Alternaria leaf spot

The causative agent of the disease is the imperfect fungus Alternaria atra, belonging to the Dematiaceae family. This pathogen belongs to a group of fungi that cause alternariosis, which are specific types of spotting that affect a wide range of vegetative organs in agricultural and ornamental crops. The type of disease is necrotic, manifesting as the death of plant tissues due to the toxic effects of the fungal mycelium.

The range of susceptible crops is extremely broad and includes cereal grains, vegetables (especially tomatoes and potatoes), oilseeds (sunflower, rapeseed), and fruit trees. The fungus is frequently found on seeds, fruits, and crop debris, where it can maintain viability for a long period as mycelium or spores, awaiting favorable conditions for activation and infection.

The primary symptoms of infection appear as the formation of dark, brown, or almost black spots with a characteristic moldy appearance. These spots often display concentric zonation, which is a key diagnostic feature for many Alternaria species. Affected areas enlarge over time, merge together, leading to premature leaf drying and shedding, as well as the rotting of fruits during storage.

The development and spread of the fungus are directly dependent on hydrothermal indicators. The most favorable conditions are moderately high temperatures (from +20°C to +28°C) and high air humidity or the presence of moisture on the leaves. Under these conditions, spores are disseminated by wind, rain splashes, or insects, causing massive outbreaks of infection in a very short period.

The harmfulness of Alternaria atra lies in the significant reduction of the leaf assimilating surface, which leads to a decrease in crop yield and a reduction in product quality. Protection measures include agronomic practices such as crop rotation, destruction of crop residues, and the use of resistant varieties. Chemical control is based on the application of contact and systemic fungicides containing copper, triazoles, or strobilurins at the first sign of infection.