In the final part of the High 5 series, Robert Harris looks at late management from full canopy to harvest to help secure yield and oil content.

Late diseases and pests can quickly undermine an oilseed rape crop’s potential, so keeping ahead of them is crucial to build yield for as long as possible.

Fewer crops than usual this season are expected to hit the 5t/ha target that many experts believe is a realistic national average. Nevertheless, sticking to high yield principles is key to maximising crop potential, experts agree.

The key yield robber during this period is sclerotinia. The disease survives in the soil for up to 10 years as sclerotia. These produce ascospores, which land on petals and green tissue – if petals fall and stick on wet leaves and stems after light rain, plant infection can occur.

Yield reductions of 50% a plant are typical, so heavy infestations can devastate output, says David Ellerton, Hutchinsons’ technical development director.

“Typically control starts at yellow bud and continues with the key mid-flowering fungicide three weeks later. This belt-and-braces approach will suit high-risk sites, especially where rape and other susceptible crops are grown in a close rotation,” he adds.

He recommends azoxystrobin, either in a tank mix or pre-formulated with cyproconazole at early yellow bud – its additional greening effect helps seed yield and oil production. Prothioconazole +/- tebuconazole may also feature, depending on risk. Boscalid mixes suit mid-flower timings, as does the new SDHI combination of bixafen + prothioconazole + tebuconazole.

For more on this: See all of the articles on High 5 OSR yields

PGR fungicides applied at green to yellow bud contribute useful early sclerotinia control, says Nick Wall of Hampshire-based Crop Management Partners. “After that you are targeting the disease every three weeks.”

He favours prothioconazole at 0.4-0.5 litres/ha at early petal fall, plus azoxystrobin at 0.3-0.5 litres/ha, depending on canopy size and timing. Prothioconazole is pencilled in again three weeks later, although strategy is based on the ADAS/BASF sclerotinia monitoring service (www.totaloilseedcare.co.uk). If disease risk is low he may extend timings, reduce rates or omit the second spray.

The azoxystrobin/prothioconazole application also has some activity against verticillium, which causes premature ripening and yield loss at the end of the season. But there is limited evidence that the spray protects yield, he notes. “The best defence is to extend the rotation and grow rape no more than one year in four.”

Alternaria, once the key late-season disease, is much less prevalent, says Dr Ellerton. “Seed dressings have been particularly effective, although many late-season fungicides offer good protectant control if wet weather favours it.”

Pests

Pollen beetle numbers can build fast, though oilseed rape does have a high threshold. Recently revised figures, based on the premise that sparser crops produce more excess flowers, show a threshold of 25 beetles a plant for a crop with a plant density of less than 30 plants/sq m, 18 beetles for 35-50 plants, 11 beetles for 50-70 plants and seven beetles for above 70 plants.

Monitoring starts when flower buds are visible until beginning of flowering, after which beetles pose no risk, says Mr Wall. “They no longer have to bite through flower buds as they have free access to pollen.”

Generally, pollen beetle does not reach threshold, he maintains. However, it can be a problem in backward crops with extended flowering periods, so it could feature more this season.

A pyrethroid, often with the yellow bud fungicide, controls the pest. No resistance to these insecticides has yet been recorded in his area, he notes.

However, in some areas resistance is spreading rapidly, says Dr Ellerton. “If you suspect it, either switch to an alternative mode of action or use tau-fluvalinate, which although being a pyrethroid has greater activity against resistant populations than others.”

Seed weevil/pod midge should be carefully monitored, Dr Ellerton advises. Seed weevils can lay eggs singly in up to 50 pods and larvae can eat a quarter of the seeds in each infested pod. Thresholds are low at one weevil a plant, half that where there is a history of the pest.

He recommends pyrethroid treatment as pods start to form and weevils begin to bore holes into which midges lay their eggs. The resulting 20-30 larvae can damage seed and cause pods to split. “In bad infestations yield losses can reach 10%.”

However, pod midges are weak fliers and damage is often confined to the headlands, Mr Wall notes. “We’ve not had to treat in the past couple of years and when we do, it is usually headlands only.”

Harvest management

Harvest management is likely to be more important this season than most, says Dr Ellerton. “Crop growth stages are all over the place so there will be quite a spread of pod maturity as we approach harvest.”

Diquat evens crops effectively, but treated plants are prone to pod shatter, he says. He prefers glyphosate, which provides additional couch control, particularly Roundup Flex for its reliability in a range of weather conditions and because it is less prone to drift.

Nick Wall also prefers glyphosate, generally good-quality generic products. Sprays should be applied when two-thirds of seeds in pods on the main raceme have turned brown, he advises. Any earlier and yield and oil content can be affected.

“As well as protecting yield and quality, desiccation saves time and fuel when cutting, and reduces drying and cleaning costs.”

Both men also favour a pod sealant such as Pod-Stik or Zip-Pod, especially useful during extended ripening. Applying sealants in a separate, earlier pass when pods are more flexible reduces shatter when applying desiccant, says Dr Ellerton. “This could be especially useful this year if pod maturity is extended.”