Oilseed rape crops should be checked for light leaf spot infections from the end of January after the disease hit record levels in spring 2011, Peter Gladders, a plant pathologist at ADAS, warned agronomists at the AICC conference.

That included crops away from the traditional light leaf spot risk areas of the north. “It is every region. Even in the sunny south east and East Anglia 70-80% crops were infected last year, according to the Crop Monitor random survey. In the north it was 100%. So there is a huge carryover of light leaf spot on stubbles this year.”

Around 65% of crops are at risk in Scotland, 56% in northern England, and around 30% in most of England, except East Anglia where the risk is lower, according to the latest Rothamsted Research risk assessment.

But that probably underestimated the threat, Dr Gladders suggested. “Those figures are for the percentage of crops that will show 25% of plants infected, and I think the threshold should be 15% infection. 15% equates to a 5% yield loss. So the crops at risk are substantially higher than those maps would indicate.”

Varietal choice was a good defence, particularly for farms struggling with the disease, he pointed out. In KWS trials, Castille treated with fungicides still had much more infection than untreated Cuillin, which is rated as a nine for resistance to light leaf spot.

“The yield benefit was 0.65t/ha from growing the right variety.”

It was worth remembering that the HGCA Recommended List didn’t highlight the advantages of disease resistance, he said. “For example, there is about a 10% yield difference between Cuillin and Castille in fungicide-treated yield, but where you put it in a disease environment the difference is about 15%,

“The value of resistance if you’re putting it into a high disease scenario is almost certainly underestimated.”

HGCA trials last season suggested that control from fungicides was at best around 60%, with only small differences between products. “If you’re in a fire-brigade situation with a lot of disease to take out you would probably use a prothioconazole-based treatment. But Sanction and Folicur are performing nearly as well in many situations.”

Yield responses in those trials were around 0.8t/ha, with two half doses of Sanction (flusilazole), the most economical treatment, he added.

A wet spring would exacerbate the risk, he said. “Light leaf spot is poised to take a tonne off yields if you don’t get there quickly.

“So I want all of you out there at the end of the January looking for light leaf spot, and reacting as soon as you see symptoms. Remember that 15% threshold is for plants at stem extension. If you see anything before then, get out and do something about it.”

Read more from the AICC conference