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First Report of a Rhabdovirus Infecting Alfalfa in Argentina

Alfalfa (Medicago sativa L.) is a major forage crop in Argentina with an estimated cultivated area of 4 million ha in the 2009–2010 season, which constitutes a primary component for the animal production chain. In early summer of 2010, alfalfa plants showing virus-like symptoms were identified in 20 commercial fields in La Pampa Province with 95% disease prevalence. Diseased plants had shortened internodes, a bushy appearance, deformations, puckering, epinasty of leaflet blades, vein enations, and varying sized papillae on the adaxial leaflet surfaces. Similar symptoms were observed in alfalfa crops in Buenos Aires, Cordoba, Santa Fe, and Santiago del Estero provinces. Electron microscopy (EM) and molecular assays were performed on leaf tissue from one asymptomatic and two symptomatic plants. EM of ultrathin sections revealed membrane-bound bullet-shaped particles associated with the endoplasmic reticulum of phloem cells from symptomatic plants only. Total RNA was extracted from symptomatic and asymptomatic plants with the RNeasy Plant Mini Kit (Qiagen, Hilden, Germany) for a template in one-step reverse transcription (RT)-PCR assays with the Access RT-PCR Kit (Promega, Madison, WI). RT-PCR assays employed degenerate primers targeting conserved regions of plant rhabdovirus polymerase (L) genes (2). An amplicon of approximately 1 kilobase pairs (detected only from symptomatic plants) was gel purified with the Wizard SV Gel and PCR Clean-Up System (Promega), cloned into pGEM-T Easy Vector System (Promega), and sequenced. Three independents clones were sequenced in both directions at Macrogen Inc. (Korea Republic) to generate a consensus sequence (GenBank Accession No. HQ380230) and compared to sequences of other plant rhabdoviruses available on GenBank. The partial L gene sequence of the alfalfa-infecting rhabdovirus shared highest nucleotide (68.0%) and amino acid (76.5%) sequence identity with the cytorhabdovirus Strawberry crinkle virus (Accession No. AY331390). A phylogenetic tree based on partial amino acid sequences of the polymerase gene indicated the alfalfa-infecting virus was more closely related to cytorhabdoviruses than to nucleorhabdoviruses. Symptoms observed resembled those reported for alfalfa plants infected with a plant rhabdovirus named Alfalfa enation virus (1), which has never been fully characterized. Collectively, the data implicate the observed rhabdovirus as the etiological agent. To our knowledge, this is the first report in Argentina (and South America) of a rhabdovirus infecting alfalfa. Additional field surveys and monitoring of vector/s and yield losses need to be conducted.

References: (1) B. Alliot and P. A. Signoret. Phytopathol. Z. 74:69, 1972. (2) R. L. Lamprecht et al. Eur. J. Plant Pathol. 123:105, 2009.