Ascochyta blight of Faba Beans

What to look for. Symptoms occur on leaves, stems and pods of infected plants, and can be confused with the early stages of chocolate spot. On leaves, small, circular, dark-brown spots appear first. As the disease develops, lesions enlarge and turn light and then change to dark grey in colour. They become irregular in shape, often zonate, and may coalesce to cover most of the leaf surface. Leaf tissue next to the lesions may become black and necrotic. Within the lesions, numerous pinhead- sized black fruiting bodies (pycnidia) of the fungus develop. These appear only under moist conditions and are often concentrically arranged.
On the stem, lesions are more elongated, sunken and darker than leaf lesions and are usually covered with scattered pycnidia. Stems may split and break at the point of infection causing plants to lodge. On pods, lesions are sunken and have pale centre and dark margins; they can be covered by numerous pycnidia. Well developed lesions can penetrate the pod and infect developing seeds causing them to be shrunken and discoloured. Badly infected seeds have yellowish brown stains on the outer seed coat, which considerably reduces its market value.
Ascochyta blight can cause seed staining in pods close to maturity even when disease levels in the crop have been too low to warrant fungicide sprays. Faba bean seed that has greater than 25 per cent seed coat discolouration can reduce the emergence of seed by 30 per cent. Seed that has less than 5 per cent seed coat discolouration will usually have normal levels of germination.
Disease Cycle. Ascochyta blight spot is caused by the fungus Ascochyta fabae. The fungus can survive on crop debris, self-sown volunteer plants, and infected seed. The disease usually becomes established when spores of the fungus, produced on old bean stubble, are carried into the new crop. Wet conditions are required for infection. During wet weather the disease can spread from infected to healthy plants by rain splash and wind-borne spores. Infection may occur at any stage of plant growth, providing there has been either rain or heavy dew. The disease is usually most severe early in wet years.