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
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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.
KeywordsPasture legumes AMV Virus vectors
Desmanthus species (desmanthus) are a group of summer growing legumes adapted to neutral to alkaline soils of medium to heavy clay texture in the drier subtropical environment. Desmanthus is a drought-tolerant perennial legume, commonly found in regions with annual rainfall 500–1000 mm (Cook et al. 2005). It is palatable to livestock with high nutritive value (Gardiner and Rangel 1994; Cook et al. 2005), but does not cause bloat in cattle due to presence of condensed tannins (Adjei et al. 1993). Desmanthus plants are defoliated by heavy frosts, but regrow from plant crowns in early spring. Plants can set large quantities of seed which have high levels of hardseededness, but desmanthus readily recruits from seed following summer rainfall (Cook et al. 2005).
Desmanthus has been identified as a productive component of sown and native pastures in tropical and subtropical regions in Australia. Two commercial cultivars are currently available that performed well in experiments (e.g.; Pengelly and Conway 2000; Boschma et al. 2012), but adoption has been slow and its potential has not yet been realised. A number of Desmanthus spp. are represented in the Australian commercial cultivars: cv. Marc (D. virgatus) and cv. Progardes (composite of 5 cultivars from 3 species: D. virgatus, D. bicornutus and D. leptophyllus). Over the last 8 years, desmanthus has also been evaluated in northern NSW (e.g. Boschma and Harris 2009) and has shown potential as a companion legume in sown tropical pastures. These findings extend the boundaries of desmanthus adaptation beyond tropical environments, thereby potentially exposing it to different abiotic (higher frequency of frost and lower proportion of summer rainfall) and biotic (insect pests and diseases) stresses.
AMV is a non-persistently transmitted virus with a very wide host range and has been reported to occur naturally in 47 species in 12 families (Hull 1969), including commonly grown pasture and crop legumes in Australia. Lucerne is widely grown in northern NSW and can harbour very high levels of AMV infection without showing obvious symptoms (van Leur and Kumari 2011). As a perennial legume it is a likely source of AMV inoculum for other legume species. Inoculum for mechanical transmission (Hull 2009) was prepared by grinding AMV positive lucerne in a pH neutral phosphate buffer using a chilled mortar and pestle. Healthy desmanthus plants, grown in pots inside a greenhouse, were inoculated by dusting young leaves with carborundum powder and gently rubbing the inoculum into the leaves. The same inoculum was applied to 3-week old greenhouse grown faba bean plants (var. Fiesta). Inoculated plants were observed for symptom development and tested for AMV presence with TBIA. Several attempts to mechanically inoculate desmanthus with AMV did not result in symptoms or in positive TBIA tests, while faba bean plants showed typical symptoms (leaf yellowing, stem necrosis) within 3 weeks of inoculation. Failure to mechanically inoculate plants with AMV have also been reported for lucerne (Hull 1969) and Cullen australasicum (Nair et al. 2009).
This is the first report of D. virgatus as a host of AMV in Australia. While its suitability as a host for cowpea aphids increases its vulnerability to AMV and other viruses, cowpea aphids are not necessarily the only vector to infect desmanthus with AMV: AMV is a non-persistent virus that can be transmitted by short probing periods of a range of aphids, not only species that are feeding on the host plant. Our field observations suggest that desmanthus productivity is greatly affected by AMV infection, but this warrants quantification. AMV is reported to be seed transmitted in Desmanthus virgatus (Mih and Hanson 1998), which would have implications for seed multiplication and distribution. We tested over 500 seedlings emerged from seed harvested from AMV positive plants for AMV presence, but did not find any positives.
Seed transmission of and susceptibility to AMV could differ between Desmanthus species. There are over 300 accessions of Desmanthus spp. held in the Australian Pastures Genebank (Pengelly and Liu 2001). Further investigations on the susceptibility of these accessions and other commercially available Desmanthus spp. is warranted.
We gratefully acknowledge technical support provided by Mark Brennan and Janine Sipple and financial support provided by the New South Wales Department of Primary Industries, Meat and Livestock Australia (MLA) and the Grains Research and Development Corporation (GRDC). We appreciated the comments of our colleagues Dr. Kevin Moore and Sean Bithell on the manuscript.
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