Winter Oilseed Rape Acreage Expansion Recorded Across Ireland
The area of winter oilseed rape planted in Ireland has increased significantly this season, according to Teagasc. Early sowing, available soil mineral nitrogen, and strong responses to organic manures have produced large canopies across many fields.
While crops are well established, Teagasc advises growers to monitor fields closely for light leaf spot and phoma stem canker this autumn. Mild and wet conditions favor disease development through rain splash, whereas cold weather slows down pathogen growth.
Light Leaf Spot Pressures and Varietal Resistance Ratings
Light leaf spot remains the most damaging disease affecting winter oilseed rape in Ireland, with potential yield losses reaching up to 1t/ha in susceptible varieties. Ambassador holds a resistance rating of 6, Aurelia is rated 7, and LG Aviron, LG Auckland, and the provisionally recommended LG Armada each carry a rating of 8.
Symptoms remain difficult to spot at this time of year. Growers should inspect crops for small white spots resembling grains of salt around affected leaf areas. Advanced infections cause leaves to curl, become brittle, and crack. Growers can place leaves inside a plastic bag in a warm room for one to two days to encourage sporulation if identification proves difficult.
Fungicide Control Strategies and Canopy Management
Crops with early sowing dates and large canopies require targeted protection. A two-spray program targeting late October or November alongside early spring provides the foundation for disease control.
The autumn application suppresses infection and limits disease cycles. A prothioconazole-based fungicide applied at 75-100% of the recommended label rate delivers effective control against both light leaf spot and phoma. Tebuconazole offers limited efficacy for light leaf spot, and its plant growth regulatory impact declines as temperatures drop.
For excessively large canopies driven by early sowing or organic manures, an application of Caryx at 0.7L/ha assists with canopy management, though maximum results occur when applied during spring growth takeoff. Growers must continue monitoring crops through the winter to evaluate the need for a follow-up treatment in February as soon as light leaf spot appears.
Phoma Stem Canker Treatment Thresholds
Phoma produces greyish leaf spots containing small black dots known as pycnidia. The primary damage occurs when the infection travels through the leaf stalk into the main stem, forming cankers that restrict water and nutrient transport and cause premature ripening.
Larger plants tolerate infection longer before it reaches the stem, while smaller crops require intensive monitoring during mild weather. Growers should apply a treatment threshold of 10-20% of plants affected, aiming for the lower end of that range for small crops and a higher threshold for large canopies. Treatments should be brought forward if phoma reaches this threshold prior to the planned light leaf spot spray.