The autumn light leaf spot (LLS) preliminary forecast, published on 18th October 2017, shows a relatively large regional variation in disease risk across Great Britain.
The forecast, which shows the proportion of winter oilseed rape (WOSR) predicted to have more than 25 per cent of plants affected by LLS in the spring, ranges from relatively low risk (13 per cent) in East Anglia to relatively high risk (66 per cent) in the North of England and Moray Firth in Scotland.
Overall, 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.
How risk compares to recent years. Based on historical weather and disease data, the average regional forecasts are shown. The estimated proportion of the oilseed rape crop (disease resistance rating of 5) with more than 25 per cent of plants affected by LLS in the spring-time for the current season is illustrated. Download as a PDF.
Dr Neal Evans, 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.
Neal said: “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.”
The preliminary forecast provides 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.
As local risk varies, the forecasts should only be used as a guide and crops should be inspected regularly on a field-by-field basis.
To provide the strongest risk assessment, plant samples should be put in polythene bags and kept at 10 to 15 degrees 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 per cent of plants affected is associated with a five per cent 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.
For further information on LLS, fungicide performance and resistance management, visit cereals.ahdb.org.uk/osrdisease