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Crop Advice & Expertise

Light leaf spot identification – the definitive guide

With annual losses exceeding £80 million, according to CropMonitor, light leaf spot is the most economically damaging disease in UK oilseed rape. Despite this, the disease is still going unidentified on some farms.

Keeping light leaf spot in check

Bayer’s SpotCheck is a free resource which provides farmers and agronomists with a definitive analysis on the presence of light leaf spot, Phoma, powdery mildew and downy mildew.

Take part in SpotCheck here.

To support farmers in identifying light leaf spot we have created a definitive guide below. Remember, for a conclusive diagnosis of light leaf spot and other diseases’ prevalence in oilseed rape crops, you can utilise Bayer’s free SpotCheck initiative.

Early disease symptoms

Light leaf spot can be found in the crop as early as October, and so it is vital to familiarise yourself with symptoms from the very start of the growing season.

As a polycyclic disease, light leaf spot continuously releases spores and can spread at an alarming rate, making early identification vital, but notoriously difficult.

Early symptoms appear as white sugar-like spores, forming in a circular pattern, on either the top or underside of the leaf. This early white ‘sporulation’ is almost microscopic, so a hand lens is useful to help inspect each leaf one-by-one.


Incubating leaves can also act as an early warning system, drawing out symptoms sometimes weeks before they appear in the field.

Generally, the optimum temperature range for light leaf spot is around 16°C, but the disease can still develop between 4 and 20°C. A period of rainfall will also cause infection to spread more readily.


Late disease symptoms

Later in the season (spring/summer), the disease spreads to stems and lateral branches. Look for small brown/purple lesions, often accompanied by black speckling. As the stem extends, the lesions expand and can become several centimetres wide.


In extreme scenarios, stems have been reported to split vertically along the lesion and even break.

In wet weather, spore droplets can be found on flower buds. Again, rain fall during flowering will cause the infection to spread more readily.

Severe cases of light leaf spot can cause infection to spread to the pods, causing twisting, distortion, browning and ultimately seed shedding.