White Leaf Spot [Mycosphaerella capsellae]

Pest Profile

Symptoms&Diagnosis

Symptoms

The most diagnostic symptoms are large circular white leaf spots (10-20 mm diameter), often with a brown margin. The centre of the lesion is white with some dark reticulation and a dark central spot. Young lesions are irregular greyish or dark spots (1-2 mm), again with darker reticulation. Symptoms are likely to be confused with downy mildew and the two may occur together. Leaf symptoms are found from autumn onwards. Susceptible varieties may show extensive stem infection, most of the upper stem becomes grey, a symptom known as “grey stem”. The pods are also affected, showing small brown lesions with dark reticulation and developing into large blotches with reticulation (reminiscent of net blotch lesions on barley).

Life Cycle

Spread to new crops of oilseed rape occurs in autumn by means of airborne ascospores produced on residues of the previous crops or weed hosts. Secondary spread by splash-dispersed conidia leads to further spread within the crop. Only 6-8 hours of leaf wetness are required for infection at 15-20ºC. Symptoms appear after accumulated mean temperature reaches 120 degree days (ie 8 days at 15ºC). At stem extension, heavy rainfall is required to splash spores from the infected basal leaves to the upper plant. Crops can therefore grow away from the disease if the weather is not conducive to splash dispersal at stem extension. Seed transmission may occur.

Treatment

Control

Azole fungicides give good control and specific treatment may be required when leaf symptoms are common.

Remarks

Importance

It is mainly a disease of the south and west in England, but there have been occasional reports in the east. Severe infection is uncommon, but leaf, stem and pod infection is capable of causing yield loss.