Bean yellow mosaic potyvirus infection of alternative hosts associated with subterranean clover (Trifolium subterraneum and narrow-leafed lupins (Lupinus angustifolius): Field screening procedure, relative susceptibility/resistance rankings, seed transmission and persistence between growing seasons

A field screening procedure was devised to determine relative susceptibility and resistance rankings for hosts of bean yellow mosaic potyvirus (BYMV) using BYMV-infected Trifolium subterraneum plants transplanted at eack end of single row test plots. Natural spread of BYMV by aphids resulted in BYMV symptoms in test lines. Four test lines were ranked as highly resistant, nine were resistant, seven were moderately resistant, eight were susceptible and two were highly susceptible to BYMV infection. Disease progress curves plotted for each test line assisted in the ranking process. Relative rankings were independent of flowering date and presence of host alkaloids. Acrythosiphon kondoi, Myzus persicae and Rhopalosiphum padi were the predominant aphid species caught in traps associated with field screening plots. Seven plant species tested were new BYMV host records. Seed of four plant species systemically infected following sap inoculation with BYMV was tested, and seed transmission detected in Melilotus indica (0.5%). When seed of 19 alternative host species that became systemically infected through natural spread was tested, seed transmission was found in Medicago polymorpha (0.9%), Medicago truncatula (0.3%), M. indica (1%), T, arvense (0. 1%), T. campestre (0.2%) and T. glomeratum (0.05%). No seed transmission was detected in T. subterraneum. It is concluded that under broadacre agriculture in the Mediterranean climate of Western Australia, seed-borne infection in naturalized M. polymorpha, T. arvense, T. campestre and T. glomeratum growing in T. subterraneum pastures probably provides the principal means by which BYMV persists over the dry summer to act as primary sources for subsequent spread. The species most likely to contribute to BYMV spread within T. subterraneum pastures and from them to Lupinus angustifolius crops were L. cosentinii, T. campestre, T. dubium and T. subterraneum itself.