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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 endpoint was 1:lO. The agent survived well in leaf material stored at -8O"C, or in sap from leaves ground in buffer with DIECA and activated charcoal and freezedried. Mechanical transmission required low dilution (1:2) in the buffer with charcoal, and chilling of materials and utensils.In lettuce, N. occidentalis-Pl 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.

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Two potyvirus isolates from endive, originating from southern France (Ls252) and from the Netherlands (Ls265), that were highly and poorly pathogenic on lettuce, respectively, were compared with a common isolate (Lsl) of lettuce mosaic virus (LMV) and with two highly deviant Greek isolates from Helminthia (Picris) echioides (Gr4) and endive (Gr5), earlier recognized as LMV. The isolates could not be distinguished by particle morphology and serology, and were all identified as LMV. Leaf curling, plant stunting and necrosis were more characteristic of the virus than mosaic. The isolates studied varied considerably on differential host species and a range of lettuce cultivars including pathotype differentials of Pink et al. [1992b]. Lsl and Ls265 reacted largely as pathotype II, including the 'common strain' of the virus, but Ls265 was least pathogenic on lettuce. Ls252 fitted pathotype IV and was very similar to LMV-E (the 'Spanish strain'). The Greek isolates were very similar to each other in causing very severe symptoms on some non-lettuce hosts and a number of lettuce cultivars. In lettuce varietal reaction Gr4 resembled pathotype I, but Gr5 severely affected 'Salinas 88', resistant to pathotypes I, II and III, and i t appears to be a novel pathotype. Genetic interaction between lettuce and LMV is not following a simple yes-or-no pattern, and it is not a mere matter of resistance versus susceptibility. Adoptior~ of a more realistic resistance terminology is proposed. None of the lettuce cultivars tested was resistant to the most pathogenic isolate Ls252, but resistance to it was found in 2 out of 12 wild Lactuca species tested (Lactuca perennis and L. tatarica) while the symptomless plants of L. perennis clearly reacted in ELISA.