Winter Oilseed Rape Update – Mid August 2020

The monitored commercial crop data for the adopted crops this week is shown in the table below. This data is generated in commercial crops in your area so is indicative of the disease pressure and growth stages on farms at the moment. It should give you a guide to the generic disease pressure in your area.

Many crops have already been swathed and some harvested.

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.

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.

We would welcome any reports of poor flea beetle/cabbage stem flea beetle control this season as it will help us 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 week and if this continues to when crops start to emerge then there may be a higher risk of TuYV than usual. 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

In ideal conditions where the soil is warm and moist, seed well covered by soil and a low risk of crop failure, then pre-emergence weed control is the most cost-effective option, especially for annual meadow-grass and for some of the more problematic weeds such as shepherd’s purse and crane’s-bill. It seems likely that pre-emergence treatments could work well this season. The crop must be treated within 48 hours of sowing. If for any reason this has not been possible, or if the seed is not sufficiently well covered by settled soil or heavy rain is forecast, then post-emergence treatments can be used. For well covered seed, post emergence treatments can be applied from the fully expanded cotyledon stage, if seed is not well covered then wait until the crop has two to three fully expanded true leaves.

Early post-emergence herbicide options are similar to pre-emergence, except that products containing clomazone cannot be applied post-emergence. Generally metazachlor mixes with quinmerac or dimethenamid-P are a better option post-emergence than straight metazachlor – if a mix of the three active ingredients is used there is root, shoot and leaf uptake, compared with just root uptake from straight metazachlor. Greatest spectrum of weeds is controlled pre-emergence. They have a more limited spectrum post-emergence and weeds that are susceptible post-emergence must be quite small – generally cotyledons stage up to 2 true leaves, depending on species.

Some early post-emergence options:

Metazachlor + Dimethenamid–p: Post-emergence controls chickweed and crane’s-bill are controlled at the cotyledon stage, common field speedwell at the 1 true leaf stage.

Metazachlor + Quinmerac: Post-emergence – chickweed, cleavers, red/henbit deadnettle, mayweeds, speedwell, corn poppy and shepherd’s purse are controlled for 1-4 leaf stage (varies with species).

Metazachlor + Dimethenamid–p + Quinmerac: Post-emergence they control chickweed, cleavers, shepherd’s purse and common field speedwell up to the 2 leaf stage. Crane’s-bill and red deadnettle are controlled at the cotyledon stage.

If a Clearfield (CL) variety has been sown, then the Clearfield herbicides can be used. These are applied early post-emergence and have a significantly broader weed spectrum than other metazachlor-based products. There are three Clearfield herbicide options – Imazamox + Metazachlor, Imazamox + Quinmerac and Imazamox + Metazachlor + Quinmerac. They are applied from the crop two expanded cotyledons stage to when the crop has eight true leaves. They are most effective when weeds are small. Control can be improved by adding the adjuvant Dash HC. Of the three options, the third product has the broadest weed spectrum. They are only for use on Clearfield varieties – they kill non-Clearfield varieties, which is useful if volunteer rape is an issue in a Clearfield crop.

If a pre-emergence or early post-emergence metazachlor treatment isn’t applied or if broad-leaf weed control isn’t 100%, then halauxifen-methyl + picloram is useful because it can be used slightly later and can control slightly bigger weeds. It is largely foliar acting on emerged weeds and is applied slightly later than metazachlor-based treatments. It controls a wide spectrum of broad-leaved weeds and can control larger weeds. It is applied as a 0.25 l/ha dose from 1^st September once the crop has 2 true leaves, or if a higher dose is needed for larger weeds then from 15^th September from when the crop has 6 true leaves. It is most effective on small weeds so a 0.25 l/ha application to smaller weeds repeated if necessary can be a good approach. It can be applied up to 8 true leaf crop growth stage. Maximum weed size varies with species, so check the label.

If grass weeds as well as broad-leaved weeds are an issue then a follow-up propyzamide treatment later in the autumn once the soil temperature has fallen below 10°C can help control grasses not controlled by earlier treatments.

Protection of water

With all residual herbicides in oilseed rape, it is important to remember that all the common rape herbicides are regularly being detected in water courses above EU drinking water limits. To help protect metazachlor, there are label restrictions – no more than 1000g/ha active ingredient applied to the same field over a three year rolling period and no more than 750g active per crop per season.

The latest VI Water Protection Advice Sheet advocates a cut-off for application of metazachlor or quinmerac to drained soils – they should preferably be applied by 30^th September, and should not be used after 15^th October. Additionally, the general VI best practice advice to protect water should be followed:

• Fill sprayer in a bunded area and clear up any spills immediately
• Ensure there is a 6m grass buffer strip next to water courses
• Wash sprayer down in the field or in a bunded area
• Do not apply when soils are cracked, dry or saturated, or if drains are flowing.
• Do not apply if heavy rainfall is expected within 48 hours of application as this can lead to significantly higher losses to water.

In high risk areas metazachlor and quinmerac should not be applied after 30^th September, and at least six of the following should be met in order to reduce the risk:

1. Soils are moist and NOT dry, cracked or saturated.
2. Field drains are NOT flowing and are unlikely to flow within 7 days of application.
3. Field slope is less than 5% (a 5% gradient is 1 metre fall in 20 metres).
4. The field is NOT bordered by a watercourse.
5. Metazachlor* is applied at less than 750g a.i./ha or quinmerac at less than 250g a.i./ha, especially in combination with other herbicides. Check required dose with your BASIS-registered adviser.
6. The field has a 5m no-spray zone or 6m grass buffer strip adjacent to water.
7. The field has NOT been deep sub-soiled (below plough layer) in the preceding 6 months.
8. The crop has been established early with minimum tillage of only the top 4-6 cm or by direct drilling.
9. There is NO risk of heavy rainfall within 48 hours of application.