THERE could be a wide-ranging threat from light leaf spot in oilseed rape this autuumn and early winter – with some Scottish crops particularly at risk – AHDB Cereals and Oilseeds has warned.

Regional variation in light leaf spot risk, is almost guaranteed and preliminary forecasts shows a relatively large variation in risk across Great Britain.

The LLS forecast, which shows the proportion of winter oilseed rape (WOSR) predicted to have more than 25% of plants affected by spring, ranges from relatively low risk (13%) in East Anglia to relatively high risk (66%) in the North of England and Moray Firth in Scotland.

Overall, said AHDB, the risk could be described as ‘moderate’ but, due to regional variation, people are reminded to calculate field-level risks by field walking and using risk-assessment tools.

Dr Neal Evans, a plant pathologist at Weather INnovations (WIN), who puts together the forecast, said: “Last season was fairly strange, with a large LLS epidemic seen in most crops during the winter but the long, dry spring limited transfer to pods.

“As pod incidence is used as a measure of inoculum in the model, this helped limit the overall risk but there are some regional hotspots and it is the field-level risk that counts.”

Results from recent AHDB-funded work show wind-blown LLS spores are produced through the summer and autumn.

Dr Evans added: “The research explains why early sown crops are at higher risk from LLS. If not treated appropriately, you can find yourself chasing the disease from day one to harvest.

“Growers should use all the tools to calculate risks and protect their crops accordingly.” Using the preliminary forecast is part of that and can provide an indication of the regional risk levels for the current season.

To take account of variety, sowing date and autumn fungicide applications, people can produce a customised forecast on the AHDB website to provide a better indication of risk.

To provide the strongest risk assessment, plant samples should be put in polythene bags and kept at 10 to 15°C for around five days to bring out symptoms.

There is no treatment threshold before stem extension and people should consider applying a suitable fungicide as soon as LLS is detected in the field.

At early stem extension, a treatment threshold of 15% of plants affected is associated with a 5% yield loss.

Paul Gosling, who manages fungicide performance work at AHDB, said: “There have been reports of decreased azole sensitivity in UK light leaf spot populations. Although this has not been confirmed and the impact on field efficacy not quantified, it is a cause for concern and the Fungicide Resistance Action Group guidance should be followed.”

Non-azoles are available for LLS control and it is recommended that a range of products, representing different modes of action groups, are used throughout the fungicide programme.

In spring, the LLS forecast will be updated to reflect deviation in actual winter rainfall data from the 30-year mean.