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Molecular and Biological Characterization of Lettuce mosaic virus (LMV) Isolates Reveals a Distinct and Widespread Type of Resistance-Breaking Isolate: LMV-Most

First, second, fourth, and eleventh authors: Equipe de Virologie, IBVM, INRA Bordeaux-Aquitaine, BP 81, 33883 Villenave d'Ornon Cedex, France; third and fifth authors: Laboratoire de Génétique Moléculaire, Immunologie et Biotechnologie, Faculté des Sciences de Tunis, Elmanar, 2092 Tunis, Tunisia; sixth author: Benaki Phytopathological Institute, Stephanou Delta Street 8, 14561 Kifissia, Greece; eighth and ninth authors: Pathologie Végétale, INRA, BP 94, 84143 Montfavet Cedex, France; seventh author: Departamento de Defesa Fitossanitária, UNESP/FCA, 18603-000, Botucatu, SP, Brazil; and first and tenth authors: Departamento de Fitopatologia/BIOAGRO, Universidade Federal de Viçosa, 36571-000, Viçosa, MG, Brazil

Lettuce mosaic virus (LMV) causes an economically important seedborne and aphid-transmitted disease of lettuce and ornamental crops worldwide. The genetic diversity among 73 LMV isolates was examined based on a 216-nucleotide sequence at the variable region encoding the NIb-coat protein junction. Three clusters of LMV isolates were distinguished: LMV-Yar, LMV-Greek, and LMV-RoW. In the latter cluster, two subgroups of isolates, LMV-Common and LMV-Most, accounted for a large proportion of the LMV isolates analyzed. These two subgroups included the seedborne isolates, consistent with this property contributing a selective advantage and resulting in widespread distribution. In addition to being seedborne, LMV-Most isolates overcome the two resistance genes commonly used in lettuce, mo1¹ and mo1², and thus represent a potential threat to lettuce cultivation. The complete sequence of an LMV-Most isolate (LMV-AF199) was determined, allowing a better definition of the genetic relationships among LMV-Most, LMV-Common, and an additional isolate of the LMV-RoW cluster.