Differentiation of pathotypes of lettuce mosaic virus

Differential interactions between six crisphead cultivars of lettuce and five isolates of lettuce mosaic virus (LMV) have identified a novel resistance factor in the cultivar Ithaca. The LMV isolates could be grouped into three pathotypes, two of which have not been identified previously. One of these, pathotype III represented by an isolate from Spain, was virulent on all of the cultivars. There were differences in aggressiveness between isolates of the same pathotype.