Light leaf spot forecast now reacts to winter wetness

AHDB’s newly improved light leaf spot forecast now reacts to rainfall during the winter, helping to focus crop monitoring, particularly in regions with higher predicted disease risk.

High winter rainfall encourages the development and spread of light leaf spot in infected oilseed rape, and movement to unaffected plants, even at temperatures lower than required for crop growth.

If long-term average levels of winter rainfall are assumed, the 2021/22 forecast suggests that disease incidence will be highest in the South West and towards the north of the UK.

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

Robert Saville, who manages disease research at AHDB, said: “As soon as the disease is observed, it is important to consider treating susceptible crops – with priority given to those sown relatively early.

“Since light leaf spot often occurs in distinct patches, whole-field monitoring is required to assess disease severity. Incubation of potentially infected leaf samples can help bring out symptoms – and some disease has already been detected this autumn, using this method.”

Until this year, the forecast was issued twice. Firstly, in autumn, based on regional average pod incidence data from the previous season (gathered as part of the Defra disease survey), site-specific deviation from the 30-year summer (July and August) mean temperature and long-term winter rainfall data. Secondly, in spring, to account for the actual deviation in winter rainfall.

Robert said: “Recent industry consultations confirmed the forecast is valued. However, one criticism was that the final forecast came too late to make a difference. As a result, we have replaced the static spring forecast with a live and dynamic winter forecast. Put simply, if the winter looks like it is going to be wetter than average at any given site, the risk will increase – and vice versa.”

The improved tool now also features a scenario-planning function to help reveal the impact of a relatively wet (rainfall 50% above long-term average) and a relatively dry (rainfall 25% below long-term average) winter on the predicted disease incidence.

The tool now also exploits weather data gathered at hundreds of UK sites. Issued earlier this year, an improved phoma leaf spot forecast uses similar site-specific weather data. It is essential that treatment plans take account of any phoma spray applied, as it may already provide adequate control of light leaf spot.

AHDB fungicide performance reports contain essential information for the optimum treatment of these two oilseed rape diseases.