Alfalfa mosaic virus (AMV): The symptoms are tip necrosis, yellow leaves and mild stunting.
Cucumber mosaic virus (CMV): Reduced internodes, bunchy appearance and plant stunting. If the virus is carried in the seed the entire plant will be stunted and may die before maturity. A plant infected late in the season may have normal sized leaves at the bottom but those near the top, above the point of infection, will be less than half normal size. The leaves on infected plants become bunched, curled down and chlorotic (pale in colour), particularly at the growing shoots. Infected plants may occur randomly throughout the paddocks. Often they will be in small patches, 1-2 metres in diameter, (Figure 10.15).
Bean yellow mosaic virus (BYMV): The initial symptoms are yellowing of leaves followed by necrosis of growth tips and death of plant, (Figure 10.16). The late infection may cause black pod symptoms. BYMV is a serious disease in narrow- leafed lupins. Two strains of the virus occur in Western Australia. These are differentiated by symptoms in narrow-leafed lupin: the common necrotic (BYMV-N) strain kills the infected plant, and the less abundant non-necrotic (BYMV-NN) strain causes stunting without killing the plant. All BYMV strains which are endemic in Australia are not seed- borne in lupins, unlike overseas strains.
Figure 10.15 Stunting and bunching symptoms of CMV in narrow leaf lupins (front plants) surrounded by healthy plants
Figure 10.16 Symptoms of BYMV in narrow leaf lupin. The initial symptoms are yellowing of leaves, followed by necrosis of growth tips and death of plants
The primary source of infection of non-persistent viruses is infected seed. If the seed infection level is one percent, one plant in every 100 will be virus-infected, and will be randomly distributed across the paddock. The secondary infection happens by aphids. Aphids acquire the virus from primary infected plants or weeds and spread them to healthy crops within a short time span from a few minutes to a few hours. Aphid lose infectivity after a few probes because these types of viruses are not kept long on aphid’s mouth parts and aphids do not remain infective throughout their life.
These viruses do not go through a lifecycle like many other pathogens. They cannot survive for very long outside the plant and they rely solely on aphids to spread them between live plants. These viruses can survive for long periods of time in seed, but do not have stages which allow them to survive in the soil or stubble as sources of infection for future crops.
The persistent viruses are phloem restricted and only transmitted by aphids. Aphids need to feed on them for hours to days to transmit the virus. The aphids remain infected for their whole life. These viruses are not seed-borne. The source of these viruses lie in over- summering perennial weeds.
- Growing virus resistant varieties is the long term best option, but the number of resistant varieties is limited. Breeding is underway to develop a faba bean with BLRV resistance and field pea with PSbMV and BLRV resistance.
- Sowing virus tested seed which has a virus level less than 0.1-0.5 percent is an effective way to control seed-borne viruses which can only be spread short distances in the field by their aphid vectors (non-persistent transmission).
- Eliminating weeds and self-sown pulses. These are a source of virus and aphids and this is usually where they survive between crop growing seasons.
- Monitoring aphids, particularly at the early growth stages of the crop. If aphids develop heavy colonies at this early stage, then it may be economical to spray the crop with an aphicide.
- Good plant stands with minimal bare ground will reduce aphid colonisation.
- Rotation of pulses with cereals and reduce self- sown hosts and weeds.
- Removal of plants with symptoms will reduce the amount of infected seed harvested (appropriate for some seed crops). The amount of virus infection in the harvested seed (percent) depends not only on the number of infected plants, but also on the rate of seed transmission of the virus.
- Seed treatment with neonicotinoid (eg. Gaucho or Cruiser Opti) insecticide prevents crops from early feeding of aphids and stops the infection of crops from viruses. It is especially useful for persistently transmitted viruses.