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Biological and Molecular Characterization of Lettuce Mosaic Potyvirus Isolates from the Salinas Valley of California. Francisco Murilo Zerbini, Department of Plant Pathology, University of California, Davis 95616; Steven T. Koike(2), and Robert L. Gilbertson(3). (2)University of California Cooperative Extension, Salinas 93901; (3)Department of Plant Pathology, University of California, Davis 95616. Phytopathology 85:746-752. Accepted for publication 9 May 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/Phyto-85-746.

Recent lettuce mosaic outbreaks in the Salinas Valley of California have been attributed to the emergence or introduction of necrosis-inducing or severe strains of lettuce mosaic virus (LMV). A combination of molecular, serological, and biological methods were used to characterize LMV isolates associated with these outbreaks, including 5 putative necrosis-inducing isolates and a “type” LMV isolate. Comparisons of the deduced amino acid sequences of the coat protein (CP) N-terminal region (referred to here as the hypervariable [HV] region) did not indicate the presence of distinct strains. The amino acid identities of CP-HV ranged from 88 to 98%, and the sequences of the 5 putative necrosis-inducing isolates were no more identical to each other than they were to those of the other LMV isolates nor did they share common amino acid residues not present in the other isolates. All isolates were classified as pathotype II based on virulence on lettuce differential cultivars. Under the conditions of these inoculations, all isolates induced necrotic symptoms on susceptible cultivars. Induction of necrosis was attributed to host and environmental factors rather than to viral variability. Taken together, these results failed to indicate that new highly virulent LMV strains (pathotypes) are responsible for the recent lettuce mosaic outbreaks. This study illustrates the usefulness of polymerase chain reaction in the characterization of viral variability and establishes that individual isolates of a potyvirus strain may show as much as 12% divergence in the CP-HV region without changes in biological properties.