Nearly half of OSR sites set to hit phoma threshold from mid-October

Nearly half of oilseed rape forecast sites across Great Britain could reach the key phoma treatment threshold in the week beginning 18 October, according to AHDB.

Disease onset is expected to be slightly later than in recent seasons, after a relatively high number of dry days slowed development across many areas.

The AHDB forecast models when 10% of oilseed rape plants are likely to develop phoma leaf spot symptoms at hundreds of sites across Great Britain.

That 10% level is an important treatment trigger, particularly for smaller plants and varieties with lower stem canker resistance ratings.

Growers in western parts of Great Britain may need to be especially alert, with the relatively small number of sites expected to reach threshold earlier in October concentrated mainly in those areas.

The forecast shows 47% of sites reaching the 10% threshold in the week beginning 18 October, followed by 27% in the week beginning 25 October.

Only 3% of sites are expected to reach the threshold in the week beginning 4 October, with a further 12% in the week beginning 11 October.

That points to a later onset than in both 2024 and 2025, when a greater proportion of sites reached threshold earlier in October.

AHDB said later disease onset should generally reduce the potential impact on yield because early phoma epidemics are typically the most damaging.

The forecast uses rainfall and temperature data from 15 July to 26 September to simulate development of the phoma pathogen.

Warm and damp conditions speed up spore release from infected stubble, crop infection and disease progression.

If left unchecked, the disease can develop into stem canker around leaf scars at the base of plants, especially in more susceptible varieties.

Growers are advised to prioritise smaller plants and varieties with relatively low Recommended Lists stem canker resistance ratings once 10% of plants show phoma leaf spots.

Treatment urgency increases as resistance ratings fall below 7, which includes a number of widely grown varieties.

Several new varieties with a resistance rating of 8 were added to the Recommended Lists for 2026/27 last December, strengthening the choice of hybrids with higher phoma resistance.

AHDB said such varieties may not need a specific phoma spray unless the 20% symptom threshold is exceeded.

Where treatment is required, timing remains important.

AHDB’s long-running fungicide trials found that “all mode of action groups authorised for use against phoma have good protectant activity”.

The trials also found that “two well-timed sprays at half the recommended label rate can deliver good phoma control”.

That finding comes from AHDB’s fungicide performance work and should be considered alongside product labels and wider spray programmes.

Reinfection can occur around four to 10 weeks after the first application, meaning a second treatment may need to be considered depending on disease pressure.

Growers should also account for any phoma control delivered by fungicides aimed at light leaf spot, which are commonly applied in November.

AHDB said fungicide mode of action groups should be alternated or used in mixtures or co-formulations across the programme to help reduce resistance risk.

With disease pressure expected to build more sharply from mid-October, the key message for growers is to monitor crops closely and base treatment decisions on symptoms, crop size and varietal resistance rather than the calendar alone.