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 of the plants, leading to reduced yield and quality.
The symptoms typically begin on the leaves and stems, often appearing as small, slightly sunken spots that gradually expand into oval or spindle-shaped lesions. These lesions range in color from brown to purple, and under humid conditions, a black mold layer develops on the surface. The affected areas may show distinct concentric ring patterns, and the tissue becomes fragile, making it prone to breaking. On onions, the lesions appear as purple-brown spots, while on garlic, they are yellowish-brown. When humidity is high, dark brown moldy material forms in a scattered or roulette-like pattern. In severe cases, the leaves and stems may wither completely.
After cutting the bulbs and necks, the tissues become semi-wet and start to rot, turning red or yellow before progressing to dark brown or black. The disease can continue to develop during storage, further worsening the damage.
The pathogen responsible for this disease is *Alternaria porri* (Ell.) Ciferri, which belongs to the genus *Alternaria*. The conidiophores are usually solitary or grouped in clusters of 5–10, with few branches. The conidia are long and rod-shaped, and the optimal temperature for its growth is between 24°C and 27°C.
The disease cycle begins when the fungus survives in the soil through mycelium, infected seeds, or plant debris. Under favorable conditions, spores are produced and spread by wind and rain. The pathogen can enter the host through wounds, stomata, or directly through the epidermis. Warm, moist environments significantly promote disease development. Poor agricultural practices such as overwatering, poor drainage, excessive planting density, and weak plant growth can all contribute to more severe infections.
To manage onion purple spot disease effectively, several preventive measures can be taken:
1. Practice crop rotation by avoiding planting onions or related crops for at least two years in the same field.
2. Use chemical control methods such as spraying 70% Mancozeb wettable powder diluted at 500 times, 75% Chlorothalonil at 600 times, 43% Fenbuconazole suspension at 3000–4000 times, 64% Trifloxystrobin at 500 times, 58% Methoxyfenozide at 500 times, and 70% Antecan at 600 times. These treatments should be applied regularly throughout the growing season to prevent and control the disease.
By combining cultural practices with timely fungicide applications, farmers can significantly reduce the impact of this destructive disease on their onion crops.
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