There are three important viruses of canola in Australia: beet western yellows virus (BWYV), synonym - turnip yellows virus (TuYV), cauliflower mosaic virus (CaMV) and turnip mosaic virus (TuMV). Infection with BWYV is common and all plants may become infected. TuMV and CaMV tend to occur at low incidences. Yield loss studies in Australia, where aphids spread BWYV virus at the seedling growth stage, caused yield loss up to 46 percent and reduced oil quality. Overseas studies indicate TuMV and CaMV can also cause significant yield losses.
Infection with BWVY causes plant stunting and purpling or reddening of lower leaves, (Figure 11.17). CaMV causes stunting, yellow ring spots and mottling, (Figure 11.18). TuMV causes stunting and mosaic which is sometimes associated with necrotic spotting and line patterns, (Figure 11.19).
Figure 11.17 Symptoms of beet western yellows virus (BWYV) in canola
Figure 11.18 Symptoms of cauliflower mosaic virus (CaMV) on canola leaves
Figure 11.19 Symptoms of turnip mosaic virus (TuMV) on canola leaf
These viruses are not seed-borne. They survive in weeds or volunteer canola host plants outside the growing season and are then spread from these infected plants into crops by aphids which act as vectors for virus transmission. BWYV is transmitted persistently, mainly by the green peach aphid. Persistently transmitted viruses are carried within the aphid’s body and remain throughout the aphid’s life. CaMV and TuMV are non-persistent and are transmitted by many aphid species. They are retained in the aphid mouthparts for short periods and lost when the infective aphid feeds on healthy plants.
Autumn is the most critical infection period for infection, so the earliest-sown crops usually have the highest infection incidence. BWYV causes most yield loss in crops that have been infected as seedlings. Infections can occur past the rosette stage of canola growth but these probably have little effect on yield.
- control broadleaf weeds (especially over summer) as they act as reservoirs for the viruses
- retain stubble at sowing to cover the ground, reducing the ability of aphids to land on young canola plants thereby reducing virus spread
- sow at recommended times; earlier sown crops usually have a greater incidence of viral infection
- seed dressing with imidacloprid insecticide will kill the green peach aphid vector and greatly reduce BWYV spread. However, commercial seed dressing procedures may not treat all seeds resulting in poor aphid control
- seed dressing insecticide application does not control the spread of CaMV or TuMV by aphids
- most Australian canola varieties carry strain specific, TuMV resistance genes which greatly limit the incidence of this virus.
WHITE LEAF SPOT