NOVEL PATHOTYPES OF LETTUCE MOSAIC-VIRUS - BREAKDOWN OF A DURABLE RESISTANCE

Resistance to lettuce mosaic virus (LMV) is derived either from cv. Gallega (g gene) or the wild accession PI251245 (mo gene). Previous studies indicated that these two genes were identical. Breeders in Europe produced numerous resistant cultivars utilising g while in the USA mo was used. The resistance has been effective for over 20 years. However, recently there have been reports of LMV isolates causing unusually severe and sometimes necrotic symptoms on cultivars with these resistance genes. Investigations of these 'severe' isolates have distinguished three new pathotypes in addition to the common pathotype (II) and identified a novel dominant gene for resistance. The mo/g genes confer resistance to pathotypes I and II but pathotype III possesses virulence for cultivars with g but not for those with mo. These two genes are therefore not identical but are probably either closely linked genes or alleles. Pathotype IV possesses virulence for all lettuce lines so far tested. Some isolates of this pathotype are seed transmitted in cultivars possessing mo or g and have caused severe crop losses in southern France. The durability of the resistance conditioned by these two genes is discussed.