Incidence of Viruses on Alfalfa in Western North America. Fazal Rahman, Research Plant Pathologist, Agricultural Research Service, U.S. Department of Agriculture, Irrigated Agriculture Research and Extension Center, Prosser, WA 99350. Richard N. Peaden, Research Agronomist, Agricultural Research Service, U.S. Department of Agriculture, Irrigated Agriculture Research and Extension Center, Prosser, WA 99350. Plant Dis. 77:160-162. Accepted for publication 6 October 1992. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1993. DOI: 10.1094/PD-77-0160.

Enzyme-linked immunosorbent assay was used to detect viruses in alfalfa leaf tissue samples taken from 216 fields at 44 widely separated locations in California, Idaho, Oregon, and Washington and southern British Columbia. Samples were collected between April 1987 and May 1988. Alfalfa mosaic virus (AMV) was found at all locations and in all but one field. Pea streak virus and pea (bean) leafroll virus were detected at 38 locations. Tobacco streak virus was found only in Idaho and Washington at seven locations, and red clover vein mosaic virus at 11 locations in the four states, but not in British Columbia. Pea seedborne mosaic virus and other members of the potyvirus group were not detected at any location. Percent of samples with AMV, pea streak virus, pea leafroll virus, tobacco streak virus, and red clover vein mosaic virus were 86, 21, 28, 4, and 3, respectively. The data indicate a higher incidence of AMV than has previously been reported over a wide geographical area. Furthermore, up to four viruses were in individual samples from several fields. Incidence of AMV in fields was not highly correlated with age of stand (r = 0.44), but generally fields 4–6 yr old contained 80% or more infected samples. Assays for pea enation mosaic virus indicated that this virus was in alfalfa samples, but these data were not included, because of variability in the reactions obtained with the infected control samples. Differences in percent infection were associated more with geographic location than with cultivar.

Keyword(s): alfalfa viruses.