Temperate Pulse Viruses: Alfalfa Mosaic Virus (AMV)

Note Number: AG1206
Published: August 2006
Updated: September 2010

Alfalfa mosaic virus (AMV) was first identified infecting lucerne in the USA, and is now distributed worldwide. The main source of this virus is lucerne, but it has a wide host range which includes the temperate pulses (chickpeas, faba beans, field peas and lentils) as well as pasture legumes and perennial weeds. The virus is spread by a number of common aphid species, as well as being seed transmitted in some species.

What to look for

Symptoms on the infected plant depend greatly on the strain of virus, host variety, stage of growth at infection and environmental conditions. In many species, a bright yellow mottle or mosaic develops. Local or systemic necrosis and stunting may also occur. Some species recover from early infections. In some species seed size may be reduced. Symptoms in the major temperate pulses, pasture legumes and host weeds are as follows:

• Chickpeas develop shoot tip necrosis (Figure 1).
• Faba beans develop chlorosis, necrosis and stunting of the new shoots. Leaf rolling, ring spots on leaves and general plant stunting may also occur (Figure 2).
• Lupins develop growth distortion, yellowing of top leaf tips and stunting (Figure 3).
• Lentils develop necrotic tip growth, twisting and deformation of leaves and stunting (Figure 4).
• Lucerne develops mottling or mosaic symptoms and reddening of leaf margins (Figure 5). Symptoms may disappear in summer.
• Field peas develop chlorosis and necrosis of the new shoots. Necrotic spots or streaking on older leaves and plant stunting may occur. Pods may be malformed and fail to develop peas.
• Subterranean clover develops vein clearing, chlorosis or necrosis of the leaves, and sometimes plant stunting (Figure 6).
• Common sowthistle develops a distinctive bright yellow mosaic on the leaves (Figure 7).

The symptoms caused by AMV can be confused with those of nutrient deficiencies, physiological disorders and herbicide damage.

Fig 1. AMV infected chickpea showing tip necrosis   Fig 7. AMV infected common sowthistle develops yellow mosaic symptoms on young and older leaves Fig 2. AMV symptoms in faba bean (centre) showing leaf rolling and stunting (Courtesy J.Van Leur, DPI, NSW) Fig 3. AMV infected lupin showing symptoms of growth distortion, yellow tip leaves and stunting Fig 4. AMV infected lentil plant showing yellowing and necrosis of shoot tips and stunting of plant (left) compared to healthy plant (right) Fig 5. AMV infected lucerne showing reddening of leaves from margins (left) and healthy leaf (right) Fig 6. AMV infected subterranean clover leaves develop interveinal chlorosis and purple or necrotic margin

Disease cycle

Transmission

AMV survives in infected seed or plant hosts. It does not persist in stubble or soil. Aphids spread the viruses from seed-infected plants to healthy plants. Seed transmission rates of 0.1-5% in lentils and 0.1-1% in chickpeas have been recorded (Jones and Coutts 1996).
The first tentative evidence for seed transmission of AMV in faba bean is reported with a transmission rate of 0.04% (Latham et al., 2004). In Western Australia, seed transmission of AMV was detected in Lathyrus cicera (2%), Lathyrus sativus (0.9-4%), Vicia benghalensis (0.9%), Vicia narbonensis (0.1%) and Vicia sativa (0.7%) (Latham and Jones 2001b). AMV is also seed transmitted in lucerne seeds (0.4-1.9% (Garran and Gibbs 1982). 
AMV is transmitted from infected plants by aphids in non-persistent manner. This means that spread of the virus is generally over short distances, as aphids only remain infective for periods from a few minutes up to a few hours. Although aphids rarely colonise chickpea plants, they still probe them as they move through the crop spreading the viruses.
There are over 20 species of aphid capable of transmitting AMV. During surveys of the Wimmera cropping region over a number of years the following aphid vectors of AMV were found, they include blue green aphid (Acyrthosiphon kondoi), cowpea aphid (Aphis craccivora), foxglove aphid (Aulacorthum solani), ornate aphid (Myzus ornatus), green peach aphid (Myzus persicae), and spotted alfalfa aphid (Thryoaphis trifolii forma maculata).

Host range

AMV has a wide host range that is not limited to plants belonging to the Fabaceae family. Temperate pulse hosts include chickpeas, faba beans, field peas, lentils, lupins, narbon beans, grass peas and vetch. Pasture legume hosts include lucerne, burr medic, other annual medics and a number of clover species. In Victorian pulse cropping areas, common sowthistle (Sonchus oleraceus) and blackberry nightshade (Solanum nigrum) were found infected. AMV can also infect a number of horticultural and vegetable crop hosts.

Economic importance

Incidence of AMV within individual infected lucerne pastures were high, with 50-98% of plants infected (Jones 2004). Late infection with AMV in faba bean cv. Fiord diminished shoot dry weight by 41% and seed yield by 45%. Plants infected early in the season recovered sufficiently so there was no significant yield losses (Latham et al., 2004). In plants of lentil cv. Matilda, infection with AMV decreased shoot dry weight by 74-76%, seed yield by 81-87% and individual seed weight by 10-21% (Latham et al., 2004). Early infection with AMV killed plants of chickpea cv. Tyson while later infection decreased shoot dry weight by 50%, seed yield by 98% and individual seed weight by 90% (Latham et al., 2004).

AMV was found in the majority of lucerne crops in surveys in New South Wales and ACT (Garran and Gibbs 1982). Seed yield losses recorded due to infection of subterranean clover were 71% with AMV (Jones 1992). Surveys of pulse crops in the last 10 years indicate that AMV is an important virus disease of lentils, chickpeas and lupins in south eastern Australia (Table 1, page 4).

Table1: Percentage of pulse crops infected with alfalfa mosaic virus in south eastern Australia and within crop virus incidence

Note: *= crop not sampled; 0= virus not found.

In 2007 in Victoria, 23% field pea crops were infected and the within crop incidence of virus was 2-13%. In 2006 in Southern NSW 13% pea crops were infected with virus, and within crop virus incidence was 1%.

Management

AMV is spread by sowing infected seed or by movement of aphid vectors from infected plants to healthy plants. Low levels of seed infection in pulses may lead to spread of AMV during years with high rainfall, and a subsequent early build up of large aphid populations.
Chemical control of aphids is not an effective method for controlling AMV. Sowing healthy seed, managing weeds and other cultural practices to minimise AMV spread are recommended. Growing crops adjacent to infected lucerne or pasture will increase the risk of crop infection. Retaining stubble is thought to reduce aphid landing rates and therefore virus spread, as aphids are attracted to bare ground. Sowing early, and at optimal seeding rates, will generate early canopy closure. Early canopy closure will shade out weak plants grown from infected seed, and reduce in crop spread of AMV by aphids. 
Some seed companies sell lentil seed which has been tested for AMV and cucumber mosaic virus (CMV). Commercial seed testing companies will also test legume seed lots for seed-borne viruses such as AMV. If farmers retain their own seed it should be selected from crops which do not have virus symptoms or evidence of aphids. See the Seed Health Testing in Pulse Crops (AG1250) Information Note, for a list of seed testing laboratories.

Further references

More information can be obtained from the DPI Information Note Series:www.dpi.vic.gov.au/graindiseases

Pulse Australia

Victorian Winter Crop Summary

Winter Pulse Disorders: The Ute Guide.

Seed Health Testing in Pulse Crops (AG1250)

Pulse Australia - Virus control in chickpea–special considerations

Pulse Australia Tech-Notes Autumn 2010 - Chickpea “sudden death” in 2009

Garran J and Gibbs A (1982) Australian Journal of Agricultural Research 33, 657-664
Jones RAC (19920 Australian Journal of Agricultural Research 43, 1229-41.
Jones RAC, Coutts BA (1996) Annals of Applied Biology 129, 491-506.
Latham LJ, Jones RAC (2001a) Australian Journal of Agricultural Research 52, 397-413.
Latham LJ, Jones RAC (2001b) Australian Journal of Agricultural Research 52, 771-790.
Latham LJ, Jones RAC and Coutts BA (2004) Australian Journal of Experimental Agriculture 44, 57-63.
Jones RAC (2004) Australian Journal of Agricultural Research 55, 757-764.

Contact/Services available from DPI

DPI Field Crops Pathology, Grains Innovation Park, 110 Natimuk Rd, Horsham 3400. Tel (03) 5362 2111, or the DPI Customer Service Centre 136 186.

Acknowledgement

This Information Note (AG1206) was originally written by Mohammad Aftab and Angela Freeman, Bacteriology & Virology – DPI Horsham, August 2005. It was reviewed by Frank Henry, BioSciences Research - Farm Services Victoria, June 2011. Financial support by the GRDC is gratefully acknowledged.