Phoma symptoms in oilseed rape

With fungicide treatment thresholds based on crop symptom levels, it is essential to recognise the classic signs of this disease. This page provides details of phoma leaf spot and stem canker symptoms to guide in-field monitoring.

The disease is caused by two closely related pathogens: Plenodomus lingam and P. biglobosus. 

Until 2024, the preferred scientific names were Leptosphaeria maculans, Lm and L. biglobosa, Lb, respectively.

Phoma goes through one infection cycle per season, as it is a monocyclic disease.

During periods of warm, wet and humid weather in the autumn, infected stubble releases airborne ascospores.

The IPM Decisions Platform phoma leaf spotting model is based on the number of rain days, 0.2 mm or greater, during August and September.

It is estimated that about 20 (16 to 23) rain days are required to trigger spore release from stubbles.

When these spores land on the leaves of young plants, infection can occur.

It takes several weeks for phoma lesions to appear on oilseed rape crops after infection, with symptoms developing faster in warmer and wetter conditions.

The AHDB phoma leaf spot forecast uses temperature and rainfall data to simulate pathogen development. 

Specifically, it predicts the date of the starting week when 10% of oilseed rape plants could potentially show symptoms of phoma leaf spot.

Each of the pathogens that cause phoma has its own distinct leaf-spotting symptoms.

• P. lingam: Typically, tawny-coloured spots with dark specks, which are asexual fruiting bodies called pycnidia
• P. biglobosus: Usually, darker spots with few or no pycnidia

Spots develop on the upper side of the leaf, with the underside clear of fungal growth, which contrasts to the white fungal growth associated with downy mildew.

Some spots may have a yellow halo and cause browning of leaf veins.

Leaf spots have minimal impact on crop growth and yield, except when severe infection of cotyledons kills seedlings.

Figure 1. Mature phoma leaf spots on oilseed rape

From the leaf, the pathogen grows along the leaf petiole to the stem, invading and killing plant tissue cells.

Classic canker symptoms often form around the leaf scars at the stem-base.

These develop further and girdle the stem, restricting water and nutrient transport, which can result in premature senescence.

In extreme cases, the stem can sever, the crop can lodge and plants can die.

The earliest infections are associated with the largest cankers. 

• P. lingam: Typically causes relatively severe stem-base cankers
• P. biglobosus: Usually causes lesions to develop on the upper stem

Symptoms of infection can also show in flowers, buds and pods.

In the latter, brown pod lesions may occur, along with pycnidia and a black margin.

After harvest, the pathogen continues to develop on the stubble.

However, symptoms gradually disappear, as the fungus enters its sexual stage on stems and roots.

Figure 2. Classic phoma stem canker symptoms

Figure 3. Phoma life cycle