Light leaf spot has been found in many crops of winter oilseed rape. The disease can significantly impact on yield and should be controlled as soon as possible.
Some crops may have received an autumn fungicide as a preventative measure. However, light leaf spot has been found in sprayed and unsprayed crops.
Light leaf spot can be difficult to detect and sometimes can be mistaken for frost damage. A preventative disease programme is often best to enforce in the case of light leaf spot. However, if growers want to see if they have a problem a method is described below.
How to determine if light leaf spot is a problem
According to Teagasc, growers should collect 30-40 leaf samples from different areas of the crop. These samples should be placed in a plastic bag at 10-15°C for four to five days. This encourages sporulation of light leaf spot.
How to control it?
Proline (prothioconazole) should be applied as soon as possible at a rate of 0.4-05L/ha. Prosaro is another option. This should be applied at 0.8-1.0L/ha.
Thin crops should get nitrogen as soon as possible. Weather conditions have delayed early nitrogen in many regions.
Growers should target a crop canopy with a green area index (GAI) of 3.5 at the beginning of flowering to achieve the maximum yield.
Nitrogen will increase GAI and can be applied at a total rate of 225kg/ha on index one soils and 140kg/ha on index 4 soils. Crops with a low GAI will require more nitrogen than those with a high GAI.
How to measure green area index
GAI is important in oilseed rape as it determines the yield of the crop. The GAI is a measure of the green plant material in a crop. Farmers can can take a sample of the crop to determine GAI.
- Cut plants along a 1m stick;
- Weigh the plants (kg);
- Divide the weight by 0.61 (in a 24in spaced crop) to take the row
spacing into account;
- Multiply the final figure by 0.8 to get the GAI.