CSFB and autumn disease best practice
Three key points:
- Drilling date can affect cabbage stem flea beetle (CSFB) risk
- Early drilled- lower risk of adult CSFB
- Late drilled- lower risk of larval damage
- Delayed control of volunteer OSR can reduce damage in nearby OSR fields
- Select varieties, cultural controls and fungicides to manage disease in line with drilling dates
Cabbage stem flea beetle (CSFB; Psylliodes chrysocephala) has been the most important insect pest of winter oilseed rape (OSR) in the UK for several years. The result of lost actives has seen large crop losses to the pest in some years. So, what can be done to best manage the pest in 2019/20? Dr Sacha White, ADAS Research entomologist summarises what the latest research shows:
Drill date
AHDB funded CSFB integrated pest management project[1] suggests adjusting drilling date can help reduce risks. Crops drilled in August tend have lower levels of adult damage than those drilled in September, likely to be because these crops are better established by the time adult CSFB migration occurs, however these early drilled crops will need careful canopy management in the spring to reduce lodging.
Conversely, crops drilled in September tend to have lower larval populations in the autumn and spring than those drilled in August (Fig. 1). This may be due to these crops emerging after peak CSFB migration has passed and because eggs laid in these crops will develop in cooler temperatures than those laid earlier in earlier drilled crops, slowing their development. While there are increased risks to late drilling, trial work suggests that the drilling window for OSR is larger than perceived. These risks will also be reduced by choosing varieties suited to late drilling.
Whichever date is chosen, drilling so that seeds have good seed to soil contact and moisture remains key.
Seed rate
Trial work in the CSFB IPM project investigating the effect of different seed rates (10, 20, 40, 80 and 120 seeds per m²) on CSFB pressure, found few benefits of increasing seed rate. There was slightly less damage from adult CSFB at 80 and 120 per m² (17% and 24% less damage respectively) compared to 10 seeds per m² at one site but not the other trial site. There was no significant difference in larvae per plant between the seed rates. We did see a benefit of increasing seed rate where the trial was drilled into dry conditions. In the other trial, where establishment was good, there was no significant difference in yield from 40 to 120 seeds per m² (the highest yield was 5.4 t/ha at 40 seeds per m²). Furthermore, as there was no difference in larval load per plant between seed rates, our trials showed that increasing seed rates resulted in higher numbers of larvae per unit area. This means that higher seed rates could result in higher pest return for the following season.
On balance, we’d recommend using a seed rate of no more than around 40 per m², unless drilling conditions are poor.
Using volunteer OSR as a trap crop
Migrating CSFB are attracted to plant volatiles given off by OSR. Leaving volunteer OSR until late September produces a trap crop that can divert migrating CSFB from drilled OSR. Trial work done in the CSFB IPM project found that delaying control of volunteers in this way significantly reduced adult numbers (Fig. 2) and damage, and significantly increased plant populations in nearby drilled OSR.
As an added bonus, any CSFB eggs and larvae in the volunteers will also be killed when the volunteers are destroyed, reducing CSFB populations for the following season.
Disease risk
Dr Faye Ritchie, Senior plant pathologist from ADAS explains that earlier drilling tends to favour the development of light leaf spot as the crop will generally be exposed to spores for a longer period of time. This can make the disease difficult to manage later in the season with fungicides, particularly on susceptible varieties.
If the plan is to drill early, using a variety with better light leaf spot resistance may be beneficial to help slow the development of the disease and allow better control with fungicides.
Smaller oilseed rape plants are more at risk from phoma leaf spot. The time it takes for the fungus to grow from leaf spots, through the leaf and petiole to the stem is much shorter in a smaller plant than in a larger plant. The risk from later drilling can be mitigated by using a variety with good resistance to stem canker in conjunction with crop monitoring to ensure fungicides are applied when the phoma epidemic is starting (10 to 20% plants affected).
Earlier drilling increases the risk from club root. Root infection occurs in the autumn when resting spores germinate and zoospores are released. This occurs at temperatures above 15°C therefore early drilling is usually not recommended at infested sites.
Follow current north_east AHDB guidance to manage club root risk; machinery hygiene, adequate field drainage and maintaining a pH >7 are several examples of the management strategies that can be used to mitigate risk.
