Didymella fabae (leaf and pod spot)

Symptoms occur on leaves, stems and pods. Where seedlings have grown from infected seeds, lesions are more obvious on the upper parts of the stem and on the older leaves. Lesions on the leaves are usually circular, dark brown and initially about 1 mm diameter. After a short time, the lesions become larger and slightly sunken with a pale-brown to dark-grey centre surrounded by a broad, dark, chocolate-coloured margin. As the spots enlarge, they become more irregular in shape and coalesce to cover larger areas of the leaf. Some zonation may occur within the necrotic area of the lesions, which may cause confusion with lesions of chocolate spot caused by Botrytis fabae. A more general browning of the vascular tissue of the leaf may occur as the lesions develop. Prominent, dark pycnidia develop within the lesions, particularly as the leaves age or when conditions are moist. The pycnidia can vary in abundance and are sometimes concentrically arranged.

On the stems, the lesions are usually smaller at the early stages of infection, but elongate up the stem and become markedly sunken. Stem lesions are usually darker than those on leaves, and contain scattered pycnidia. When the lesions are deeply sunken, either the stems of the plants may break at the point of infection, causing the plants to lodge or, if infection occurs at an early stage, the stems may bend upwards producing a kink where the stems regrow vertically. At the seedling stage, when infection originates from the seed, the combination of stem and leaf infection may result in the death of the plant.

As the pods develop, lesions can be produced over the surface. They become very deep with dark brown centres containing abundant pycnidia. In damp conditions, the conidial masses produced are pale pink to yellow. Well-developed lesions may penetrate the pod wall and affect seed set or may blemish the developing seeds within the pod. However, seed staining does not always indicate infection by the pathogen, because other saprobic organisms may invade the damaged tissue of the pod. Colonies of D. fabae can also frequently be isolated from unstained seed during routine seed health tests in the laboratory (A Biddle, [address available from CABI], personal observation, 2000).

Ascochyta blight is a common and occasionally destructive disease that has been reported from all six continents (Gaunt, 1983). The extent of the damage caused by the disease depends on weather conditions (McKenzie and Morrall, 1975; Maurin and Tivoli, 1992) and cultivar resistance. Madeira et al. (1988) reported that the disease reduced the leaf area index and dry matter production and that a significant seed weight reduction of 15% was incurred. Yield losses of 32-41% were reported in several years in the Czech Republic (Ondrej, 1991); similar levels were observed in New Zealand (Hampton, 1980). In the drier areas of eastern England, crop losses are relatively low although pod infection can become severe later in the season, especially in autumn-sown crops, resulting in high levels of seedborne infection. Worldwide yield losses can be as high as 90 to 100% in susceptible varieties (Yu, 1947; Hawtin and Stewart, 1979; Hanounik and Robertson, 1988).