OSR phoma symptoms may show a little later this autumn

Our phoma leaf spot forecast indicates that many oilseed rape crops could reach a key spray threshold around the middle of October. Catherine Harries explains the results and the implications.

• The online forecast models when 10% of oilseed rape plants may develop symptoms at hundreds of GB sites
• As the forecast shows trends in disease onset, it can guide field monitoring and fungicide treatment timings
• Disease onset is expected to be a little later in autumn 2026, compared to recent seasons, with the treatment peak starting around the middle of October

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

Damp and warm conditions speed up spore release from infected stubble, crop infection and disease progression.

If the disease is left unchecked, classic stem canker symptoms can form around the leaf scars at the base of stems, especially in varieties that are relatively susceptible to the disease.

It is important to treat small plants and varieties with relatively low Recommended Lists (RL) stem canker disease resistance ratings first when 10% of plants develop phoma leaf spots.

Treatment urgency increases as the disease resistance rating falls below 7, which includes many popular varieties, according to our planting and variety survey results.

Last December, we added several new varieties with disease resistance ratings of 8 to RL 2026/27.

It strengthened the line-up of hybrid varieties with a high level of phoma resistance.

Such varieties may not need a phoma spray, unless the 20% symptom threshold is exceeded.

A relatively high number of dry days in many areas underpinned a slightly later predicted disease onset in this autumn’s forecast, compared to recent years, with the risk period consolidated – see Table 1.

The forecast predicts that:

• Nearly half of the sites will reach the 10% threshold during the week commencing 18 October 2026
• Just over a quarter of the sites will reach this threshold in the following week
• A relatively small number of sites will reach this threshold in the first two weeks of October, which are mainly in the western half of GB

As early phoma epidemics are the most damaging to yield, a later start to onset will generally reduce the impact of the disease.

Table 1. The proportion of forecast sites reaching the 10% phoma symptom threshold during seven starting weeks in autumns 2024, 2025, and 2026

As part of our fungicide performance project, we evaluate the efficacy of products against stem canker on relatively susceptible oilseed rape varieties.

Our first dose-response trials for major diseases of oilseed rape took place 20 years ago in a harvest 2006 trial.

We now have a powerful over-year dataset that shows:

• All mode of action groups authorised for use against phoma have good protectant activity
• Two well-timed sprays at half the recommended label rate can deliver good phoma control

As reinfection often takes place about 4 to 10 weeks after the first application, the second application may need to be considered.

The potential phoma control from a spray targeted at light leaf spot, usually in November, also needs to be reflected in the overall fungicide programme.

It is also critical to use mode of action groups in alternation or as mixtures/co-formulations across the fungicide programme to minimise resistance risks, along with an appropriately low dose.