Basil (Ocimum basilicum L.) plants collected from three fields in Imperial County, CA in May, 2011 were found to be exhibiting yellowing, chlorotic sectors and spots on leaves, resulting in unmarketable plants. Dodder (Cuscuta spp.) was present in one of the fields, but was not visibly associated with symptomatic plants. Total nucleic acid was extracted from four symptomatic and three asymptomatic basil plants, as well as from the dodder plant with the RNeasy Plant Mini Kit (Qiagen, Valencia, CA). Nucleic acid extracts were tested by reverse transcription (RT)-PCR for the presence of Alfalfa mosaic virus (AMV) using primers designed to amplify a 350-nt region of the AMV coat protein gene (3). RT-PCR produced bands of the expected size in extracts from all symptomatic plants and the dodder sample. No amplification was obtained from symptomless plants. A 350-nt band amplified from one plant was gel-extracted, sequenced (TACGen, Richmond, CA), and confirmed to be AMV by comparison to sequences available in GenBank (Accession No. K02703). Although serological tests on an initial basil sample were negative for AMV by ELISA using antiserum produced against AMV by R. Larsen, USDA-ARS, Prosser, WA (unpublished), AMV was confirmed by ELISA and RT-PCR in symptomatic Nicotiana benthamiana, N. clevelandii, and Malva parviflora plants following mechanical transmission from basil source plants. The fields with AMV infections were located at opposite ends of the production region from one another, indicating widespread dispersal of AMV in the region. All AMV positive plants were adjacent to alfalfa. Two additional basil plantings in shade houses open to the outside environment did not have AMV symptomatic plants and were also confirmed negative by RT-PCR, but these plantings were at the extreme north end of Imperial Valley agriculture and well away from any alfalfa fields. At the time the basil plantations were sampled for AMV, no aphids were found in any plantations, but during the several weeks prior to finding the AMV-positive plants, cowpea aphid, Aphis craccivora Koch; pea aphid, Acyrthosiphon pisum Harris; blue alfalfa aphid, Acyrthosiphon kondoi Shinji; and spotted alfalfa aphid, Therioaphis maculata Buckton were colonizing Imperial Valley alfalfa fields, producing winged adults. AMV is transmitted by at least 14 aphid species (1), and most aphid populations increase during the late spring in this important desert agricultural region. The acquisition of AMV by dodder suggests the parasitic plant may serve as a vector of AMV within basil fields, although further study will be necessary for clarification. Significant acreage of basil is grown in the Imperial Valley. This acreage is surrounded by extensive and increasing alfalfa production totaling 55,442 ha (137,000 acres) in Imperial County and representing a 21% increase in acreage over 2009 for the same region (2). To our knowledge, this is the first report of basil infected by AMV in California. The proximity of basil production to such a large alfalfa production region warrants the need for enhanced efforts at aphid management in basil production to reduce vector populations and reduce transmission to basil crops.
References: (1) E. M. Jaspars and L. Bos. Alfalfa mosaic virus. No. 229 in: Descriptions of Plant Viruses. Commonw. Mycol. Inst./Assoc. Appl. Biol., Kew, England, 1980. (2) C. Valenzuela. Imperial County California Crop and Livestock Report, 2010. (3) H. Xu and J. Nie. Phytopathology 96:1237, 2006.