Fungicide Preharvest Application Strategies and Their Effects on Crop Yield, Quality, and Sprouting of Dried Onion Bulbs

Postharvest losses in onion (Allium cepa L.) bulbs constitute a major economic challenge globally, primarily driven by fungal pathogens and premature sprouting during long-term storage. Addressing these issues with effective preharvest strategies is critical for market stability and supply chain integrity. This study evaluated the effects of two preharvest fungicide strategies, i.e., T1 (dimethomorph + pyraclostrobin) and T2 (metalaxyl + mancozeb + copper oxychloride), on the crop yield, postharvest quality, and sprouting behavior of dried onion bulbs. Both treatments significantly reduced the incidence of foliar disease in the field and improved the crop yield of commercial bulbs compared to the control in two consecutive seasons. T1 achieved the highest yield (~76 and 88 t ha−1 in ‘Mata Hari’ and ’Recas’ onions). During storage at 20 °C for 84 days, in the ‘Mata Hari’ cultivar, the T1 bulbs exhibited the lowest weight loss and respiration rate, the lowest sprouting incidence (1%), and superior firmness retention and higher total soluble solids. In contrast, control bulbs exhibited accelerated weight loss and tissue degradation, with up to 95% sprouting. Pyruvic acid content, an indicator of pungency, was highest in T1 bulbs and increased significantly in sprouted controls, likely due to internal enzymatic activation and tissue senescence. The fungicides indirectly delayed dormancy release by delaying sprouting and internal stem axis formation. Overall, T1 was the most effective strategy for preserving onion quality during storage without using synthetic sprout inhibitors. These findings support the integration of specific fungicide programs into preharvest management to improve onion storability, reduce postharvest losses, and maintain commercial value in intermediate-dormancy dried onion cultivars, such as ‘Mata Hari’.