Winter Oilseed Rape – late August 2020

Most crops are now harvested.

Yields to date seem reasonable and some of the new season crop is already drilled. Phoma risk is still low in our region and decisions around light leaf spot fungicides are some way off yet.

Clubroot

Clubroot is a major issue in rape in arable rotations with over 50% of fields used for rape carrying the disease – and in areas like Aberdeenshire the incidence is higher with the majority of fields affected.

Knowing which fields are infected and making basic decisions about good hygiene when moving machinery around the farm and perhaps finishing operations in infected fields. Cleaning down equipment remains important and applies to both farm owned and contractor equipment – the risk of spreading infection between fields and farms is very high and some machinery can potentially carry tonnes of soil if it isn’t cleaned off. Contractors should be particularly careful obviously. Starting in an infected field and then trundling straight into a clean field is asking for trouble. There is an AHDB funded project running at the moment which is mapping fields for clubroot and the infection maps show really clearly how often there is a tell-tale patch of infection at the gate, followed by tails of infection as this is spread out through cultivations.

Testing fields adds to the information you have to plan with. SRUC’s Crop Clinic and other providers offer soil testing. If a field is infected, you can of course drill a variety listed as clubroot resistant such as Mentor. However, Mentor carries the same resistance mechanism as previous resistant varieties such as Cracker and Mendel and evidence shows that, after two or three uses in the same field, strains of clubroot which can overcome resistance emerge. The current project shows these strains are common throughout the UK and if fields where resistance has been deployed several times before it ceases to be an effective strategy. Consequently, planning rotations is really important so that you keep fields profitable for the long term and don’t solve the problem temporarily for this year and next, but end up in a worse position than ever within the decade. Clubroot persists for up to 20 years in the soil which is clearly way beyond what can be handled by sensible rotation, but even stretching oilseed rape rotations out in infected fields from three out to four or five years is enough to make a difference to infection levels and reduce pressure on that resistance mechanism. Clearly the one year in two type approach to growing rape in some fields will really accelerate the risk.

Current advice on clubroot is therefore:-

• Test soils for clubroot and pH and use results to plan strategy for farm.
• Pay attention to hygiene and soil movement.
• Rotations of greater than 1 year in 5 are likely to be beneficial.
• Avoid early sowing on infected sites
• Avoid over-reliance on resistant varieties in short rotations
• Only deploy resistant varieties where justified by disease level to avoid over use and selection of virulent clubroot strains
• Maintain higher pHs on infected sites and use long rotations.
• Spot treat infected patches in fields with lime
• Always investigate patches of poor emergence or growth to see if clubroot is the cause

Pests

The key pests to consider in winter oilseed rape in the autumn are flea beetle, cabbage stem flea beetle, peach-potato aphid (and turnip yellows virus – TuYV) and later in the autumn rape winter stem weevil. Slugs are also a perennial problem, and reliance on fine seed beds and slug pellets (metaldehyde and ferric phosphate) remains for this pest, with careful use of metaldehyde a necessity to avoid contamination of water. Slug management may well be needed this season due to conditions favouring slugs, although it is a race with crops emerging rapidly and slugs inflicting damage before crops reach the 4 leaf stage.

Flea beetle damage is seen as small holes in the cotyledons and first true leaves of the emerging rape crop. The beetles are mainly active during dry soil conditions, so be prepared to spray with a pyrethroid insecticide if feeding punctures are present on germinating plants. Once 3-4 leaves have emerged, there is no need for treatment.

Cabbage stem flea beetles also cause shot-holing of leaves, but in addition they 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.

One way to assess the risk from cabbage stem flea beetle is to look at the trailers during the harvest of this seasons winter oilseed rape – the beetles will be caught up in the harvest and can be found on the trailer and in the harvested seed. They will cause no harm to the seed if taken back into the store, but their presence is a ‘heads up’ that the beetle poses a threat on the farm to the next winter rape crop. We would welcome any sightings of these beetles during the rape harvest.

As with the smaller flea beetles, pyrethroid insecticides (see list below) can 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.

SRUC would welcome any reports of poor flea beetle/cabbage stem flea beetle control this season as it will help them gauge the spread of resistance to pyrethroids in Scottish populations.

Peach-potato aphids may well be carrying turnip yellows virus (TuYV), which, if transmitted into rape seedlings can reduce yields by up to 30%. There has been a jump in the numbers of peach-potato aphids caught in yellow water traps over the last ffew weeks which may well lead to early emerging crops being infested and at risk of TuYV. 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 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 late September-mid-October, so would not be controlled by a pyrethroid spray targeting flea beetles or cabbage stem flea beetle. 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.

Weeds

For crops not treated pre-emergence, there are a number of metazachlor-based herbicides that can be applied once the crop has two expanded cotyledons. Generally speaking, formulated mixes are a better option post-emergence than straight metazachlor as they give root or shoot uptake rather than only the root uptake with straight metazachlor.

Halauxifen-Methyl + Picloram is a useful new autumn post-emergence option that can be applied slightly later. It can be used as a stand-alone treatment, or as a follow-up if weeds haven’t been fully controlled by the residual. It controls a wide range of broad-leaved weeds post-emergence between 1 September and 31 December. Weeds still need to be quite small when treated – at the 0.25 l/ha dose rate between 1 and 5 cm high/across, weed size depending on species. At the 0.5 l/ha dose rate, shepherd’s purse and cleavers are controlled up to 10 cm high/across and corn poppy up to 8 cm high/across. The 0.5 l/ha dose rate can’t be applied until 15 September.

For grass weeds, a graminicide applied when volunteer cereals and grass weeds are small is a good strategy. Controlling them early enables a lower dose to be used and allows the crop to grow away.

If problem grass weeds are an issue, then the early graminicide can be followed by a propyzamide-based product later in the autumn once the soil temperature has fallen below 10°C.