Onion smudge

Onion smudge, caused by the fungus Vermicularia circinans (also known as Colletotrichum circinans), is a fungal disease that primarily affects bulbous crops. It is a common problem in onion and garlic production, particularly in regions with high soil moisture and warm temperatures during the late stages of crop development.

The disease specifically attacks onions and garlic. While the infection can occur in the field, it is most destructive during the storage period, where it can spread rapidly from infected bulbs to healthy ones, causing significant post-harvest losses and economic damage.

Symptoms of the infection typically appear as dark green to black circular spots on the outer scales of the bulbs. These spots often develop small, black, cushion-like structures in concentric rings, which represent the fungal fruiting bodies. In advanced stages, the affected scales become discolored, sunken, and eventually rot completely.

The fungus thrives in warm and humid environments, especially where there is persistent moisture in the soil or air. It survives in the soil as sclerotia or on infected debris. Poor ventilation and high humidity in storage facilities significantly accelerate the spread of the pathogen, leading to severe bulb deterioration.

Effective management and prevention of onion smudge rely on a combination of cultural and chemical practices. Growers should prioritize the use of disease-free sets and ensure proper drainage in fields to prevent waterlogging. After harvest, onions should be dried quickly and stored in cool, well-ventilated conditions to inhibit fungal growth.

It is also crucial to rotate crops and avoid planting onions or garlic in fields that have previously experienced high levels of infestation. Implementing a strict sanitation program, which involves removing and destroying crop residues, helps reduce the initial inoculum level and limits the risk of recurring infection in future seasons.