Understanding How Physicochemical Properties of Mancozeb and Metalaxyl Shape Onion ( Allium cepa L.) Production Outcomes: Experimental Stability Studies and Molecular Modeling

This study aims to elucidate the impact of biostimulants and fungicides on onion yield and quality, utilizing a combined experimental and molecular modeling approach. The biostimulants (Humiblack ® , Agasi ® , and Tifi ® ) and fungicides (mancozeb and metalaxyl) were applied to onion crops, resulting in significant improvements in onion quality and yield. The stability and environmental impact of mancozeb and metalaxyl alone and in conjunction with biostimulants were investigated. The stability of the fungicide mixture was assessed in ultrapure water and rainwater, revealing high resistance to hydrolysis. Solar stability assessments, conducted using a sun simulator to mimic environmental conditions, highlighted differences in stability between mancozeb and metalaxyl in the presence of biostimulants. Metalaxyl showed higher photostability owing to the benzene ring. It was also less susceptible to biostimulant effects and remained stable in solution. Density functional theory descriptors and frontier orbital analysis rationalized the higher photoreactivity of mancozeb (smaller HOMO–LUMO gap and broader orbital delocalization). At the same time, molecular dynamics simulations supported stronger solvation of mancozeb and short-range water structuring, consistent with enhanced aqueous susceptibility. The results link fungicide physicochemical properties with field performance and aqueous stability, supporting the use of the fungicide mixture together with a single biostimulant as a practical approach for balancing crop productivity and environmental persistence.