Start monitoring oilseed rape for phoma leaf spot symptoms

Our phoma leaf spot forecast suggests that many oilseed rape (OSR) crops could reach the spray threshold for the disease in the next few weeks.

The online forecast uses rainfall and temperature information to estimate when 10% of OSR plants may develop phoma leaf spot symptoms at hundreds of sites across Great Britain.

The guidance is to treat varieties with relatively low Recommended Lists (RL) stem canker disease resistance ratings (7 and below) and small crops promptly when 10 to 20% of plants develop phoma leaf spots.

Over half of the sites are forecast to reach the 10% threshold during a two-week period in October (12 to 25 October).

As usual, the forecast predicts a large spread of dates, so it is important to monitor crops to determine the actual field-level risk.

The forecast’s main strength is to indicate the overall trend, with the distribution of forecast dates similar to last autumn.

A small number of sites have already passed the earliest threshold dates (mainly located towards the South West).

Conversely, a few sites do not reach threshold until November (mainly located towards the North East).

Sacha White, who helps manage the forecast at AHDB, said:

“The forecast mainly considers temperature peaks and dampness from mid-summer to early autumn (15 July to 26 September) because warm and wet conditions favour phoma development.

“Although summer was dry, it was the warmest on record, and the more recent showers will have encouraged disease progression.

As a rule of thumb, symptoms show on infected leaves after at least 20 rainfall days in August and early September.”

Most RL 2025/26 OSR varieties have moderate levels of resistance to phoma stem canker (disease ratings 5, 6 or 7), with some even more susceptible to the disease. These should receive prompt treatment when the 10% threshold is reached.

Of the most widely planted OSR varieties (according to the AHDB Planting and Variety Survey), many have a stem canker resistance rating below 7.

Some hybrids have high levels of resistance (disease ratings 8 or 9), and these may not need a phoma treatment, unless the higher (20%) threshold is exceeded.

The efficacy of products against stem canker on relatively susceptible OSR varieties has been evaluated as part of our fungicide performance project for over a decade.

The results show that two well-timed sprays at half the recommended label rate can deliver good control.

The second application should be considered when re-infection is evident. This is typically 4 to 10 weeks after the first application.

Potential phoma control from a spray targeted at light leaf spot (usually in November) also needs to be considered in the overall fungicide programme.

The Fungicide Resistance Action Group (FRAG) has cited reports of decreased sensitivity to azole (DMI) fungicides in some phoma populations in western Europe, including in the UK, as well as other countries.

There are several modes of action (MoA) that control phoma leaf spot/stem canker, which include azole and non-azole options.

MoA should be used in alternation or as mixtures/co-formulations across the fungicide programme to minimise resistance risks, along with an appropriately low dose.

All MoA also offer protection when applied prior to infection.

The charts capture the distribution of starting weeks (when 10% of oilseed rape plants could potentially show symptoms of phoma leaf spot) at the forecast sites for the current (2025) and the previous two (2024 and 2023) seasons.