Alfalfa mosaic virus: occurrence and variation among isolates from forage legumes in Ethiopia

Field samples and seedlings of 51 leguminous forage species were tested for alfalfa mosaic virus (AMV) in routine monitoring of seed multiplication fields of the International Livestock Research Institute (ILRI), Ethiopia. The virus was detected in 24 species and a solanaceous weed Solanum nigrum . Of these, the following 17 species are being reported for the first time as hosts of the virus in world literature: Aeschynomene falcata, Centrosema pascuorum, Chamaecrista rotundifolia, Desmanthus virgatus, Desmodium intortum, Leucaena leucocephala, Macroptilium atropurpureum, Macroptilium lathyroides, Macrotyloma axillare, Medicago truncatula, Neonotonia wightii, Sesbania sesban, Stylosanthes scabra, Trifolium calocephala, T. steudneri, T. tembense and Vigna parkeri. The solanaceous weed S. nigrum was suspected to play a role in the virus epidemiology. The level of seed infection varied from 0.00% to about 13.00%. Infectivity studies on 5 isolates of the virus revealed heterogeneity in prevalent AMV isolates. Two major strains were identified, based on the development of necrotic local lesions on Vigna unguiculata accessions. One of the isolates which failed to incite local necrotic lesions on V. unguiculata infected Phaseolus vulgaris cv. Top Crop systemically. Differences among isolates were also noticed in their concentration in different hosts as indicated by ELISA absorbance values. Introduction Leguminous forages are an important source of protein for supplementation of low quality native grassland and crop residues for livestock production. They are also important for soil stabilisation and nitrogen fixation. The International Livestock Research Institute (ILRI) maintains over 13 000 accessions of grass and legume forages in trust in its genebank with the aim of making seeds freely available for evaluation and subsequent incorporation into sustainable croplivestock systems (Hanson 1995). Distribution of seeds has been identified as a cause of longdistance spread of seed-borne viruses, especially in legumes (Hampton et al. 1982; Bos and Makkouk 1993). Among the seed-borne viruses infecting legumes, alfalfa mosaic virus (AMV) is one of the most important, occurring worldwide and recorded as naturally infecting over 150 herbaceous and woody species in 22 plant families (Jaspars and Bos 1980). Loss of herbage yield due to AMV amounts to 13–35% in Medicago polymorpha (Jones and Nicholas 1992), over 50% in M. truncatula (Dall et al. 1989) and 20–49% in Trifolium subterraneum (Jones 1992). The parallel losses in seed yield are 7–32%, 58–76% and 15–30%, respectively. The virus has also been implicated in reduced nitrogen fixation in nodules of M. polymorpha (Wroth et al. 1993). These effects usually depend on virus strain among other factors (Dall et al.