Early detection and timely treatment will be essential for getting on top of light leaf spot this season, oilseed rape growers are being advised.

Waiting to treat heavily diseased crops at the stem extension stage won't be effective, warns Paul Fogg of plant protection product manufacturer Adama, who points out that once the disease is established in the leaf it can't be killed. A key reason why the disease pressure was so bad last year was that growers took preventative action too late and were then chasing the disease.

"Growers need to be one step ahead of light leaf spot," he explains. "It moves into crops in the autumn and it keeps cycling all the way through the winter and spring."

That's why spotting the first signs of the disease is so important, he continues. "Control has to be preventative – there's no way of beating it with a curative strategy. So spraying decisions shouldn't be made based on the phoma threat, or to tie in with other crop passes, which is what has tended to happen in the past."

Likewise, applying the first fungicide in November and expecting it to last until March is unrealistic, stresses Dr Fogg, regardless of how good the fungicide product is.

"A well-timed fungicide application will give 3-4 weeks protection, so with a two spray programme you can expect to achieve 6-8 weeks security.. The disease pressure that some crops are now coming under means that this is the minimum requirement."

For this reason, growers and agronomists should be out looking for the first signs of disease so they can treat if necessary. "If it's there in January or February, you need to spray. There's no flexibility with this, so don't be tempted to wait until stem extension."

He accepts that it can be difficult for growers to get on to field with a sprayer in January and February, which is when many oilseed rape crops must receive their second light leaf spot treatment for control to be successful.
"There's no doubt that missed spray timings are part of the problem. We've seen higher levels of the disease, in all parts of the country, with growers struggling to get more than 50% control, despite using a two spray programme."
Crop Monitor disease information service results show that light leaf spot was the most prevalent disease on oilseed rape crops in 2014, affecting 86% of crops and 38% of plants.

"That was markedly higher than both the previous year and the long-term mean. It has overtaken phoma as the main disease of oilseed rape," points out Dr Fogg.

The reasons for this are believed to be weather conditions, lack of varietal resistance and poor use of fungicides, he notes. "Everything is moving in the disease's favour."

Triazoles are still effective against light leaf spot but mixes of actives with light leaf spot activity, such as in Orius P, are a better solution, he believes.

"They have given some of the best results in the HGCA-funded fungicide performance work and they reduce the industry's reliance on using the same mode of action for phoma, light leaf spot and sclerotinia control."
Adama's Orius P, which contains tebuconazole and prochloraz, will be a particularly good choice this spring, because crops are forward, he explains.

"The tebuconazole also brings some growth regulation, which is likely to be needed this year."  
Trials suggest a slight benefit of using higher rates of Orius P, between 0.75-1.0l/ha, although the final rate used will depend on the amount of PGR activity required, he ends.