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Onion blight
Onion diseases can cause significant damage to crops, leading to reduced yield and quality. One common issue is a fungal infection that primarily affects the leaves and stems of onions, including green onions and shallots. Initially, there may be no visible signs on the green or white parts of the plant. However, as the disease progresses, grayish-white spots appear and gradually expand. These lesions cause the leaves to wilt from the top down, resulting in what is known as "dry tip" in the field.
Under high humidity conditions, a thin layer of white mold may develop on the infected areas, while under dry weather, this mold disappears. Upon examination of the inner walls of the long, conical leaves, white mycelium can be observed, which helps distinguish this condition from physiological "dry tip" caused by environmental stress.
The pathogen responsible for this disease is *Phytophthora allii* Saw., which is also considered to be *P. nicotianae* var. *parasitica* (Dast.) Waterhouse. The disease cycle involves the survival of mycelium, oospores, and chlamydospores within infected plant tissues. In southern regions, zoospores are the primary means of infection and reinfection, spreading through rain splash or irrigation water. They enter through stomata and thrive in wet, low-lying, poorly drained areas. Excessive nitrogen fertilizer application can also contribute to disease development.
Currently, there is limited research on the resistance of onion varieties to this particular pathogen. Some varieties that show strong resistance to purple spot disease may also be resistant to this condition, but further field testing in different locations is needed to confirm this.
To manage the disease effectively, it's important to select resistant varieties. Deep trench cultivation and proper drainage systems can help reduce moisture buildup. After heavy rains, ensure timely drainage. Balanced fertilization and foliar nutrient applications should be used to avoid excessive vegetative growth, which can weaken the plant's natural defenses.
In terms of chemical control, spraying should be done based on weather conditions. Recommended fungicides include 25% metalaxyl WP, 64% mancozeb at 600 times dilution, 66.5% propamocarb at 800 times, 72% chlorothalonil wettable powder at 600–800 times, or a mixture of 69% oxathiapiprolin + 75% chlorothalonil (1:1) at 1000 times. Apply 2–3 times with an interval of 7–10 days, alternating between different products for better efficacy.