Alfalfa (Medicago sativa L.) is one of the most important forage legumes in the United States (2). In Ohio, alfalfa is grown on approximately 600,000 acres. Virus-induced diseases play an important role in alfalfa production (3). Many viruses do not cause significant damage to the crop; however, alfalfa can be a reservoir for viruses that may cause damage to other hosts. Of the viruses that do cause damage to alfalfa, the most important is alfalfa mosaic virus (AMV).
Alfalfa plants infected with AMV express a variety of symptoms (Fig.1). This report describes a survey of the alfalfa-growing areas in Ohio in an effort to identify the viruses present in alfalfa and their frequency.
Seventeen counties located throughout Ohio (1990 and 1991) plus the test plots at the Ohio State University were sampling sites (Fig. 2). Sampling was conducted by taking plants showing virus-like symptoms, as well as symptomless plants. A total of 168 samples were collected from 42 alfalfa fields. The samples were analyzed for virus infection using a variety of methods. Enzyme-linked immunosorbent assay (ELISA) (Agdia, Inc., Elkhart, IN) was used to detect cucumber mosaic virus (CMV), AMV, and tobacco streak virus (TSV) (1). Dot-blot immunobinding assay (D-BIA) was used to detect red clover vein mosaic virus (RCVMV). Samples were also analyzed using viral-associated double-stranded RNA (dsRNA) analysis (4) and electron microscopy (EM) using the leaf-dip method. AMV was the most common virus identified in this survey, occurring in 61% or 102 of the samples tested. None of the samples analyzed with ELISA tested positive for TSV or CMV. Of a total of 168 samples, 69 or 41% tested positive for RCVMV. Of the 168 plants tested approximately 20% had a mixed infection of AMV and RCVMV. AMV was the most common virus identified using dsRNA analysis (50 samples or 30%). RCVMV was detected in 33 samples (20%). All samples were analyzed with EM; however, virus particles (long flexous rods with characteristics similar to RCVMV) were identified in only two of the samples. Those samples tested positive for RCVMV in D-BIA.
Results of this survey indicate that approximately 75% of the alfalfa samples tested were infected with either AMV and/or RCVMV. This is the first report of these viruses in Ohio alfalfa. AMV was the most common virus identified (61%). AMV was more likely to be identified in samples using ELISA than those tested with dsRNA or EM. RCVMV virus was identified in 41% of the samples tested using D-BIA, 69 of the 168 samples using dsRNA analysis, and in only two samples using EM.
AMV has a wide host range and alfalfa fields containing AMV-infected plants could serve as a source of this virus for other hosts (2). In Ohio, alternative host crops are being produced in many areas immediately adjacent to alfalfa production fields. Though this was a small scale survey, the results give growers in Ohio baseline information from which to work.
1. Clark, M. F., and Adams, A. N. 1977. Characteristics of the microplate method of Enzyme-Linked Immunosorbent Assay. J. Gen. Virol. 34:475-483.
2. Jaspers, E. M. J., and Bos, L. 1980. Alfalfa mosaic virus. Descriptions of Plant Viruses No. 229. Commow. Mycol. Inst., Assoc. Appl. Biol., Kew, Surrey, England.
3. Stuteville, D. L., and Erwin, D. C. 1990. Compendium of Alfalfa Diseases. American Phytopathological Society, St. Paul, MN.
4.Valverde, R., Nameth, S. T., and Jordan, R. L. 1990. Analysis of dsRNA for plant virus diagnosis. Plant Dis. 74:255-258.