Lettuce big‐vein virus: mechanical transmission and relationships to tobacco stunt virus

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Abstract
Summary: Big‐vein diseased lettuce plants contained an agent that could consistently be transmitted mechanically to Chenopodium quinoa, in which it caused characteristic local lesions. Mechanical transmission was also possible to five other plant species including Nicotiana benthamiana, N. clevelandii and N. occidentalis, but not to lettuce. Symptoms in N. occidentalis were reminiscent of those of tobacco stunt disease. With zoospores of originally virus‐free Olpidium brassicae, subcultured on the roots of N. occidentalis‐P1, sap‐inoculated either from lettuce or via C. quinoa, the agent could be transferred back to lettuce in which characteristic symptoms of big‐vein were reproduced.Infectivity in sap at room temperature was reduced by half after 2 h, and was practically lost after one day. Thermal inactivation was considerable at 45°C and complete at 50°C. Most infectivity was lost at dilution 1:5, and the dilution end‐point was 1:10. The agent survived well in leaf material stored at ‐80°C, or in sap from leaves ground in buffer with DIECA and activated charcoal and freeze‐dried. Mechanical transmission required low dilution (1:2) in the buffer with charcoal, and chilling of materials and utensils.In lettuce, N. occidentalis‐P1 and C. quinoa, with all isolates tested but one, infection was always associated with the presence of rod‐shaped particles which in the literature have been associated with lettuce big‐vein, and are similar to those described for tobacco stunt. Results obtained corroborate the assumption that these particles are the virions of lettuce big‐vein virus. The virus also resembles tobacco stunt virus in mechanical transmissibility, instability in sap and symptoms on N. occidentalis.