Alternaria blight manifests as dark, brown, or black spots that often have a characteristic concentric pattern resembling a target. On leaves, the spots eventually coalesce, causing necrosis, while on stems and fruits, depressed areas form, which, in conditions of high humidity, are accompanied by a velvety olive-colored sporulation coating.
Disease development is most intense at temperatures from +20 to +30°C and high air humidity, including the presence of free moisture. The risk of infection increases significantly with dense planting, mechanical tissue damage, and mineral nutrient deficiencies, as the pathogens often act as a secondary infection attacking weakened plants.
Varietal resistance to Alternaria blight is achieved through a combination of passive barriers, such as tissue structure features, and active responses, such as hypersensitivity, which blocks pathogen development. Resistance evaluation of varieties in breeding is conducted through field trials on natural infestation sites and provocation methods using artificial inoculation, which allows for the differentiation of varieties by the degree of infection.