Diagnosing bean yellow mosaic virus - early symptoms in narrow-leafed lupins

Bean yellow mosaic virus (BYMV) is an aphid-borne virus that commonly causes necrotic symptoms that kills infected lupin plants before pod set (BYMV-early symptom). When plants are infected after pod set, black pods develop (Black pod syndrome, BPS). BYMV is found predominantly in high rainfall wheatbelt zones.

 

Youngest growth bends over causing a 'shepherd's crook' appearance
Necrosis of the shoot tip
Plants infected early (right) die quickly and do not produce seed
Plants are often affected first and most severely on paddock edges

Contributing factors

The extent of BYMV spread in crops depends on many factors. The most important include:

  • 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.
  • 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.
  • Other factors include: lack of groundcover, sparse stands, poor canopy development, heavy grazing of nearby pasture, paddocks with large perimeter to area ratios, and extended growing seasons.

What to look for

    Paddock

  • Dead or dying plants, or plants with bent ‘shepherd’s crook’ upper stems.
  • Plants are often affected first and most severely on paddock edges or other bare areas, particularly on the windward side and adjacent to legume pastures.
  • Patches are common with infection spreading out from the centre.

    Plant

  • Initial symptoms are necrotic streaking of the youngest portion of the shoot, which bends over causing a characteristic ‘shepherd’s crook’ appearance.
  • The growing tip dies, and leaves become pale, wilt and fall off.
  • Necrotic streaking and blackening then spreads along the stem causing the plant to die.
  • Plants infected early die quickly and do not produce seed.
  • Varieties differ in susceptibility.

What else could it be

Condition Similarities Differences
Diagnosing anthracnose in narrow-leafed lupins 'Shepherd's crook' stem bend Brown lesions and pink spores in the crook of the bend.
Diagnosing frost in narrow-leafed lupins Bent and discoloured stems Plants wilted, rapid symptom development, and symptoms vary with landscape position.

Where does it occur?

  • BYMV strains that are endemic in south-western Australia are not seed-borne in lupins, unlike overseas strains.
  • The main source of BYMV is infected clover plants in adjacent pastures. The virus is seed-borne in clover and survives from one growing season to the next in pasture seed bank. Clover weeds in lupin crops are an additional source if not controlled. Perennial native legumes in adjacent bush may occasionally act as BYMV reservoirs.
  • If there are no internal clover weed sources, BYMV infection is highest near the edge of a lupin crop close to pasture, especially at its windward edge. There is only a brief period between initial symptom formation in young plants and their death, so incoming aphids can only acquire the virus from infected lupin plants for one to two weeks. Infection incidence declines rapidly with increasing distance into the crop.

Management strategies

Stubble management
Stubble management
Grass pasture control
Grass pasture control
Spraying insecticide
Spraying insecticide
important
  • 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 to minimise virus infection sources within the crop.
  • Note: Insecticides applied to crops are ineffective at controlling BYMV.

See also

Bean yellow mosaic virus in lupins
Monitoring establishing crops for leaf diseases

Where to go for expert help

DDLS Seed Testing and Certification
+61 (0)8 9368 3721
Email DDLS Seed Testing and Certification

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Page last updated: Thursday, 16 April 2015 - 4:29pm