Nearly two-thirds of OSR crops show light leaf spot symptoms

Almost two-thirds of oilseed rape crops tested are showing signs of light leaf spot, and if mild weather continues, infection levels from the crop’s most destructive disease could rise.

Mild winter temperatures with no prolonged cold spells, coupled with heavy rainfall, have driven the incidence of the disease in leaf samples assessed by the free SpotCheck initiative, run by agrochemical group Bayer.

This project, in partnership with crop consultant Adas and the Association of Independent Crop Consultants (AICC), tests samples of oilseed rape leaves for light leaf spot and other diseases.

Of the leaf samples sent to Adas during December, some 62% showed light leaf spot symptoms after the leaves were incubated for three days to encourage the disease to develop on the leaf surface. More than 50 samples were tested in December, and close to 200 in the last three months of 2019.

See also: Autumn disease alert for well-established rapeseed crops

Mild weather

Philip Walker, arable plant pathologist at Adas, said, “If conditions remain mild, there will likely be more and more infection events in previously symptomless fields”.

However, he adds that if there is a significant drop in temperatures, growers are unlikely to see new infections, and any pre-existing diseases will pause in development.

A crop with high levels of disease when it moves into stem extension will be hit at this vital yield-building stage of development. However, he points out that the challenge is finding an opportunity to travel, given the current wet weather

Ground conditions

Grant Reid, Bayer’s commercial technical manager for northern Scotland, said ground conditions have not been ideal for travelling, and he suspects there are a lot of crops that have not received a fungicide spray yet.

“Having said that, it is important to remember that oilseed rape is a resilient crop that branches quite a lot. If you have a crop of about 15 plants/sq m, don’t write it off and be patient,” he added.

Fungicide control for light leaf spot is largely based on using one of two azoles, prothioconazole or tebuconazole. The first is the most effective, while the second is less expensive and has a plant growth regulatory effect on crops.

You can find more information on the free SpotCheck initiative online.


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