Anthracnose (Onion smudge) (Colletotrichum circinans)
It usually appears in fields just before harvest and continues to develop during storage period. Under warm and wet soil conditions, it can cause seedling damping-off. The most common symptom is the small dark green or black stains (dots) on outer scales of bulbs. The dots develop concentric rings. In severe cases, the fungus attacks the living tissue causing a collapse of fleshy scales. On coloured onions, the fungus is restricted to the neck of the bulbs making the flattened leaves colourless. The fungus survives on onions, sets and in the soil. Warm moist conditions favour development of the disease.
Optimum temperature for infection is from 23.9 to 29.4 °C. White onions are very susceptible to the disease. Reduced market value results from marred bulb appearance and bulb shrinkage. It also attacks leeks and shallots.
- Disease management involves growing coloured varieties where smudge is a persistent problem.
- Harvesting the crop promptly.
- Avoiding exposure to rain between harvest time and storage.