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.

From the 2021-22 winter, a dynamic forecast was introduced. This uses actual rainfall data during winter to indicate how risk is changing. The final forecast is produced at the end of the last day of winter (28 February).

Light leaf spot forecast now reacts to winter wetness (November 2021)

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.

4 March 2022

  • Added monthly rainfall totals for subset of Scottish rain gauges
  • Added a quality control component to omit rain gauges that recorded excessive 15 minute rainfall total

How risk changed over the 2021–22 winter

Our dynamic light leaf spot forecast allows the assessment of changes to risk during the winter period. This month-by-month analysis is based on Met Office climate/weather summaries.

Met Office climate/weather summaries

December 2021

Overall, the UK had 90% of average December rainfall. With rainfall close to average, overall risk was similar to the preliminary forecast. However, at the site-specific level, there were some clear changes to risk – with either increases or decreases forecast at some locations.

January 2022

Overall, the UK had 50% of average January rainfall.  Most areas received less rainfall than average and forecast risk decreased in many places – with large areas featuring in the lowest-risk category, particularly towards the central, eastern, and south-eastern parts of England.

February 2022

Stormy spells with heavy rainfall occurred in the middle of February. Most areas received more rainfall than average, doubling in many areas. This resulted in jumps in risk at many sites. Some sites even moved from the lowest to the highest risk categories – most notably around the Welsh borders. Some areas of England, especially towards the southern coastline, still recorded less rainfall than average.

Conclusion

At the end of January, it looked like it was going to be a relatively low-risk year for light leaf spot. In the end, the risk map ended up closer to the preliminary forecast at the start of winter – with the risk greater towards the usual places (towards the North and Southwest).

Many areas ended up in the lowest-risk category (dark green). Even in such scenarios, people should expect to see disease and consider treatment as soon as possible (especially on relatively susceptible varieties). Current strong rapeseed prices have also shifted the cost-benefit equation in favour of protection.

The main purpose of the forecast is to guide crop monitoring, with inspection frequency upped as forecast risk increases.

Light leaf spot management tips

How to manage light leaf spot in oilseed rape

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.

Light leaf spot forecast archive

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