Increased begomovirus disease incidence and severity in cucurbits associated with the emergence of CuLCrV in mixed infection with SLCuV in Baja California Sur, Mexico

Abstract

Begomoviral diseases in cucurbits have been observed in Baja California Sur (BCS), Mexico recurrently since the 2017 detection of the Squash leaf curl virus (SLCuV) in squash. Two years later, samples of symptomatic plants were collected in three arid agricultural regions, where the most frequently observed symptoms were chlorosis, reduced leaf size, stunting and severe leaf deformation, observed mainly on squash and cantaloupe. The mean incidence of begomovirus-like symptoms differed among cucurbits, with 20% in cucumber, 28% in cantaloupe, 30% in watermelon and 50% in squash. Samples were tested by polymerase chain reaction for begomovirus infection with degenerate primers, and later the virus identities were confirmed by specific primer pairs. All symptomatic cucurbit samples tested positive for SLCuV, and in a second analysis of these previously tested samples, squash, and cantaloupe samples tested positive for Cucurbit leaf crumple virus (CuLCrV). Thus, these samples showed a mixed CuLCrV/SLCuV infection. Pairwise identity analysis of the DNA-A component of the viruses indicated the presence of CuLCrV at 99%, and SLCuV at 99.1%. Recombination analyses between these viruses were negative, indicating that they belong to well-defined phylogenetic groups. Additionally, the 5´-TGGTGTCC-3´ iteron was consistently observed in all SLCuV and CuLCrV variants, including in other close species of the SLCuV clade. However, an additional iteron was observed in CuLCrV-BCS that is not in the Arizona and California variants, indicating that more studies are needed to elucidate the role of the additional iteron and the importance of CuLCrV/SLCuVSLCV mixed infection in emerging viruses in Mexico and other parts of the world.