Light leaf spot, an oilseed rape disease more familiar to northern and Scottish growers, could be costly for unwary producers further south this season.

Heavy carryover from last year’s crops raising infection levels in this season’s sowings, less autumn fungicide spraying and the cold winter look set to make it the worst year for light leaf spot since 1995, warn specialists.

“We’re seeing very high levels in crops – even where they’ve had fungicide in the autumn,” says ADAS‘s Peter Gladders. “There’s certainly a bigger threat than usual.” Infection can be more damaging than phoma, he adds.

Last autumn’s CSL Crop Monitor survey highlighted this season’s increased risk, notes DuPont’s Mike Ashworth.

Unchecked, the disease can slash yields by up to 1.5t/ha (12cwt/acre), though 0.5-0.75t/ha (4-6cwt/acre) is more typical, he says. Untreated infections continue to release spores, causing fresh infections throughout the season.

“It likes the cold,” adds Dr Gladders. “It keeps growing at 0C even when the plants have stopped. So it gains ground on crops all the time.”

Light leaf spot develops particularly well under snow cover, which retains its spores, says Elsoms Seeds’ Mark Nightingale.

Infection in the firm’s Lincolnshire trials is “considerably worse and earlier” than in other seasons, and highlighting differences in variety susceptibility. “I think we could see some of the resistance scores go down – and I think we could even perhaps be dealing with a new race.”

Cambridgeshire NIAB pathologist Jane Thomas confirms the unusually high incidence of light leaf spot, but plays down suggestions of a new race. “In our inoculated field trials, using samples from Scotland to the south, there’s no evidence of that from our early scores.”

Although infection is most noticeable in early (late August to mid September) sowings all crops remain at risk. Many have been more stressed than usual weakening their disease defences, says Mr Ashworth.

ProCam‘s Nick Myers agrees light leaf spot incidence in the south and east is much worse than usual. “It’s hardly been seen down here in recent years, but has been picked up in crops over the past two to three weeks.”

He believes much reduced autumn spraying is partly to blame. “Many of last season’s crops had two autumn phoma sprays. This year most had only one – or none.”

While most anti-phoma fungicides also control the disease, many went on some time ago and will have run out of steam, adds Mr Ashworth.

Given the potential risk and increased value of this season’s crops Dr Gladders suggests that the traditional treatment threshold of 25% plants infected be reduced to 15%.

The strongest weapons are prothioconazole-based, like Proline and Presaro, but others such as Punch C (flusilazole) and Folicur (tebuconazole) also offer good control, he says.

“The important point, if there’s a lot of disease, is to keep the dose above half rate. That’s especially so where there’s been a lot of rain and you want to maximise kick-back activity.”

andrew.blake@rbi.co.uk