Sustainable Control of Onion Purple Blotch through Integrated Disease Management: A Review

Onion (Allium cepa L.) is a globally important vegetable valued for its culinary, nutritional, and medicinal properties. Its production, however, is severely constrained by biotic stresses, among which purple blotch, caused by Alternaria porri, is a major foliar disease. The pathogen induces elliptical purplish lesions on leaves and flower stalks, leading to chlorosis, necrosis, early senescence, and yield losses of up to 60–70% under favorable environmental conditions. Disease prevalence is influenced by high humidity, moderate temperatures, prolonged leaf wetness, and dense crop canopies, with polycyclic conidial spread exacerbating epidemics. Effective management of purple blotch requires an Integrated Disease Management (IDM) approach that combines cultural practices (crop rotation, field sanitation, optimal spacing), host resistance (moderately resistant varieties like Arka Kalyan and Bhima Super), judicious chemical application (fungicides such as mancozeb, difenoconazole, and azoxystrobin), and biological control (antagonistic fungi and bacteria, as well as botanical extracts). Recent advances, including molecular diagnostics, marker-assisted selection, and decision support systems, enhance early detection, precision management, and development of resistant cultivars. Despite these strategies, challenges such as climate variability, limited availability of resistant varieties, and low farmer awareness persist. Future directions emphasize climate-resilient IDM modules, eco-friendly alternatives, and capacity building to promote sustainable onion production. This review consolidates current knowledge on the etiology, epidemiology, and integrated management of purple blotch, highlighting practical and innovative approaches to mitigate its impact on yield and quality