Oilseed rape agronomy as crops approach flowering

Covering weeds, pests, diseases and nutrients, AHDB’s Ollie Johnson blogs about the agronomic focus for oilseed rape as crops approach flowering.

Most oilseed rape crops will reach stem extension soon, with some already having flower buds well above the canopy. It leaves an extremely tight window to apply those last herbicides – clopyralid or clopyralid and halauxifen-methyl – to manage thistles, mayweed, crane’s-bill, poppies and cleavers. Weeds left after this window closes will be with us until harvest desiccation.

Pollen beetle is also in focus. Typically, the pest emerges from mid-March to mid-May, particularly when daytime temperatures climb above 15°C. If bud formation is slower than expected, this extends the at-risk period.

Monitoring pest populations is key. AHDB’s treatment-threshold guidance is based on the number of beetles per plant at the green-to-yellow bud stage and the plant population density.

Treatment thresholds

• Fewer than 30 plants/m² = 25 beetles per plant
• 30–50 plants/m² = 18 beetles per plant
• 50–70 plants/m² = 11 beetles per plant
• More than 70 plants/m² = 7 beetles per plant

If beetle numbers are under these thresholds, then it is not economic to spray. Avoiding sprays also helps to maintain beneficial insect populations and slows the spread of insecticide resistance, which is already widespread in this pest (pyrethroid resistance).

Of course, pollen beetles are unusual: they can be a pest one day and beneficial the next. This is because pollen beetles are impatient. They want to get to the pollen and will bore through the bud, damaging the developing flower. Once flowers open, the beetle no longer needs to bore, and its feeding activity can lead to crop pollination.

Oilseed rape needs an adequate supply of sulphur and boron during flower development. As some of the leachiest nutrients, the wet weather means that preventing deficiencies – before symptoms (and the flowers) are visible – is likely to be more important this year. A tissue test can help reveal potential deficiencies. Speed is of the essence at this time of the year, so corrective nutrients are best applied as a foliar treatment.

As temperatures rise, light leaf spot symptoms develop. The pathogen produces more than one cycle per season (it is polycyclic), which means you will often have all stages of its life cycle present and full control is rarely possible.

Where risk is high or symptoms are present, a fungicide treatment should be considered. Azoles and non-azoles are available for light leaf spot control, which is important for fungicide-resistance management. The latest annual reports from our fungicide-performance project provide excellent information on how to balance the chemical and non-chemical control of light leaf spot, as well as phoma.

Sclerotinia stem rot also needs to be considered during flowering. Although the infection cycle of the pathogen is complex, a good understanding of the three main risk factors – the presence of inoculum (spores), weather (warm and humid conditions), and crops in flower – will help you target control.

Our sclerotinia-infection risk alerts are back for the 2024 flowering season. The interactive tool highlights when weather is conducive for infection at over one hundred locations.