Difficulties controlling light leaf spot infections in oilseed rape crops, even where well-timed fungicide programmes have been used, were being reported at Cereals.

As a result, doubts about the continued effectiveness of triazoles on the disease were being aired, with comparisons being made between light leaf spot control problems and the deteriorating septoria situation in wheat.

However, researchers and agronomists considering the possibility of fungicide resistance also highlighted other factors which they believed had played a role in cases of disappointing control, including high disease pressure, poor spray timings, key fungicide shortages and a lack of good varietal resistance.

Light leaf spot isolates with reduced sensitivity to triazoles were first identified two years ago, confirmed Nichola Hawkins of Rothamsted Research, who acknowledged that their numbers would have increased in that time.

But she stressed that it was important to recognise that poor control under high disease pressure – which had been the case this year – was not always due to resistance.

“We know there are isolates with target-site mutations, and some with overexpression, and some with both,” she said. “They affect the performance of fungicides in different ways, but they have been at low levels up until now and had little effect on the disease control achieved.”

However, the current level of concern means it will be important to test more field isolates this year to confirm whether resistance mechanisms are present in sites reporting poor control, she said.

“We also need to understand more about control levels in the field where these mutations are present, relative to the products and rates used.”

That’s a view shared by Dr Fiona Burnett of Scotland’s Rural College (SRUC). “There’s not enough testing being done for us to be certain about what is happening with the pathogen. But we do know some sites are hotspots and there are many more reports of problems, so we can expect the same outcome as we’ve seen with septoria over time.”

However, growers shouldn’t be too hasty in putting all the blame on triazole failures, she cautioned. “Remember that light leaf spot has become more prevalent in the South and there’s always been a tendency to aim fungicides at the phoma timing in this region, rather than thinking about light leaf spot as the target.”

Variety resistance ratings aren’t very good either, she said. “If there’s one thing growers can do to help the situation, it’s to look for more resistant varieties and make good use of them.”

Niab Tag agronomist Patrick Stephenson aid the industry was right to question whether the extent of resistance to triazoles had been recognised.

“Fungicides based on the active ingredient prothioconazole are front line products in oilseed rape,” he said. “If there are changes afoot, we need to know.”

His view is that the 2013-14 season provided perfect conditions for the disease, making it more difficult to get good control and putting greater emphasis on spray timing.

“We always see more problems in a high disease pressure year. Growers need to ask whether it’s realistic to put a low dose of fungicide on in November and expect it to last until March”

He added that the shortage of prothioconazole could have had an effect too. “There were situations where inferior products had to be used. You won’t get any effect from carbendazim on light leaf spot, for example.”

Andrew Flind of Bayer CropScience agreed that product choice would have been an issue this year. “Put very simply, there are some fungicides that don’t work well on light leaf spot, even if they do control phoma. Flusilazole is a good example.”

The disease is tougher to control than phoma and spray timing is critical, both in the autumn and the spring, he said. “Control has to be preventative. There’s no way of beating it with a curative strategy.”

Growers also need to learn to recognise the disease, he continued. “It gets confused with other things, including scorch. And because it used to be confined to the North, it hasn’t been seen enough in the South for growers to be confident in its identification.”

He accepted that resistant isolates had been identified, but pointed out that prothioconazole-based fungicides had continued to give good control in Bayer’s trials.

“For best results, you have to use a minimum of a three-quarters dose at the right time. That usually means spraying in November and then again in late winter.”