Light leaf spot identification
Light leaf spot symptoms are starting to appear on oilseed rape crops. It is often seen first in susceptible varieties, particularly those not sprayed in the autumn or winter. But given protective sprays may have been applied in October, it is not surprising to see new symptoms develop three months following this treatment.
Text books pictures tend to give the typical symptoms, but what you are more likely to see is what looks like winter kill, dead lower leaves, random white marks on the leaves. It is usually complemented by obligatory pigeon damage.
Winter kill usually accompanies light leaf spot infections. Affected crops are, however, more prone to winter kill, so the two are commonly seen together. White marks on leaves may not always be caused by this disease. Phoma leaf spot is usually easily differentiated from light leaf spot, but downy mildew and white leaf spot are two other likely suspects you can see on leaves. Both will be well adapted to the mild wet winter. Scorch from fertiliser following application is not unusual either.
To find out what you’ve got, why not take the airing cupboard test (a warm room will do just as well)? Place a few suspect leaves in an inflated polythene bag with a piece of wet kitchen towel. After a few days, you may see the typical white “spray deposit” symptoms appear on the green parts of the leaf.
Mealy cracking symptoms and leaf distortion are typical symptoms of light leaf spot. The white spray deposit symptom is not always present. Where it does occur, the spray deposits may turn brown.