Purple blotch of onion

Alternaria porri is a necrotrophic fungal pathogen belonging to the Ascomycota phylum. It is the primary causative agent of purple blotch, a destructive disease affecting various Allium species. This pathogen is globally recognized as a significant constraint to onion and garlic production, particularly in tropical and subtropical regions where conditions favor its rapid proliferation.

The fungus affects all aerial parts of the plants, including leaves, stalks, and seed stems. Initial symptoms appear as small, water-soaked lesions that rapidly expand into elongated, oval spots. These spots often develop a purple or brown center, frequently surrounded by a yellow halo. As the disease progresses, the leaf tissue dries out, causing the foliage to collapse, which directly inhibits bulb development.

The life cycle of Alternaria porri is characterized by the survival of mycelium and conidia in infected plant debris or in the soil. Under favorable conditions, conidia are dispersed by wind, rain splashes, or irrigation water onto healthy tissue. The pathogen enters the plant through natural openings, such as stomata, or wounds caused by insect feeding, such as thrips, or by physical damage to the leaves.

The development of the disease is highly dependent on environmental factors. The optimal conditions for spore germination and infection include temperatures between +20°C and +25°C and high relative humidity (above 90%) or prolonged periods of leaf wetness. These conditions allow the fungus to complete its cycle rapidly, often leading to severe secondary infections throughout the growing season.

Economic damage caused by this pathogen is manifested in significant yield reduction, stunted growth, and poor bulb storage quality due to secondary rot. Management strategies should prioritize an integrated approach: using healthy, certified seeds, maintaining strict crop rotation, and implementing proper field hygiene by removing crop residues. Chemical control through regular application of fungicides, especially during periods of wet weather, is essential to minimize losses.