Purple blotch

The disease known as purple blotch is caused by the fungus Alternaria porri. This pathogen is a significant economic threat to the Allium genus worldwide. The fungus survives as mycelium or conidia in crop debris left in the field, in the soil, or on infected onion sets and seeds, acting as a primary source of inoculum for subsequent seasons.

The primary hosts of this pathogen include onions (bulb onions, scallions, leeks) and garlic. While it primarily affects the foliage and floral stalks, it can also infect the bulbs, leading to severe decay. The disease is particularly damaging in tropical and subtropical regions, as well as in temperate climates during warm and humid summers.

Symptoms initially appear as small, water-soaked lesions on the leaves, which later enlarge and turn white, gray, or light brown. As the disease progresses, these lesions develop a characteristic purple or dark brown center, often surrounded by a yellow or chlorotic halo. Under high humidity, dark, velvety spores (conidia) become visible within the lesions, causing the tissue to collapse.

Environmental conditions play a critical role in the development and spread of Alternaria porri. Optimal temperatures for fungal development range from 20°C to 30°C. Frequent rainfall, prolonged leaf wetness, and high humidity facilitate the production and dissemination of spores via wind and water splashing. Wounded tissue, often caused by onion thrips or mechanical damage, is highly susceptible to rapid infection.

The impact of purple blotch includes severe defoliation, which reduces bulb size and prevents the crop from reaching its full potential. To manage this disease effectively, farmers should adopt an integrated pest management (IPM) approach:

• Rotation with non-host crops to reduce soil-borne inoculum
• Ensuring proper drainage and aeration in the field
• Control of insect vectors like thrips
• Application of preventative and systemic fungicides when weather conditions favor disease spread