Light leaf spot will be the number one threat to oilseed rape crops this autumn with growers urged to strike quickly against the yield-robbing disease.
The disease now effects all regions of the UK with 33% of plants affected this spring, and the national yield loss is estimated at 10%.
Peter Gladders at crop consultants ADAS says a 15% threshold of affected plants at early stem extension has been used as a guide as to when to spray, but he suggests any signs of infection could be a signal to act.
Dr Gladders say if this threshold is reached then the yield loss could be about 0.2t/ha, costing the grower £60-70/ha, so they should be looking seriously at controling the disease.
Certainly, high-risk sites where the disease has been seen before will need an autumn spray in order to keep the disease in check with prothioconazole proving the most effective fungicide.
Growers in southern England who have used a phoma treatment will have seen some control of light leaf spot, but further north phoma is not such a threat and the key disease is light leaf spot.
“In the north they might see a bit of phoma but they are not likely to cause yield loss so they are holding back until the middle of November onwards,” he says.
Control is likely to be improved by increasing the number of applications rather than by increasing the dose according to Dr Gladders.
Dr Gladders says after autumn treatments, all crops need careful monitoring in January and February and if any light leaf spot is found the disease needs to be dealt with promptly.
Unlike phoma, light leaf spot is still a threat to yield in the spring and losses could be 30% if all plants are affected at early stem extension stage,
Meanwhile, the phoma risk this autumn has increased significantly over the past week and Dr Gladders urges action as soon as the weather allows, with early November being critical.
“It’s developing all the time with 70-80% infected plants, so getting out and checking the crops will be key.”