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Some winter oilseed rape crops may already have 10 per cent of the crop showing phoma symptoms and could need treating, according to AHDB's forecast for the disease.

Compiled by Rothamsted Research, the 2020 forecast reveals a wide range of predicted dates (September 13 to November 10) for crops to reach this threshold across the UK.

But compared with 2019, most sites (19/29) are likely to reach the threshold earlier. The 2020 forecast predicted eight sites would reach it in September, in contrast to just two sites in 2019.

Catherine Harries, who manages disease research at AHDB, says: "Phoma spores mature on stubble - a process that is favoured by warm, wet weather. Many sites monitored experienced such conditions in 2020, so it is important to look for symptoms in the field, especially when the local forecast suggests the threshold date is near. Backing up the forecast findings, we have received reports of early-sown crops showing phoma symptoms."

Ten top tips for phoma management

1. Infected crop debris is the main source of infection - use cultivation and rotation to reduce disease pressure.
2. Select varieties with strong resistance to diseases, including phoma (see the AHDB Recommended Lists).
3. During autumn, monitor oilseed rape for phoma leaf spots (prioritise susceptible varieties and small crops).
4. Note that warm and wet conditions favour the disease. Most crops breach treatment thresholds in October, although it may be earlier (check the AHDB forecast).
5. A fungicide applied as close as possible to a threshold helps maximise its effect.
6. Check AHDB fungicide performance data, which includes information on product efficacy against phoma.
7. Treat varieties with lower resistance ratings for stem canker (7 and below) and small crops first, when 10-20% of plants have phoma leaf spot.
8. Only treat varieties with high resistance ratings for stem canker (8 to 9) if more than 20% of plants have phoma leaf spot.
9. When reinfection occurs, consider a second spray - typically, four to 10 weeks after the first spray.
10. Adjust spray programmes to account for any late-autumn fungicide (typically, November) required for light leaf spot control.

Source: AHDB