The arrival of two new hybrid winter oilseed rape varieties with good light leaf spot resistance is being hailed as a breakthrough in Scotland.

To have two varieties with exceptional light leaf spot resistance is very exciting for Scottish growers, says Elaine Booth of SAC.

“Light leaf spot is a big issue up here,” she says. “Growers have really struggled to control the disease with fungicides for last couple of years, so this is the development that they’ve been waiting for.”

Cuillin from KWS and Emerson from Nickerson both have 9s for light leaf spot resistance and their inclusion on the HGCA Recommended List has come at just the right time, she adds.

“We had Elan, with an 8 rating for light leaf spot, but that’s gone now. So these two new varieties are very welcome indeed.”

However, neither has been widely grown yet and Dr Booth believes they need another couple of years before it is known whether their resistance will hold up.

“There’s no doubt that it gives the crop a head start. But in high disease pressure conditions, we would still recommend using an autumn spray. It’s critical to give crops protection over the six-week period from infection until symptoms are seen.”

However, she suggests that it should be possible to cut back on fungicide rates and to reduce the need for a further application in the spring.

“One of the issues with this disease is that you have to spray before you can see any signs of it. So we’re not able to say that growers won’t need to use an autumn treatment.”

Dr Booth adds that Cuillin and Emerson will have an important role in workload management. “Spray days can be very limited in Scotland. Having one of these more resistant types in the rotation allows growers to get their other, more susceptible varieties covered.”

They will also help to take the pressure off pesticides, she notes. “We will be increasingly looking to resistance for this help. It would be great if we could have the same development with club root resistance.”

Andrew Gilchrist of Scottish Agronomy points out that both Emerson and Cuillin are very unresponsive to fungicides.

“Both in two years of Scottish Agronomy trials and on farm, they haven’t shown any response to fungicides. So we’re not using fungicides as a matter of course in the autumn and both varieties are still exceptionally clean at this stage of the season.”

Omitting the autumn treatment saves the cost of one pass and around £15-20/ha in fungicide costs, he adds.

“The normal course of action is to use two fungicides, the first in the autumn and the second at early stem extension in the spring. With this level of light leaf spot resistance, it’s possible to abandon the autumn spray.”

Mr Gilchrist believes the second spray may still be required. “But this is to help regulate the crop rather than control disease.”

He stresses that he doesn’t expect the resistance to hold forever. “We’re confident about the advice for this season, but we will be reviewing it on an on-going basis.”

Les Daubney of Nickerson confirms that there’s never been any light leaf spot seen in Emerson. “It’s the same resistance as in Elan, so we know that it’s very robust.”

While he accepts that it’s a big leap of faith not to use an autumn fungicide, he does point out that Emerson gives growers enormous flexibility. “And that’s something which has been a big bonus this year.”

Allen Scobie of Bridgend Partners is also very enthusiastic about the introduction of Cuillin and Emerson. “They’re needed in the north. Let’s hope this is just the beginning of better resistance – sowing a variety with a 4 or 5 rating is a disaster.”

He acknowledges that growers need to have confidence in the new varieties before they agree to omit the autumn spray.

“Don’t forget we’ve had high rating varieties before and it was necessary to spray them. The work needs to be done before we can say that it’s fine to step back completely.”

Spring spray advice

Don’t wait until stem extension to spray unprotected oilseed rape crops, warns Dr Booth.

Growers who didn’t manage to get an autumn spray on should go as soon as they can, she advises.

“Light leaf spot thrives in cool, damp conditions and plants are struggling after such a hard winter,” she says. “So it’s all going in the disease’s favour.”

Although the autumn spray is the critical one, there will still be some benefit from a spring treatment, she notes.