Light leaf spot disease is being detected in oilseed rape crops from Berwickshire to Essex after plant leaves were analysed in the laboratory, giving growers a warning that it may soon be seen in the field.

The crop’s most yield-damaging disease is being identified a lot earlier than anticipated this season, showing how endemic it is becoming, although it is not yet being seen in growing crops.

Julie Smith, plant pathologist at crop consultant Adas, points out that light leaf spot can be present in  oilseed rape crops a lot earlier than it can be detected in the field.

“The disease is not appearing in the field, but it is being seen after incubation and under the microscope in the laboratory,” she said.

See also: Oilseed rape light leaf spot risk higher in the North

Samples of oilseed rape plants have tested positively for light leaf spot in the laboratory from the Scottish Borders down through Lincolnshire and into Norfolk and Essex.

Free diagnosis scheme

This testing is part of a free disease diagnosis scheme, called SpotCheck, being undertaken by Adas and funded by agrochemicals group Bayer, which is looking to test 500 samples throughout the winter.

The scheme, which started at the beginning of October, has tested about 150 samples, which have shown plenty of phoma and some downy mildew, but no visual signs of light leaf spot.

Disease spores circulate as early as the start of August from previous oilseed rape crops, but it is unclear how long it takes from initial infection to symptoms appearing in the field.

“This is giving growers a heads-up that the disease is likely to be in crops, and if they are looking to spray they should not wait until the new year,” she added.

Early signs of light leaf spot © Blackthorn Arable

No disease in field 

Lincolnshire-based crop consultant Sean Sparling says he has sent in 15 crop samples and only one has come back showing 3% of leaves infected with light leaf spot, while he has seen no sign of the disease in the field.

Many of his crops have had a fungicide spray for phoma, such as tebuconazole or difenoconazole, and he adds that a big mistake would be to delay a light leaf spot spray to go with a much later propyzamide (Kerb) herbicide application.

“Now is the season for light leaf spot, so growers need to keep their eyes open, especially with this mild weather,” said Mr Sparling, who is also chairman of the Association of Independent Crop Consultants.

Moderate risk

Neal Evans, plant pathologist at independent forecaster at Weather Innovations, says the risk of light leaf spot is “moderate” this year, as the long, dry spring weather limited disease transfer from leaves to pods.

But there are big regional variations, with the risk highest in the Moray Firth area of Scotland and northern England, and lowest in East Anglia, he adds.

There is no disease threshold for fungicide treatment, and the general advice is to spray as soon as you see symptoms to protect newly expanding leaf tissue and growing points.

Growers are advised to use azole fungicides based on either prothioconazole or tebuconazole, or a mix of the two to protect against the disease.