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Light Leaf Spot

Pyrenopeziza brassicae

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General Description

Light leaf spot is the main foliar disease to attack winter oilseed rape in the north of Britain.

Symptoms are not generally seen until December onwards. The plants show either light green or bleached irregular blotches on the leaves. Later the areas take on a mealy appearance covered with fine cracks. In humid conditions, white speckles can be seen around the lesions. These are the spores of the fungus. If it is not clear if symptoms are due to light leaf spot or fertiliser scorch, if leaves are left overnight in a polythene bag, you can see the white deposits of light leaf spot around the lesions clearly. light leaf spot is most serious before stem extension.

It can cause severe damage if it attacks the flower buds before stem extension. At this stage, it can cause the flower buds to rot.

Later it can be seen on the stems and pods, but it is not such a problem. It only causes superficial lesions on the stems, and does not cause lodging (unlike Phoma).

There are varietal differences to Light leaf spot. There are also differences between varieties in the yield benefit from fungicide treatment, but it does not always follow that the most susceptible varieties respond best to treatment.

Biology

One key source of the disease is oilseed rape stubble and trash. Cool wet weather in the autumn  favour the spread of the fungus onto new crops.  High humidity is also favourable for infection.  Symptoms generally first appear  in January, and slow growth of the crop during leaf production can lead to extensive symptoms developing on the leaves and developing flower buds.  Once the crop starts to grow rapidly  at stem extension, the disease becomes less important.  It can however continue to develop on the upper leaves, the stems and eventually the pods later in the season.

Advice

Light leaf spot is controlled with fungicides. A two spray programme of a fungicide in the Autumn (late October - November) and Spring (March) is effective. Symptoms will not be seen at the time of the first spray, but the fungus will be present on the plants.

Diary

Resistance Issues

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Forecasting

Use this section to help manage the disease throughout the season.