Oilseed rape growers are being alerted to the threat of light leaf spot as the industry’s most serious disease has been seen from Scotland to Herefordshire.

The disease, which can cut yields by up to 1.5t/ha, is being picked up on susceptible varieties, and experts say where the disease is seen, growers should look to use a fungicide when they can travel on the land.

Faye Ritchie, plant pathologist at crop consultant Adas, reports seeing signs of the disease across England, while the disease is being seen at low levels in Scotland.

“We are starting to see symptoms on susceptible varieties in Yorkshire and Herefordshire, and also one suspected case in Norfolk at low levels,”  she tells Farmers Weekly.

See also: Light leaf spot sightings prompt call for vigilance

The tell-tale signs of the diseases – white spore droplets around a suspected lesions – are being seen in crops that received a fungicide in November.

These symptoms are visible in susceptible varieties with disease resistance scores of 4 to 5 – many advisers say growers should only pick varieties with a 6 or above on the 1-9 scale, where 9 is good resistance and 1 is very susceptible.

Disease identification

Identification of the disease in the growing crop is often difficult because symptoms are not always visible and are easily confused with other plant disorders.

To confirm the disease, growers are advised to take a few leaves, put them in a plastic bag and leave for 24 hours at room temperature. Symptoms should become clearer after this time.

The advice from Adas is to treat crops with a fungicide as soon as the first disease symptoms are seen.

David Leaper, oilseed rape specialist at agronomy group Agrii, says the use of more resistant varieties and good growing conditions in the autumn have kept a lid on the disease at this stage compared with the past two years.

Cold days and nights are holding the growth of the crop and also the disease back, but crops could become susceptible as they look to grow as spring approaches.

“A well-timed spray at the onset of growth will protect the new spring growth from the disease,” MrLeaper says.

Three main options

The three main options for fungicide control at this stage include the two azoles prothioconazole (Proline) and tebuconazole and also the SDHI-strobilurin Refinzar.

Tim Nicholson, commercial technical manager at Bayer, which markets Proline, says he is seeing the disease at the group’s trial site in Oxfordshire and also across quite a few sites where autumn fungicides have not been used.

“There is likely to be latent infection in crops, so growers should be monitoring them every week,” he says.

Mr Nicholson adds growers should be particularly vigilant with varieties with a light leaf spot resistance score of less than 6.

He argues the most consistent yield response comes from prothioconazole, and warns that tebuconazole has plant growth regulatory properties, so should not be used on backward crops.

Mike Ashworth, fungicide specialist at Refinzar maker DuPont, says this SDHI-strobilurin product matches prothioconazole for light leaf spot control and also has the advantage of not being an azole.

“We believe it is as effective as prothioconazole and it is good from a resistance management viewpoint,” he says.

The product contains the SDHI penthiopyrad and strobilurin picoxystrobin. It has the limitation that only one application can be used per crop and it can only be used up to the start of stem extension (GS30) which, depending on the season, can be mid- to late March .