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Mycosphaerella blight (Mycosphaerella pinodes)

Mycosphaerella in peas

Mycosphaerella blight: biology

Mycosphaerella blight is the most common disease of field peas in western Canada and is also prevalent on processing peas. In the field, stem symptoms are difficult to distinguish from those of ascochyta foot rot, so the two diseases are treated together. Severe epidemics develop under cool, wet conditions.

Fungi can survive for several years in seed or as resting spores in soil. Infection occurs when the emerging seedling comes in contact with resting spores, or the pathogen grows from the seed into the stem. Infested crop residue is the most important source of inoculum. In spring, spores are produced on residue on the soil surface. They may be carried over long distances by wind, and can infect all above-ground parts of plants. Spores are also produced during the growing season on diseased leaves and on the lower stem.

Mycosphaerella blight: Damage description

Mycosphaerella blight can produce yield losses of more than 30% in field peas and 50% in processing peas. The pathogens attack leaves, stems, flowers and pods:

  • On leaves, early symptoms of infection are small, purplish spots with irregular, indefinite margins. Lesions on the stem may extend 10 mm (3/8 in.) above and below the point of leaf attachment.
  • On older leaves or under moist conditions, the lesions enlarge, causing the tissues to dry up. Lesions on petals are small, but infected blossoms may drop.
  • On pods, lesions are initially small, and are typically lighter in colour. Lesions may expand on ripe pods and produce extensive purplish brown discolouration.
Infected seeds from diseased pods may show no visible symptoms, or may be shrunken and discoloured. Seedlings from infected seed often develop disease symptoms, but the lesions are confined to the stem base where the cotyledons are attached to the stem.

Mycosphaerella blight: Management

Some field pea cultivars appear to be tolerant to Mycosphaerella blight, but high levels of resistance have not been found. Loss can be reduced by crop rotation and by using disease-free seed. Other management practices include:

  • Peas should be grown no more than once in 4 years in a field and planted as far as possible from the previous year's pea fields.
  • If possible, infested crop residue should be cultivated under the soil surface after harvest.
  • Fungicide seed treatments may prevent the introduction of the disease to new areas, and may reduce other diseases such as stem rot. Application of foliar fungicides often reduces disease severity.

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