Green-synthesized silver and metal oxide nanoparticles for in vitro antifungal activity against Alternaria solani in tomato

Abstract Background Early blight, caused by Alternaria solani, is a destructive tomato disease, and repeated use of synthetic fungicides raises concerns regarding resistance development, residues, environmental contamination, and human-health risks. This in vitro study evaluated Eucalyptus camaldulensis Dehnh.-mediated silver nanoparticles (g-AgNPs), copper oxide nanoparticles (g-CuO NPs), and zinc oxide nanoparticles (g-ZnO NPs) against A. solani isolate ASIB-15. Nanoparticles were synthesized using leaf extract and characterized by UV–Vis spectroscopy, FTIR, XRD, EDX, SEM, and TEM. Antifungal activity was assessed at 10, 25, 50, and 100 µg/mL using inhibition-zone diameter and colony-diameter-based mycelial-growth inhibition as complementary endpoints. Results Mean particle sizes were 14.0 ± 9.5 nm for g-AgNPs, 6.93 ± 1.92 nm for g-CuO NPs, and 28.5 ± 6.5 nm for g-ZnO NPs, supporting their nanoscale range within this assay system. The synthesized nanoparticles showed nanoscale morphology and concentration-dependent antifungal activity. At 100 µg/mL, g-AgNPs produced the strongest response, with a 16 mm inhibition zone and about 72% mycelial-growth inhibition. At the same concentration, g-CuO NPs produced an approximately 13 mm inhibition zone and about 60% inhibition, whereas g-ZnO NPs produced an approximately 15 mm inhibition zone and about 50% inhibition. The different rankings obtained from inhibition-zone diameter and percentage mycelial-growth inhibition reflect that these endpoints measure different aspects of fungal suppression. Conclusion Overall, the results identify E. camaldulensis-mediated g-AgNPs, g-CuO NPs, and g-ZnO NPs as preliminary in vitro antifungal candidates against A. solani. However, phytotoxicity testing, greenhouse and field validation, formulation-stability assessment, mechanism studies, and commercial fungicide comparisons are required before practical tomato early blight management recommendations can be made. Graphical Abstract