Phoma leaf spot and stem canker in oilseed rape

Caused by two closely related fungal pathogens, this disease can progress from virtually harmless leaf spots to highly damaging stem cankers. As affected stems are less able to move water and nutrients throughout the plant and can lodge, it is important to manage the disease effectively.

Yield-robbing cankers make phoma one of the most important diseases of winter oilseed rape in the UK, especially in Central, Southern and Eastern England. Despite fungicide treatment, infection is estimated to cause economic losses of about £100,000,000* each season.

Early phoma epidemics on small plants are associated with the greatest yield losses – with typical reductions of 0.5 t/ha in susceptible varieties. Generally, the later that leaf spotting occurs, the lower the risk of stem canker. Late epidemics (January–March) have minimal impact on yield, unless plants are small. The disease also affects vegetable brassica crops.

*Economic loss estimates based on annual survey data published by CropMonitor.

With fungicide treatment thresholds based on crop symptom levels, it is essential to recognise the classic signs of this disease. This page provides details of phoma leaf spot and stem canker symptoms to help guide in-field monitoring.

Through understanding the drivers of risk, treatment thresholds and fungicide efficacy, it is possible to reduce crop losses to phoma. This page also includes information on the annual phoma leaf spot forecast.