ALMOST epidemic levels of light leaf spot are being found in oilseed rape crops, according to Bayer’s SpotCheck service.

The latest results from 160 samples evaluated in February have shown extremely high levels of light leaf spot infection after three days incubation, compatible with an epidemic, said Philip Walker, an arable plant pathologist at ADAS, who helps evaluate the project.

While high infection levels are usually observed in February, the latest results are higher than normal, almost certainly as a consequence of the continued recent mild weather.

“With 88% of samples showing symptoms – and, of those, 70% showing infection levels above 25%, – I would expect this to carry on increasing and have a detrimental effect on yield, as we generally estimate that a 5% yield loss occurs when more than 15% of a crop is infected,” pointed out Mr Walker.

“Since last November, the average temperature has not dropped below 5˚C, which is 1-2˚C above the long-term national average. But even if temperatures drop, the damage is done because – we have already got infection in the crop. While Yorkshire had the highest levels of infection, we are now starting to see LLS symptoms in the west side of the country as well.”

Applying a fungicide, if it is possible to travel, has now become urgent, he added. “Fungicides work better protectantly, but the quicker you are able to spray once you see symptoms, the higher level of control you will get.”

Ella Crawford, Bayer's commercial technical manager for Suffolk, agreed that the recurrent wet weather with mild temperatures have affected the crop growth and increased disease pressure, including LLS.

Comparing the results from the SpotCheck with what she saw in her area, she said symptoms of LLS are starting to become more visible, particularly in more susceptible varieties and/or crops drilled in early August.

“Typical symptoms showing tiny white spore droplets on the leaf surface have been seen locally but these lesions can still be difficult to identify, particularly in wet weather, without incubating the leaves to encourage further development of the infection," she said.

“If there is any doubt about disease symptoms in field, growers and agronomists should consider using the Bayer SpotCheck service to attain an accurate diagnosis. They should continue to monitor crops carefully, keeping in mind the resistance scores of your varieties and prioritising those with a weaker resistance rating for LLS.

“There is no threshold for LLS and fungicides should be applied at the first signs of the disease. Proline (prothioconazole) is the strongest product for control of this disease and will also be favoured in the more backward crops as it avoids unwanted PGR effect," she pointed out.

"Application timing is essential for effective control of LLS as fungicides need to be applied preventatively. However, weather conditions will dictate how reactive growers can be when applying fungicides at this time. Fingers crossed it dries up soon,” she said.


Crop losses valued at £80m per year

Light leaf spot is the No 1 threat to oilseed rape crops around the country, but it is still going undiagnosed on some farms – and the average annual losses exceeds £80m.

Traditionally associated with Scotland and northern England, the incidence has increased over the last decade and the disease is now widespread.

Of the 632 samples assessed between October, 2018, and April, 2019, 49% were infected with light leaf spot. However, light leaf spot remains hard to identify and ADAS identified the disease in 36% of samples, despite it not being seen by the grower when sampling in 2018/19.

Claire Matthewman, the oilseed rape fungicide campaign manager for Bayer, believed the difficulty identifying light leaf spot and its changing geography has created an extremely challenging scenario for growers: “With light leaf spot, even if you can’t see symptoms, it doesn’t mean it isn’t there. When symptoms do manifest, it is difficult to recognise them, particularly in the early leaf stages.

“That means fungicide applications may be poorly timed, leading to inadequate control of the disease, which can spread at an alarming rate in these circumstances.”

She advised looking for very small, white, sugar-type spores, which can appear on the top or underside of the leaf. Using a hand lens to magnify the leaf surface may also help.