Oilseed rape growers are being urged to monitor crops for signs of light leaf spot, as leaf testing has revealed the main disease epidemic has started.

Almost half (45%) of oilseed rape leaf samples assessed by Bayer’s SpotCheck initiative between the 4 and 11 January showed symptoms of the crop’s most yield-damaging disease. 

The initiative found varying levels of disease across the country, with the highest level seen in Northamptonshire, where 47% of plants showed signs of light leaf spot infection after being incubated for three days.

See also: Drought and pests give rapeseed growers a tough start

Experts say the mild winter weather has been favourable for latent light leaf spot infection to develop into visible leaf symptoms.

Philip Walker, arable plant pathologist at Asas, says: “The time period from initial infection to visible lesions is quicker when temperatures are higher.

“The average UK temperature in December was 6C, so it would take about 30 days for an infection occurring in mid-December to be visible now.” Therefore, he advises farmers to monitor their crops.

Ben Giles, commercial technical manager at Bayer, highlights that as light leaf spot symptoms can be difficult to identify, “this is a key time to vigilantly walk and examine crops as regularly as possible”.

Farmers can also use the free SpotCheck service to know for sure which diseases are present in their oilseed rape.

If farmers do find disease in crops that have yet to receive a fungicide with activity against the disease, he suggests the good ground conditions could allow a spray application of 0.4–0.5 litres/ha of prothioconazole.

However, he adds that those crops that received a robust fungicide with light leaf spot activity in the autumn should still be protected at this stage of the season, especially if it went on from mid-November onwards.

To benefit from Bayer’s free SpotCheck initiative, visit the dedicated webpage.