INFECTION OF ALTERNATIVE HOSTS ASSOCIATED WITH ANNUAL MEDICS (MEDICAGO SPP) BY ALFALFA MOSAIC-VIRUS AND ITS PERSISTENCE BETWEEN GROWING SEASONS

Under conditions of natural alfalfa mosaic virus (AMV) spread, five pl ant species found associated with annual Medicago spp. (medics) were i nfected commonly and another seven sporadically. Ten of these were new records. Because seed of herbaceous plant hosts provides a possible r oute for virus persistence through dry summer conditions, AMV seed tra nsmission was tested for in alternative hosts. Of ten species systemic ally infected by sap inoculation with AMV, seed transmission was detec ted in Melilotus indica (10%), Ornithopus compressus (0.1%) and Stachy s arvensis (2%). Seed of seven naturally infected potential alternativ e host species was tested, and seed transmission found in Crassula dec umbens (0.1%), M. indica (3%), O. compressus (0.2%), S. arvensis (0.4% ) and Trifolium subterraneum (2%). Carry-over of AMV through seed tran smission was detected in seedlings of Hypochaeris glabra (0.2%) and M. indica (0.6-0.8%) that germinated naturally in the field. In grazed, self-regenerated Medicago murex and M. polymorpha swards sown in 1987, the virus persisted for at least seven growing seasons. Levels of inf ection in M. polymorpha seed produced each year declined, but levels i n the general seed bank remained higher due to presence of older seed. It is concluded that under the conditions of broadacre agriculture in the Mediterranean-type climate of Western Australia, seed transmissio n in C. decumbens, H. glabra and volunteer O. compressus is an alterna tive means by which AMV can persist over summer to act as sources of A MV for spread within annual medic pastures. However, persistence throu gh seed of the annual medic cultivars sown and of naturalized annual M edicago species is the principal means of persistence. AMV persists re adily from year to year following sowing of infected seed and is likel y to cause a recurrent disease problem in annual medic pastures.