Onion purple spot disease is one of the most significant and widespread diseases affecting green onions, as well as other Allium crops such as garlic and leeks. This fungal infection can cause serious damage to both the foliage and bulb structures, leading to reduced yield and poor quality produce. The disease is characterized by distinct symptoms that develop over time, making early detection crucial for effective management.
Initially, small, slightly sunken lesions appear on the leaves and leaf sheaths. As the disease progresses, these spots grow larger, taking on an oval or spindle-like shape, and turn from brown to purplish in color. Under high humidity conditions, a black mold layer forms on the affected areas, often displaying concentric ring patterns. The infected tissue becomes weak and easily breaks, especially when the plant is under stress. On onions, the lesions appear as purple-brown spots, while on garlic, they are yellow-brown with dark brown, moldy material arranged in a circular pattern. In severe cases, the entire leaves and stalks may wilt and die.
After cutting the bulbs and necks, the internal tissues become soft and discolored, turning red or yellow before gradually darkening to brown or black. The disease can continue to develop during storage, further reducing the marketability of the crop.
The pathogen responsible for this disease is *Alternaria porri* (Ell.) Ciferri, a species of fungus belonging to the genus *Alternaria*. The conidiophores are usually solitary or occur in clusters of 5 to 10, with few branches. The conidia are long and rod-shaped, with optimal growth occurring at temperatures between 24°C and 27°C.
The disease cycle begins with the survival of mycelium in infected plant debris, seeds, or soil. It can also persist through the winter in diseased plant material. When environmental conditions are favorable—such as warm, humid weather—the fungus produces spores that are spread by wind and rain. These spores can infect the host through wounds, stomata, or directly through the epidermis.
Factors that contribute to disease development include excessive irrigation, poor drainage, continuous rainfall, overcrowded planting, and weakened plant health. These conditions create a perfect environment for the fungus to thrive and spread rapidly.
To manage onion purple spot disease effectively, several preventive measures can be taken:
1. Practice crop rotation by avoiding planting onions or related crops in the same area for at least two years.
2. Use resistant varieties where available.
3. Ensure proper field drainage and avoid overwatering.
4. Maintain good air circulation by spacing plants appropriately.
5. Remove and destroy infected plant material to reduce inoculum levels.
6. Apply fungicides as a preventive measure. Recommended options include:
- 70% Mancozeb wettable powder diluted at 500 times
- 75% Chlorothalonil wettable powder diluted at 600 times
- 64% Trichoderma viride 500 times
- 58% Methomyl 500 times
- 43% Propineb suspension diluted at 3000–4000 times
- 70% Antecan wettable powder diluted at 600 times
Regular monitoring of fields and early intervention are essential to prevent outbreaks and minimize economic losses. By combining cultural practices with timely chemical control, growers can significantly reduce the impact of onion purple spot disease on their crops.
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