FURTHER OBSERVATIONS ON VARIATION OF LETTUCE MOSAIC-VIRUS IN RELATION TO LETTUCE (LACTUCA-SATIVA), AND A DISCUSSION OF RESISTANCE TERMINOLOGY

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 (Ls1) 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]. Ls1 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 it 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. Adoption 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.