Silver nanoparticles (AgNs) are known as a promising alternative tool to control fungal diseases. AgNs were biologically synthesized using Trichoderma harzianum filtrate as an ecofriendly approach. The presence of AgNs was confirmed by changing the color to brown, followed by UV-Vis spectroscopy, transmission electron microscopy (TEM), and Energy-dispersive spectra (EDS). TEM studies showed that the size of AgNs average was 31.13 nm and the shape was spherical. In vitro assays of AgNs showed a significant inhibitory effect on the growth of Sclerotinia sclerotiorum (S. sclerotiorum) and Sclerotium rolfsii (S. rolfsii). The percentage inhibition on mycelial linear growth, dry weight, and sclerotia formation of S. sclerotiorum and S. rolfsii at 100^−L were 87.8, 82.7, 96.4, 52.8, 55.1, and 85.4%, respectively. The obtained results suggested that the biosynthesized AgNs have antifungal activity against S. sclerotiorum and S. rolfsii. Foliar spray of bean and sunflower plants with AgNs caused a decrease in disease severity, which promoted the plant protection against S. sclerotiorum and S. rolfsii, respectively. Substantially, this study will extend our understanding of the AgNs antifungal action for suppressing fungal diseases.