Light leaf spot forecast

Producing more than one infection cycle per season, light leaf spot is an important disease of winter oilseed rape. Although chemistry is available, sufficient control often depends on a combination of cultural measures and well-timed sprays.

About the forecast

Issued each autumn, the preliminary light leaf spot (LLS) regional forecast shows the proportion of the oilseed rape crop (see 'Notes') estimated to have more than 25% of plants affected by the disease in the spring.

The forecast uses previous season pod incidence data and deviation from the 30-year mean summer (July and August) temperature. 

It also uses historic average winter rainfall data and the actual deviation in winter rainfall from the 30-year mean.

The forecast provides a guide to seasonal risk levels. However, as local risk varies, it is important to inspect crops on a field-by-field basis, with prioritisation on early-sown susceptible varieties.

Latest light leaf spot forecast

Notes on fungicide use, varietal resistance and sowing date

Autumn fungicide

The forecast is based on no autumn fungicide.

Typically, a well-timed application of an appropriate autumn fungicide – at a sufficient dose – has a relatively large impact on the final light leaf spot levels in a crop.

However, when disease risk is low, the effect is much less.

In very low-risk situations (determined by observations of field-level risk), there is potential to reduce fungicide dose or omit some sprays – provided the strategy adopted also controls phoma sufficiently.

Varietal resistance

The forecast is based on a resistance rating of 5.

Cultivar resistance has a significant effect on predicted light leaf spot prevalence.

When the forecasts were first developed (based on data from 1987–99), ‘5’ was a moderate resistance rating.

However, most current varieties are more resistant.

Sowing date

The forecast is based on a sowing date in the week centred on 1 September.

The effect of sowing date on predicted light leaf spot levels is moderate.

Early sowings (mid-August) increase prevalence and later sowings (mid-September onwards) decrease prevalence.

Once again, the effect is smallest when risk is relatively low.

Tool updates

23 December 2021

For a given location, the tool now uses data from the nearest weather station to calculate the deviation of the previous summer temperature from the historic 30-year mean summer temperature.

A new quality-control mechanism identifies rain gauges with 'unexpected' results and omits them from the map in rainfall scenario 1 (actual rainfall so far, 1961–90 for remainder). Compared to all rain-gauge data, this identifies rain gauges with:

  • An unusually low number of rainy days or dry days
  • An unusually high number of rainy days
  • Missing data exceeding 15%

 Note: Due to a SEPA security issue, no rainfall data for Scotland is available.

Historic light leaf spot forecasts

From 2021–22 onwards, an updated model and new dynamic visualisation tool were used to present the forecast.

For earlier forecasts (harvest years 2013–21), visit the light leaf spot forecast archive.

These forecasts present risk across defined light leaf spot risk regions (see blue map).

Light leaf spot forecast archive

Light leaf spot management tips

How to manage light leaf spot in oilseed rape