Lettuce big-vein is a soil-borne viral disease characterized by vein clearing and leaf shrinkage resulting in plants of poor quality and reduced marketable value. The disease commonly occurs at temperatures below 20^°C, typical of temperate climates. Recently, Mirafiori lettuce virus (MiLV), a possible ophiovirus, has been found to be the causal agent of the disease whereas the historically associated Lettuce big-vein virus (LBVV, genus Varicosavirus) causes only a latent infection (Roggero et al., 2003). No synergism has been observed between these two serologically unrelated viruses although both are transmitted by Olpidium brassicae. MiLV has been detected in samples from several European countries, USA, and Japan. Mixed infection by both MiLV and LBVV are common in samples from lettuce (Lactuca sativa L.) crops in northern Italy and France (Roggero et al., 2003). Molecular sequences for both viruses are now available (Sasaya et al. 2002; Van Der Wilk et al., 2002).

During recent years typical big-vein symptoms (Fig.1) have been observed in São Paulo State, Brazil, on various lettuce cultivars grown both directly in soil or hydroponically. Symptoms were observed mainly during the cooler season with day temperatures ranging from 18 to 22^°C and night temperatures from 10 to 16^°C. Samples collected in different areas of São Paulo State (Biritiba-Mirim, Embu-Guaçu, Guarulhos, Itapecerica da Serra, Mogi das Cruzes, Monte Alegre do Sul, Vargem Grande Paulista) were tested by DAS-ELISA with antisera specific for MiLV and LBVV (Roggero et al. 2003) and examined by electron microscopy (EM) using negative staining for the presence of viruses. Most samples with big-vein symptoms tested positive by ELISA for both viruses and a few only for one of the two. Using EM, only LBVV virus particles were observed; virions were observed in only a few samples confirming the difficulty in EM visualization of both LBVV and MiLV particles. Thus, our surveys in this subtropical area of Brazil had similar findings to those in the temperate climate of Europe, where both viruses are commonly found. The presence of O. brassicae was already reported in Brazil (Lin, 1979). To our knowledge, this is the first report of the occurrence of lettuce big-vein disease and the associated viruses MiLV and LBVV in a sub-tropical area.