The management of fungal diseases, particularly those caused by Alternaria alternata, requires efficient and environmentally friendly approaches due to the substantial economic losses and environmental concerns associated with chemical fungicides. One innovative strategy is the green synthesis of metal nanoparticles using plant extracts. In this study, two treatments—“copper nanoparticles synthesized using an aqueous extract of Calotropis procera” and “copper nanoparticles synthesized using a hydroalcoholic extract of Calotropis procera”—were prepared and compared. Transmission electron microscopy (TEM) revealed that the nanoparticles of both treatments had an average diameter of approximately 48 nm, spherical morphology, and relatively uniform size distribution. Antifungal activity against A. alternata (strain PTCC 5224) was assessed using a serial dilution method in potato dextrose broth (PDB) to determine the minimum inhibitory concentration (MIC) and on potato dextrose agar (PDA) to determine the minimum bactericidal concentration (MBC). Both treatments showed the same MIC value of 50 mg mL⁻¹. The MBC was 60 mg mL⁻¹ for the aqueous extract-derived nanoparticles and 73.3 ± 5.8 mg mL⁻¹ for the hydroalcoholic extract-derived nanoparticles. The results indicate that both types of nanoparticles exhibit significant inhibitory effects on fungal growth, with the aqueous-based sample achieving complete inhibition at a lower concentration. These findings suggest that copper nanoparticles synthesized via a green method using C. procera extract can serve as an eco-friendly and effective candidate for developing biofungicides in sustainable plant disease management.