Light leaf spot risk increases in response to winter rainfall

Thursday, 4 March 2021

Our updated light leaf spot forecast shows large jumps in oilseed rape disease risk, following a relatively wet winter.

Light leaf spot forecast page

This tallies with field reports, where light leaf spot infection is most evident on susceptible varieties – with some crops showing symptoms without the need to incubate leaf samples to bring out symptoms.

The increase in risk (compared with the autumn preliminary forecast) is generally larger at locations further away from the South East of England.

The regional disease forecast highlights the proportion of susceptible (disease rating of 5) oilseed rape crops predicted to have more than 25% of plants affected by the spring.

Catherine Harries, who manages disease research at AHDB, said: “Some regions experienced rainfall levels way above the long-term winter averages. Parts of eastern Scotland stand out, in particular. “Frustratingly, following limited disease incidence on pods at the end of the 2019/20 season, the UK was on track for a relatively low-risk year. Now, the pendulum has swung towards a more traditional risk pattern – with risk relatively high towards the West and the North.”

Disease development will be encouraged by rising spring temperatures and farmers should closely monitor crops, especially susceptible varieties.

If light leaf spot is present, a fungicide should be applied as soon as possible. According to AHDB fungicide performance data, half-doses generally give good control. However, crops in higher-risk areas may require higher doses.

How to manage light leaf spot in oilseed rape

About the light leaf spot forecast

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)

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)

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