The onion (Allium cepa L.) stands as a significant vegetable crop globally, including Egypt. However, its susceptibility to white rot, a severe disease caused by the soil-borne fungus Sclerotium cepivorum, poses a substantial threat to Allium spp. In our study, we illustrate a bio-based approach to synthesizing silver nanoparticles (Ag-NPs) using neem extraction (Azadirachta indica) leaves and Trichoderma reesei (formerly known as Hypocrea jecorina). The reaction process is monitored using UV-visible light spectroscopy, while fluorescence emission spectroscopy provides detailed insights into the nanosecond time frame of Ag-NP creation. Additionally, Fourier transforms infrared spectroscopy (FTIR) aids in quantitatively analyzing the reaction products. Throughout the reaction period, the surface Plasmon band in the silver nanoparticle solution remains consistently near 429 nm. Furthermore, measurements reveal an average size distribution (DLS) of 66.2 nm for the Ag-NPs, with those synthesized from neem leaf extract exhibiting a negative zeta potential of -22.93mv.In vitro experiments demonstrate the inhibitory effect of T. reesei-produced silver nanoparticles (Ag-NPs-Tr) and neem extract-derived nanoparticles (Ag-NPs-Ne) on S. cepivorum mycelial growth at a concentration of 250 µL/L. However, it is noteworthy that the T. reesei strain used in this study displayed tolerance to Ag-NPs at all tested doses. Moreover, the application of Ag-NPs-Tr and Ag-NPs-Ne, or fungicides, via dipping onion transplants led to a reduction in onion white rot disease incidence. Notably, Ag-NPs-Tr at a concentration of 250 µL/L exhibited similar efficacy to Flumid 24%, resulting in the most significant reduction in onion white rot incidence (11.1%) and increased onion bulb yield (256.3 and 258.8g/pot, respectively).In conclusion, our findings suggest that biosynthesized silver nanoparticles offer promising potential in managing onion white rot disease.