Risk of LLS is high
22 Jan 2019
In this blog, ADAS plant pathologist, Philip Walker, discusses the current risk from light leaf spot and how best to control the disease.
“Generally, oilseed rape crops are looking well; however, the current risk to crops from light leaf spot is high, with visible light leaf spot symptoms observed in early January in field crops in counties in the West, North and East of England.
Because the risk of infection is high more regular crop monitoring is advisable, once a week, particularly if your variety is susceptible. Early detection is important, as light leaf spot can spread rapidly within the crop, going from first signs of infection to the whole crop infected within several weeks.
Spring LLS control
There is no threshold for light leaf spot prior to stem extension, so fungicides should be applied as soon as possible at the first signs of the disease.
Light leaf spot is polycyclic, so is continually producing spores. These are splashed on to new leaves which makes it difficult to control curatively, especially in a high-pressure situation. Protectant applications will protect the newly emerged leaf layers before they are exposed to the disease. Keeping these new leaves clean will prevent the production of new spores, decreasing a further source of infection for leaves and pods.
If light leaf spot is not seen prior to mid-February or stem extension (GS 30), then a preventive mid-February spray, in an area or crop considered at high risk of the disease may be advisable in order to protect the crop as it extends, and to decrease pod infection. Fungicide applications made in a highly curative situation tend to be less effective and this decreases the overall effectiveness of fungicides.
The relative contribution of a fungicide application to protecting yield will differ from year to year. Previous work suggested for every 15% plants affected at stem extension the associated yield loss would be 5%. Therefore, the potential for substantial yield loss is greater in a crop that is heavily infected at stem extension. Using varieties with better resistance and keeping the crop disease free through regular monitoring and using fungicides at the appropriate time will allow the disease to be controlled most effectively.
Light leaf spot is a difficult disease to control, so an integrated approach, using a combination of variety resistance and chemical control, is the best strategy. We need to use fungicides wisely, implementing fungicide resistance management strategies, to prevent the selection for strains with the potential to be more problematic to growers in future.
Best practice starts by thinking about the fungicide programme as a whole and using effective co-formulations and tank mixes (single applications of two modes of action that are both effective against light leaf spot) and alternating different modes of action throughout the whole programme. We are limited to azole and azole co-formulations to control light leaf spot in the autumn, so opportunities to alternate azoles with non-azole options as well as use azoles in mixtures for other parts of the fungicide programme, should be considered.”
Real Results Circle
Philip Walker is on our OSR expert panel, as part of the Real Results Circle, and will be providing tips and advice throughout the season.
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