Aphid vectors
BYMV is spread by aphid species that colonise lupins, including green peach (Myzus persicae), blue green (Acyrthosiphon kondoi) and cowpea (Aphis craccivora) aphids, as well as by migrants of common non lupin-colonising species, especially oat (Rhopalosiphum padi) and turnip (Lipaphis erysime) aphids. It is transmitted non-persistently, that is, an aphid picks up the virus within one to two seconds while probing on an infected plant, but the virus is then lost when the aphid probes one or two healthy plants.
Yield losses
Infection of plants with BYMV when they are young causes greater yield losses than infection that occurs when they are older. On an individual plant basis, BYMV-N causes high yield losses as it kills young plants so no seed forms on them. On the whole crop scale, BYMV-NN has greater potential to cause losses as it spreads to more plants. When all plants in a crop become infected with either type of BYMV, yield losses can reach 80%. Plants that develop black pods resulting from late season infection will not produce any seed.
Influence of weather
Rainfall events in late summer and early autumn are associated with greater spread of BYMV. This is because rainfall stimulates plant growth before the crop growing season and provides hosts (clovers, weeds and volunteer crop plants) that encourage rapid aphid multiplication and BYMV spread in clover plants. When crops emerge, aphid flights invade the young plants bringing BYMV with them and spread begins early. When there is little or no rain, few plants are available to support aphids before crops are sown, so they take much longer to build up and arrive much later. In low risk areas, this scenario occurs more frequently than in high risk areas.
Management
The extent of BYMV spread in crops depends on many factors. The most important include:
- Time of aphid arrival. Early aphid arrival favours earlier and more extensive spread and greater yield losses.
- The source of the virus. As spread declines rapidly over distance, infected pastures adjacent to crops are much more important sources than infected pastures further away. If abundant clover weeds remain uncontrolled within the crop, they may result in greater spread than external sources.
- Abundance of colonising and non-colonising aphids during the growing season. This increases BYMV spread in the crop.
- Lack of groundcover; sparse stands; poor canopy development; heavy grazing of nearby pasture; paddocks with large perimeter to area ratios; and extended growing seasons. All these factors increase BYMV spread.
An integrated disease management approach is needed to control BYMV in lupin crops:
- Sow early at high seeding rates using narrow row spacing to promote early crop canopy coverage. This deters aphids from landing and shades over early infected plants, denying aphids access to them. High plant densities dilute the proportion of plants that become infected and increase compensatory growth of healthy plants.
- Direct drill into retained stubble. Groundcover reduces aphid landing rates before a crop canopy develops.
- Sow a non-host crop (for example, cereal) border strip between crops and adjacent pasture. Incoming aphids lose the virus when they probe the non-host which helps to decrease spread into the crop from an external source.
- Avoid paddocks with large perimeter to area ratios. This reduces exposure of the crop margin to adjacent BYMV-infected pasture.
- Control clover weeds effectively. This minimises potential virus infection sources within the crop.
Note: Insecticides applied to crops are ineffective at controlling BYMV.