Early fungicide sprays for phoma are not cost-effective in Scotland except in areas where the disease is prevalent, and although the disease is sometimes recorded in Scottish crops, especially around the Borders it is usually at trace levels and not economically damaging to the crop.
Do watch for and investigate any patches of poor emergence in case this is clubroot. Where clubroot is detected planning future strategies can’t start soon enough. Controls are very limited and a sole reliance on resistant varieties not very sustainable as overuse in short rotations erodes its efficacy and fields become unsuited for oilseed rape altogether. Lengthen rotations in affected fields and if patches are limited so think about leaving them uncropped if it is a small area say around a gate. This will prevent inoculum building up and spreading further into the field. Work really hard to avoid spreading the infection between fields. Cleaning equipment between fields would be the ideal but if this is impractical try and plan operations so that infected fields are dealt with after clean fields. It would also be worth sharing this preference with contractors if you use them.
Slug damage has been reported in many oilseed rape crops, but if crops have reached 4 true leaves they will be able to grow away from any further damage. Due to the risk of metaldehyde ending up in water, use of metaldehyde slug pellets should be avoided unless strictly necessary, especially bearing in mind that metaldehyde pellets should not be allowed to fall within a minimum of 10 metres of any field boundary or watercourse. Ferric phosphate slug pellets are a viable alternative.
Damage from flea beetles has been reported in several crops, but as for slugs, once crops have 3-4 true leaves they can usually grow away from any further damage.
Cabbage stem flea beetle (CSFB) damage has been reported in several crops. We would welcome any reports of poor flea beetle control after insecticide treatments and may be able to sample these crops for beetles to confirm their resistance through lab tests.
Cabbage stem flea beetles as well as causing shot-holing of leaves, they also lay eggs near plants and the larvae burrow into the stem which can lead to winter kill, no stem elongation or lodging in spring. In Scottish crops the adult beetle feeding damage tends to be worse than the much less problematic larval damage, whereas in England both stages of the pest can cause significant damage.
As with the smaller flea beetles, pyrethroid insecticides (see list below) can be applied if the following damage thresholds for cabbage stem flea beetle have been exceeded:
>25 % of the leaf area damaged at the 1 to 2 true leaf growth stage
>50 % of the leaf area damaged at the 3 to 4 true leaf growth stage
Note that there is some concern that cabbage stem flea beetles in Scotland may have some resistance to pyrethroid insecticides (alpha-cypermethrin, beta-cyfluthrin, cypermethrin, deltamethrin, lambda-cyhalothrin, tau-fluvalinate and zeta-cypermethrin), as resistance is widespread in English populations. Only use an insecticide treatment if these damage thresholds have been exceeded but bear in mind that control may not be absolute due to resistance.
Peach-potato aphids may well be carrying turnip yellows virus (TuYV), which, if transmitted into rape seedlings can reduce yields by up to 30%. Peach-potato aphids are resistant to pyrethroid insecticides, so will not be controlled by the pyrethroid insecticides used against flea beetles mentioned above. Look for aphids on the leaves (including the underside of the leaves) from crop emergence and if aphid colonies are present then intervention may be necessary. There are just two options available to reduce the threat from TuYV:
- thiacloprid (various products – note that 3 August 2020 was deadline for sale and distribution, with on-farm stocks to be used up by 3 February 2021)
- flonicamid (various products)
If growers had problems with plants in this seasons’ crop that produced extra lateral shoots, or were stunted when you were expecting stem extension, then that could be a sign of rape winter stem weevil infestation on the farm. Adult weevils don’t tend to move into crops until now, so would not be controlled by a pyrethroid spray targeting flea beetles or cabbage stem flea beetle applied in September. Consequently, growers will usually have until the end of October/early November to apply a pyrethroid insecticide to control the weevil to prevent them laying eggs, as once the grubs hatch out and burrow into the stem of a plant they are beyond any insecticidal control. A pyrethroid insecticide treatment can be tank-mixed with the light leaf spot fungicide treatment (check label for compatibilities) and gives good control of rape winter stem weevil if applied before any eggs are laid. Delaying treatment into November will allow egg laying and hatch to happen, and the grubs will be protected within the rape stem.
Volunteer cereals and grass weeds
Volunteer cereals are particularly competitive with young oilseed rape and tend to reduce its growth, so it is a good strategy to control volunteer cereals early. Also, as oilseed rape is an excellent break for cereal diseases such as take-all and for grass weeds, controlling volunteer cereals and grass weeds is essential for the break to be fully effective.
Commonly used ‘fop’ or ‘dim’ graminicides are the obvious choice in oilseed rape – they are most cost-effectively used at reduced dose early when the cereal volunteers are small and not shielded by the oilseed rape canopy. Going early creates space for the rape plants to establish and grow thus optimising the benefit from the good growing conditions in early autumn. There may be further germination and low dose split sequences are advised on some labels to cope with staggered weed emergence but note that only one application of a product is generally permitted per crop so different products need to be used in sequence. If there are problem weeds such as bromes, then a sequence is advisable to try to achieve 100% control and zero seed return.
These ‘fop’ and ‘dim’ graminicides are generally very effective on other grass weeds such as brome grasses, rye-grasses and non-resistant wild-oats and black-grass, but a higher dose is needed than that used for volunteer cereals. They are not generally effective on meadow-grasses, although clethodim will control annual meadow-grass from 2 leaves to before 3 tillers and propaquizafop may control small annual meadow-grass. Additionally, clethodim has been shown to be more effective on black-grass than other graminicides.
There are numerous ‘fop’ and ‘dim’ products available, for example:
- Propaquizafop and other propaquizafops
- Cycloxydim and other cycloxydims.
Note some of these herbicides require the addition of an adjuvant – read the label to check which adjuvants are suitable.
If there are grass weeds present, a follow-up treatment with soil-acting propyzamide will help ensure that a high level of control is achieved. Propyzamide is applied once soils cool to 10°C and falling. Propyzamide relies on the weeds germinating in the top 2 cm of soil so it is most effective in min-till or no-till systems rather than where weed seeds have been buried by ploughing. Stewardship guidelines should be followed when using propyzamide or carbetamide. Recent trials have shown that lowest losses to drainage water are in crops established with minimal soil disturbance.
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