Kura clover (Trifolium ambiguum M. Bieb.) has been reported to be resistant to several viruses, including Alfalfa mosaic virus (AMV), Bean yellow mosaic virus (BYMV), Clover yellow vein virus (CYVV), Peanut stunt virus, Red clover vein mosaic virus (RCVMV), and White clover mosaic virus (WCMV) (2). Furthermore, 54 of 61 kura clover plants were resistant to Clover yellow mosaic virus (CYMV). Field-grown kura clover plants had no visual symptoms of virus infection, but a small proportion of plant introductions tested positive for CYVV, WCMV, and RCVMV (1). These and similar studies have given kura clover the reputation of being highly resistant to most viruses that affect other forage legumes. Viral-like symptoms, characterized by mosaic, crinkling, and reduced size of leaflets were observed on 53 kura clover plants in an 88-plant collection (plant introductions and cultivars) growing in the field. A 20-plant subset was screened for AMV, Bean pod mottle virus, BYMV, Soybean mosaic virus, Tobacco ring spot virus, and Tobacco streak virus using enzyme-linked immunosorbent assay (ELISA). Only AMV was found, and it was detected in nine plants. To our knowledge, this is the first report of AMV in kura clover. The remaining 68 plants were tested for AMV. In total, 70 plants were positive, and 18 plants were negative. AMV was detected in leaves and rhizomes of kura clover. Simultaneously, plants were scored on two occasions for interveinal mosaic, yellowing, curling of leaves, and general chlorosis. There was no correlation between visual symptoms and ELISA results. Results from ELISA were confirmed with a local lesion assay on ‘Bountiful’ beans (Phaseolus vulgaris L.). Leaves from three AMV-positive plants were bulked and ground in phosphate buffer (pH 7.0). Leaves of challenge plants were dusted with Carborundum, and infected sap was rubbed on the youngest leaf of each plant. This procedure was repeated with leaves from three AMV-negative plants. All 12 bean plants inoculated with sap from AMV-positive kura clover developed local lesions or systemic reactions. None of the 12 negative controls developed local lesions. The transmission of AMV from one kura clover plant to another was attempted with the inoculation procedure described above, except that a phosphate-sulfite buffer was used and with soybean aphids (Aphis glycines Matsamura). Seven virus-free ‘Endura’ kura clover plants were inoculated with sap from AMV-positive kura clover plants, and five negative control plants were included. Ten other plants were inoculated with AMV using soybean aphids. Aphids were allowed to feed for 3 min on AMV-positive kura clover plants, then allowed to feed, 10 per plant, on AMV-negative plants. There were six negative controls for this treatment. Three weeks after inoculation, top-growth was clipped, and 9-week-old regrowth was tested for AMV. Two of the mechanically inoculated plants tested positive for AMV using ELISA, and infection was further confirmed by the local lesion assay described above. Therefore, it is demonstrated that AMV can be mechanically transmitted to kura clover. AMV was not transmitted by the colony of soybean aphids, which previously transmitted AMV to soybean (3). This suggests virus-strain aphid specificity and possibly host specificity for phid transmission of AMV to kura clover.
References: (1) R. Alconero. Plant Dis. 67:1270, 1983. (2) O. W. Barnett and P. B. Gibson. Crop Sci. 15:32, 1975. (3) J. H. Hill et al. Plant Dis. 85:561, 2001.
