First Report of Lettuce Big Vein Disease Caused by Olpidium spp., Mirafiori Lettuce Big-Vein Virus, and Lettuce Big-Vein Associated Virus in Israel

Lettuce big-vein disease (LBVD) is an important disease of lettuce (Lactuca sativa L.) worldwide, which was found in 2014 in Saudi Arabia (AL-Saleh et al. 2015). LBVD is associated with a complex of two viruses, lettuce big-vein associated virus (LBVaV) and Mirafiori lettuce big-vein virus (MLBVV), which are transmitted by motile zoospores of the soil-borne fungus Olpidium spp. In lettuce, this viral complex causes clearing of the leaf lamina along the veins, resulting in enlarged leaf veins, and delay in heading with reduction of plant size. A disease with these typical symptoms was first observed in Israel in December 2010 in two commercial fields in the Besor region in the southwest of Israel. Further surveys for LBVD and assays for the presence of associated pathogens were carried out in three lettuce growing cycles during August, October, and December 2011 in two commercial fields in Sde Nitzan (N31°1308.03, E34°2503.32) and Ein HaBesor (N31°1554.74, E34°2814.33) in the southwest of Israel. From each field, 39 plants were sampled, and the presence of Olpidium spp. was assayed from roots after DNA extraction, by polymerase chain reaction (PCR) using species specific forward primers OLPbraF, OLPvirF, and OLPborF for Olpidium brassicae, O. virulentus, and O. bornovanus, respectively (Herrera-Vásquez et al. 2009). O. virulentus was found more prevalent in the lettuce roots compared with O. brassicae and O. bornovanus during winter (100, 28, and 28%, respectively) and autumn (90, 76, and 0%, respectively). On the other hand, in the summer, O. brassica was more prevalent than O. virulentus (23 and 3%, respectively), although symptoms were not seen on the lettuce plants. Associated viruses were identified after RNA extraction from lettuce leaves with LBVD symptoms, using reverse transcription PCR with specific primer pairs for the coat proteins of LBVaV and MLBVV (Navarro et al. 2004). MLBVV was found at higher incidence in the lettuce leaves compared with the LBVaV during winter (100 and 10%, respectively), in agreement with reports from other studies (Maccarone 2013). During summer, the viruses were not found in the lettuce plants. Naturally infested soil taken from a commercial field in Sde Nitzan, in which LBVD symptoms were observed, was used for four inoculation tests. Typical LBVD symptoms developed in lettuce plants planted in the infested soil compared with plants planted into sterile sandy soil. The presence of Olpidium spp. and MLBVV pathogens in the lettuce samples was confirmed by laboratory methods as described above, and both were found in 83 to 100% of the plants planted in the infested soil, compared with 0% in the uninfested soil.