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.
How to manage light leaf spot in oilseed rape

Issued each autumn by AHDB, the preliminary light leaf spot (LLS) forecast shows the proportion of the oilseed rape crop (disease resistance rating of 5, sowing date in the week centred on 1 September) estimated to have more than 25% of plants affected by LLS in the spring for the current season. 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. In spring, the forecast is updated to reflect deviation in actual winter rainfall data 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.

Light leaf spot forecast 2020/21 (final)

Light leaf spot risk increases in response to winter rainfall (spring 2021 news) A UK map showing forecast light leaf spot risk (2020-21)

0–14%

15–29%

30–44%

45–59%

>60%

Light leaf spot forecast 2020/21 (preliminary)

Light leaf spot final forecasts (2013–20)

2019/20

2018/19

2017/18

2016/17

2015/16

2014/15

Regional forecast

How to account for the effects of using an autumn fungicide, altering sowing date and increased varietal resistance.

NOTE: This is based on the preliminary forecast.

Autumn fungicide

The forecast (scenario 1) 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.

Sowing date

The forecast (scenario 1) 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 – with early sowings (mid-August) increasing prevalence and later sowings (mid-September onwards) decreasing prevalence. Once again, the effect is smallest when risk is relatively low.

Varietal resistance

The forecast (scenario 1) 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.

Forecast scenario

1

2

3

4

5

Location

Resistance rating of 5

Sowing date during week commencing 1 September

No autumn fungicide

Scenario 1 with autumn fungicide

Scenario 1 with early-sown crop (mid-August)

Scenario 1 with later-sown crop (mid-September)

Scenario 1 with relatively disease resistant crop sown (rating of 8)

Grampian

37%

13%

42%

31%

15%

Aberdeenshire

40%

15%

45%

34%

16%

Fife

37%

13%

42%

31%

15%

North of England

26%

7%

31%

21%

8%

West of England and Wales

31%

10%

36%

25%

11%

East Anglia

6%

1%

10%

2%

3%

East

3%

1%

4%

1%

1%

South

6%

1%

10%

2%

3%

South East

4%

1%

5%

1%

2%

South West

14%

4%

18%

11%

4%

0–14%

15–29%

30–44%

45–59%

>60%

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