Phoma leaf spot forecast

Use the phoma leaf spot forecast to guide crop monitoring and help time fungicide sprays in the autumn. Based on temperature and rainfall information, the model predicts treatment threshold starting-week dates at hundreds of locations.

The 2026 forecast will be published in late September or early October.

Page 1 shows the main forecast, which predicts the date of the starting week 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 stem canker disease ratings of 7 and below or small plants.

The tool also shows how the current year compares to recent years to indicate 'earliness' or 'lateness' of disease onset.

Page 2 shows the predicted infection status, which is based on the proportion of the crop forecast today with ‘no symptoms’, ‘infections taking place’ and ‘10% incidence'.

• To highlight sites associated with a specific forecast date, click on the date in the horizontal date list at the top of the GB map
• To view data for an individual location, click on the relevant coloured circle in the map
• To remove site-specific highlights and reset the map view, click on the map but avoid location circles

Phoma is caused by two closely related pathogens: Plenodomus lingam and P. biglobosus.

The phoma leaf spot model is based on P. lingam.

The phoma leaf spot forecast is based on Equation 2.1 in Evans et al. (2007):

S = 196 + (216.5 – 0.24R – 4.55T)

S = Start date, Julian Day, of 10% phoma leaf spot incidence

196 = Julian Day for 15 July

216.5 = A constant value that originates from phoma modelling research

‘T’ = Mean maximum daily temperature over the period 15 July to 26 September

‘R’ = Rainfall** (mm) over the period 15 July to 26 September

To calculate the ‘T’ value, the forecast uses mean maximum daily temperature data from the nearest Met Office weather station.

To calculate the ‘R’ value, the forecast uses rain-gauge data for each location from the Environment Agency, Natural Resources Wales and Scottish Environment Protection Agency.

 Timing of S (infection status)

• At least 8 days away = no symptoms
• In the next 7 days = infections taking place
• Today or in the past = 10% incidence

AHDB-funded research in 2021 found that the original model developed in the early 2000s, which used total rainfall information, predicted threshold breaches around two weeks earlier than seen in the field.

Since autumn 2021, the model only considers daily rainfall events up to 10 mm and total rainfall up to 200 mm, with additional rainfall having no influence.

This change was made because dampness, rather than wetness, is more critical.

As a result of the changes, the revised model tends to predict later start dates for threshold breaches.

‘Capped’ rainfall data has also been used retrospectively for the historic data in the above forecast, allowing forecast years to be compared.

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 is likely to be most accurate in regions with no/limited variability in forecast dates.

The forecast provides a general indication of the risk trends for the season and can only support field-level data and monitoring.