A survey for viruses in open field lettuce crops was carried out in March 2014 in the Al-Uyaynah area, central region of Saudi Arabia. In one plot, more than 50% of the lettuce plants (Lactuca sativa; hybrid: Romaine), with the majority of the affected plants in the edges of the plot, were showing virus-like symptoms such as necrotic lesions, necrosis of the lamina of the younger leaves, and leaf curling, indicating a possible infection by a Tospovirus, possibly Tomato spotted wilt virus (TSWV). Most of them were dead when the field was visited again 3 weeks later. Samples from 10 symptomatic and two asymptomatic plants were collected. Five of the samples from symptomatic and two from asymptomatic plants were mechanically inoculated onto Nicotiana benthamiana and N. glutinosa (three indicator plants of each species were used for each sample) using 0.1 M phosphate buffer (pH 7) containing 0.01M Na2SO3 mM. All the symptomatic lettuce samples were also tested serologically using polyclonal antisera (3) against TSWV, CMV, and by using monoclonal antibodies against potyviruses. Moreover, total RNA was extracted (1) and detection of TSWV was also attempted with reverse transcription (RT)-PCR using species specific primers (4) for a 276-bp fragment of the L RNA segment. In both serological and molecular methods, positive and negative controls were included. All the mechanically inoculated plants with tissue from the symptomatic lettuce plants of N. benthamiana showed chlorotic local lesions followed by systemic top necrosis 2 to 3 weeks post inoculation. Similarly, all inoculated N. glutinosa plants showed necrotic local lesions followed by systemic chlorosis. However, all the indicator plants mechanically inoculated with tissue from asymptomatic lettuce plants gave no reaction. All the symptomatic lettuce samples reacted positively, while asymptomatic samples reacted negatively in ELISA tests with TSWV antiserum and the presence of the virus was further confirmed by RT-PCR by using specific primers (method A) (4). PCR products of two randomly selected positive samples were directly sequenced and BLAST analysis of the obtained sequences (Accession Nos. KJ701035 and KJ701036) revealed 99% nucleotide and 100% amino acid identity with the deposit sequence in NCBI from South Korea (KC261947). Regarding mechanical inoculation, 10 days post-inoculation, both indicator plants showed typical symptoms of TSWV infection, such as necrotic local lesions, systemic necrotic patterns, and leaf deformation. None of the symptomatic plants was found to be infected with either CMV or potyvirus. To our knowledge, this is the first report of TSWV naturally infecting lettuce in Saudi Arabia; therefore, insect vector and weed management are necessary measures to control the virus spread to other crops such as tomato and pepper (2).
References: (1) E. Chatzinasiou et al. J. Virol. Meth. 169:305, 2010. (2) E. K. Chatzivassiliou. Plant Dis. 92:1012, 2008. (3) E. K. Chatzivassiliou et al. Phytoparasitica 28:257, 2000. (4) R. A. Mumford et al. J. Virol. Meth. 46:303, 1994.