Onion

Allium cepa

1. Identification and origin

The onion (Allium cepa L.) is a biennial herbaceous species of the Amaryllidaceae family, probably originating from Central Asia. It is one of the most widely cultivated vegetables in the world, used both fresh and in industrial processing.

2. Economic importance

The onion has high global economic importance, being consumed fresh, dehydrated, preserved or processed. It is a key crop in intensive and extensive horticulture, with strong weight in the domestic market and in exports.

3. Botanical characterisation

Biennial plant grown as an annual. The harvested organ is a bulb formed by modified leaves (scales) that accumulate reserves. The leaves are cylindrical, fistulous and erect. The inflorescence is a spherical umbel with numerous white flowers. The root system is fibrous and shallow.

4. Soil and climate requirements

It prefers temperate and dry climates, with good light. It develops best in loose, well-drained, fertile soils with pH between 6.0 and 7.0. It is sensitive to waterlogging and benefits from controlled irrigation, especially during the bulb-swelling phase. Photoperiod sensitivity varies according to the cultivar (short, intermediate or long days).

5. Main pests

• Onion fly (Delia antiqua): destruction of seedlings and tunnels in the bulbs.
• Thrips (Thrips tabaci): leaf silvering and reduced productivity.
• Stem and bulb nematodes (Ditylenchus dipsaci): deformations and rotting.
• Caterpillars (Lepidoptera): foliar damage.
• Mites (Tetranychidae): chlorosis and loss of vigour.

6. Main diseases

• Downy mildew (Peronospora destructor): yellow spots and foliar necrosis.
• Botrytis (Botrytis squamosa, B. allii): foliar and storage rots.
• Fusarium wilt (Fusarium oxysporum f. sp. cepae): basal rot.
• Anthracnose (Colletotrichum circinans): foliar lesions and reduced growth.
• Bacterial rots (Pectobacterium, Burkholderia, Pseudomonas): post-harvest deterioration.

7. General crop management

Management includes crop rotation, use of certified material, adequate planting density, balanced fertilisation and controlled irrigation. Monitoring of thrips and downy mildew is essential. Harvesting should be carried out when 70–80% of the plants show natural lodging, followed by adequate curing to prolong storage.

Bibliographic references

• EPPO Global Database – Allium cepa.
• CABI Crop Compendium – Allium cepa.
• Brewster, J. L. (2008). Onions and Other Vegetable Alliums. CABI Publishing.
• Rabinowitch, H. D., & Currah, L. (2002). Allium Crop Science: Recent Advances. CABI Publishing.
• Schwartz, H. F., & Mohan, S. K. (2008). Compendium of Onion and Garlic Diseases and Pests. APS Press.