Alternaria Black Spot and Fruit Rot
This fungus causes two main groups of symptoms in pomegranate that do not necessarily appear concurrently. Commonly called black spot and fruit heart rot, they are often dependent on the variety of pomegranate. Black spot disease is characterized by the appearance of small, reddish brown of black circular spots (1-3 mm) surrounded by a green yellow halo on fruits and leaves. As the disease progresses, these spots coalesce to form larger patches that can cover up to 50% of the fruit surface. On leaves, they become chlorotic and may lead to premature shedding. The outer part of the fruit starts rotting, while the edible tissue remains undamaged. A slightly abnormal skin color or changes in shape of the fruit can be external signs of heart rot but often the fruit retain their healthy appearance until harvest. As they are cut open, the decay of the arils is obvious.
Black spot and heart rot symptoms can be triggered by many fungi of the Alternaria family but the main cause is Alternaria alternata. These fungi commonly survive on plant debris, mummified fruits or on the soil. The spores are then carried to the flowers by the wind. Insects and birds are alternative vectors. The infection is favored by frequent rain or humid weather during the late flowering stages or early fruit development periods. Often, heart rot is is only recognized after the harvest, during storage or transportation. The fungus grows inside the pomegranate fruit which rots and is no longer marketable.
No biological treatment seems to be available to antagonize Alternaria alternata. However, products based on copper oxychloride have been proved highly effective in controlling the disease on pomegranate.
Always consider an integrated approach with preventive measures together with biological treatments if available. Two preventive sprayings during the blooming period or when the first symptoms appear on the fruits give good control of the disease. Product based on propiconazole, thiophanate methyl or azoxystrobine have been proved highly effective. It is important to follow the specified concentrations and to use fungicides with different mode of actions to prevent resistances.