Mycosphaerella blight (Mycosphaerella pinodes)

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 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.