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18 December 2013 10:34:16 |

New research to shed light on leaf spot

New research at Scotland’s Rural College is seeking to discover better ways to predict an outbreak of light leaf spot in oilseed rape crops. Light leaf spot is the number one disease threat to oilseed rape crops and can cut yields by up to a tonne per hectare.
SRUC researchers will also look to pin down the best timings and treatments to manage this perennially tricky problem. The £114,000 project is funded by industry body HGCA. It will be run by SRUC in conjunction with Weather Stations, Rothamsted Research and ADAS, in order to study the spread of the disease over a range of sites and varieties.
In 2005 just over £20 million of oilseed rape in the UK was lost due to LLS; in 2012 that figure was over £150 million. Like many pathogens Leaf Spot evolves constantly to try and get around the defences thrown at it. Typically it will erode between half a tonne and a tonne of yield from an infected crop. It was once considered a northern UK disease but has spread south in the last two seasons so that the SRUC crop clinic in Edinburgh has been inundated with requests to confirm the symptoms at sites right down to the south coast.
Fiona Burnett, SRUC’s Crop Protection Team Leader, says: “With light leaf spot now making its presence felt both south and north of the border this research will be fundamental to enhancing growing conditions for farmers. After the project we should be able to predict possible epidemics of the disease and improve our guidance around timing of fungicide use.”
Trial plots have been established in three sites in England. In Scotland SRUC will be monitoring two sites close to Edinburgh and Aberdeen. Researchers will therefore be able to assess a range of types and varieties, with different levels of resistance at different geographic locations, over the course of the three year project.
While the research could help farmers better predict epidemics and better target their protection strategies Fiona is warning that, in the meantime, farmers should not become complacent.. While it looks like this year could see a lower LLS level than usual, the incidence of the disease has shot up in the last decade as established varieties have declined in resistance to this very variable pathogen, something one good year will not change.
Prevention and treatment of Light Leaf Spot relies on resistant varieties, however, inevitably the disease has adapted and varieties like Cracker which were rated a nine when first introduced have now been downgraded to an eight and at some sites behave more poorly than that.
The other key defence against the disease is fungicides, but some are now proving less effective against the disease than previously. Sensitivity to some of the older fungicides has declined and there are often quite large differences in sensitivity between sites and between seasons. A further difficulty is that oilseed rape is very vulnerable to the stressful effects of some fungicides. This effect can be useful to regulate the growth but in a stressed crop the effect of growth regulation can often be negative to yield.
According to Fiona: “This year, with so many crops early drilled in fine conditions, growth stages are very advanced and a decision might be made in the spring to slow the growth of the crop using a treatment such as tebuconazole. However, growers have to be careful as too much could be detrimental to the crop in the long term. This makes decisions about using chemicals very hard as they need to protect the crop and check its growth without damaging it. We hope this research will make such decisions easier and so reduce yield loss in the long term.”


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