Alfalfa mosaic virus infects the tropical legume Desmanthus virgatus in Australia and the potential role of the cowpea aphid (Aphis craccivora) as the virus vector

Abstract

Severe yellowing and stunting of plant growth was observed in experimental plots of Desmanthus virgatus (desmanthus) at the Tamworth Agricultural Institute during the 2015/16 summer season. Both symptomatic and non-symptomatic plants were tested for the presence of a range of viruses by Tissue blot immunoassay and symptomatic plants consistently reacted positive to Alfalfa mosaic virus (AMV), while non-symptomatic plants were negative to all viruses tested. AMV was not detected in seedlings grown from seed collected from AMV-positive desmanthus plants. AMV was readily transmitted from AMV-positive desmanthus plants by mechanical inoculation to faba bean, but attempts to transmit the virus from AMV-positive Medicago sativa (lucerne) plants to desmanthus by mechanical inoculation were unsuccessful. However, AMV was successfully transmitted from lucerne to desmanthus by Aphis craccivora (cowpea aphid). Aphid feeding studies showed that the cowpea aphid, but not Acyrthosiphon pisum (pea aphid), could colonise and multiply on desmanthus. AMV could become a limiting factor for the adoption of desmanthus as a pasture legume in NSW, particularly as AMV has been reported to be seed transmitted in desmanthus.