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United States Department of Agriculture

Agricultural Research Service

Research Project: BIOLOGY, EPIDEMIOLOGY, PATHOGENESIS, AND VECTOR SPECIFICITY OF SUGARBEET AND VEGETABLE VIRUSES Title: First report of Alfalfa mosaic virus infecting basil (Ocimum basilicum L.) in California.

Authors
item Wintermantel, William
item Natwick, Eric -

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 2, 2011
Publication Date: February 1, 2012
Citation: Wintermantel, W.M., Natwick, E.T. 2012. First report of Alfalfa mosaic virus infecting basil (Ocimum basilicum L.) in California. Plant Disease. 96(2):295.

Interpretive Summary: Basil (Ocimum basilicum L.) plants collected from a field in Imperial County, CA in May, 2011 were found to exhibit yellowing, chlorotic sectors and spots on leaves, resulting in plants being unmarketable. Total nucleic acid was extracted from plants and tested by 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, producing bands of the expected size in extracts from symptomatic test plants, but not in healthy basil or alfalfa samples. A 350 nt band was gel-extracted and sequenced and confirmed to be AMV by comparison to sequences available in GenBank. Serological tests (ELISA) using AMV-specific antiserum provided by R. Larsen confirmed infection by AMV. The virus was transmitted mechanically from the original source to indicator hosts Nicotiana benthamiana and N. clevelandii. Two additional basil fields in Imperial County were identified with similar symptoms and also confirmed positive for AMV by RT-PCR. Two additional basil plantings in shade houses open to the outside environment did not have AMV symptomatic plants, but these fields were at the extreme north end of Imperial Valley agriculture and well away from any alfalfa fields. The fields with AMV symptomatic plants and basil plants found to have plants positive for AMV were located at opposite ends of the production region from one another, indicating widespread incidence of AMV in the region. The basil fields that contained AMV positive plants were adjacent to alfalfa, although less than 1 percent of the plants exhibited AMV symptoms. At the time the basil plantations were sampled for AMV, no aphids were found in the 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, and when weather conditions favored flight all of the aforementioned aphid species were actively emigrating. 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. Significant acreage of basil is grown in the Imperial Valley. This acreage is surrounded by extensive and increasing alfalfa production totaling 137,000 acres in Imperial County, and representing a 21% increase in acreage over 2009 for the same region (2). The proximity of basil production to such a large alfalfa production region warrants the need for enhanced efforts at aphid management in basil production in order to reduce vector populations and reduce transmission to basil crops.

Technical Abstract: Basil (Ocimum basilicum L.) plants collected from a field in Imperial County, CA in May, 2011 were found to exhibit yellowing, chlorotic sectors and spots on leaves, resulting in plants being unmarketable. Total nucleic acid was extracted from plants and tested by 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, producing bands of the expected size in extracts from symptomatic test plants, but not in healthy basil or alfalfa samples. A 350 nt band was gel-extracted and sequenced and confirmed to be AMV by comparison to sequences available in GenBank. Serological tests (ELISA) using AMV-specific antiserum provided by R. Larsen confirmed infection by AMV. The virus was transmitted mechanically from the original source to indicator hosts Nicotiana benthamiana and N. clevelandii. Two additional basil fields in Imperial County were identified with similar symptoms and also confirmed positive for AMV by RT-PCR. Two additional basil plantings in shade houses open to the outside environment did not have AMV symptomatic plants, but these fields were at the extreme north end of Imperial Valley agriculture and well away from any alfalfa fields. The fields with AMV symptomatic plants and basil plants found to have plants positive for AMV were located at opposite ends of the production region from one another, indicating widespread incidence of AMV in the region. The basil fields that contained AMV positive plants were adjacent to alfalfa, although less than 1 percent of the plants exhibited AMV symptoms. At the time the basil plantations were sampled for AMV, no aphids were found in the 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, and when weather conditions favored flight all of the aforementioned aphid species were actively emigrating. 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. Significant acreage of basil is grown in the Imperial Valley. This acreage is surrounded by extensive and increasing alfalfa production totaling 137,000 acres in Imperial County, and representing a 21% increase in acreage over 2009 for the same region (2). The proximity of basil production to such a large alfalfa production region warrants the need for enhanced efforts at aphid management in basil production in order to reduce vector populations and reduce transmission to basil crops.

Last Modified: 12/19/2014
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