Phoma leaf spot forecast

As one of the most important diseases of winter oilseed rape in England, it is important to keep on top of the phoma threat. Varietal resistance is the foundation of control. However, a well-timed fungicide spray(s) is sometimes necessary too.

The timing of fungicide applications is important to achieve phoma control. Use fungicides in response to disease forecasts, crop monitoring and thresholds.

Temperature and rainfall information (1 July to 26 September) is used to simulate the development of Leptosphaeria maculans, a key pathogen responsible for phoma leaf spot and phoma stem canker.

Accounting for subsequent crop infection, the forecast (published in October) predicts the date when 10% of oilseed rape plants could potentially show symptoms of phoma leaf spot*. This level of infection relates to a treatment threshold for varieties with lower disease ratings for stem canker (7 and below).

*Note: Localised rain events are highly influential. Dates for 10% leaf spotting can vary tremendously (by a month or more), even at sites close to those monitored in the forecast. The forecast provides a general indication of the risk trends for the season and can only support field-level data and monitoring.

1. Select varieties with strong resistance to diseases, including phoma (see the AHDB Recommended Lists).
2. During autumn, monitor oilseed rape for phoma leaf spots (prioritise susceptible varieties and backward crops).
3. Look on the underside of leaves; if white tufts (mycelium and spores) are present, the symptom is downy mildew, not phoma.
4. Note that crops usually start to breach treatment thresholds in October.
5. A fungicide applied as close as possible to a threshold helps maximise its effect.
6. AHDB fungicide performance data includes information on product efficacy against phoma.
7. Treat varieties with lower resistance ratings for stem canker (7 and below) and backward 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 ten weeks after the first spray.
10. Adjust spray programmes to account for any late-autumn fungicide (November) required for light leaf spot control.