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.

We use temperature and rainfall information to simulate the development of a key pathogen* responsible for phoma leaf spot and phoma stem canker.

The forecast page (<1 of 2>) 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 lower disease ratings for stem canker (7 and below) or small plants.

The tool also shows how the current year compares to recent years – allowing ‘earliness’ or ‘lateness’ to be assessed.

The predicted infection status page (<2 of 2>) shows the proportion of the crop currently (today) forecast to have ‘no symptoms’, ‘infections taking place’ and ‘10% incidence'.

*Phoma is caused by two closely related pathogens – Plenodomus lingam* and P. biglobosus** – and goes through one infection cycle per season (it is a monocyclic disease).

The pathogens are known by other scientific names: *Leptosphaeria maculans and **L. biglobosa, which were the preferred scientific names until 2024.

The phoma leaf spot model, which is based on P. lingam, uses temperature and weather data for the period 15 July to 26 September.

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 (originating 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

**A note on rainfall

AHDB-funded research (completed March 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.

The phoma leaf spot forecast for each location uses rain-gauge data from the Environment Agency, Natural Resources Wales, Scottish Environment Protection Agency and Starling Roost (to calculate the ‘R’ value) and mean maximum daily temperature data from the nearest Met Office weather station (to calculate the ‘T’ value). Locations associated with no/low oilseed rape areas are excluded from the analysis. Other locations were excluded when manual data quality control detected missing or unexpected data (e.g. rainfall values significantly out of line with neighbouring stations for several days).

 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)