A simple procedure for copper nanoparticle synthesis has been studied by manipulating ascorbic acid as a reductant and pectin as a stabilizer by conventional/sonication-assisted heating. The dependence of copper nanoparticle formation on precursor, reductant, stabilizer, sonicating temperature, and sonication-mediated duration was investigated by ultrasound-assisted and conventional heating. The copper nanoparticles were produced at a lower sonicating temperature 40 °C compared to heating at 50 °C in the same synthesis conditions: 20 mM (CH₃COO)₂Cu, ascorbic acid/(CH₃COO)₂Cu molar ratio of 2, and a 30-min reaction. The experimental findings suggested that synthesized materials were generated with phase-pure coppers and a maximum wavelength of 604 nm. Copper-based nanoparticles using opposed spheres with an average particle diameter of 7.1–7.7 nm and negative zeta potential ranging from – 14.3 to – 11.3 mV. Sonication-mediated copper nanomaterials were more stable than non-sonicated copper particles with a storage time of 20 days instead of 8 days. The produced copper nanoparticles exhibited high inhibition activity against fungi growth with the IC₅₀ value of 10.5 and 22.7 mM for conventional heating and 2.4 and 18.9 mM for sonication treatment against Fusarium solani and Rhizoctonia solani, respectively. Even though the difference in the particle size (lower 10 nm) and the synthetic efficiency (approximately 91%) for synthesizing copper nanoparticles were insignificant, the sonication-synthesized copper nanoparticles with higher antifungal activity were considered outstanding benefits of the sonication technique. The synthesized copper nanoparticles in this work were recommended as a promising procedure with environmentally safe and offered an antifungal agent to substitute harmful insecticides in agricultural output.