Antifungal activity of zinc oxide nanoparticles against Phytophthora capsici and their phytotoxicity in Piper nigrum

Phytophthora capsici is a destructive oomycete pathogen responsible for severe yield losses in black pepper ( Piper nigrum L.), particularly in tropical production systems. Limitations associated with conventional fungicides, including resistance development and environmental concerns, necessitate alternative disease management strategies. This study evaluated the antifungal activity of zinc oxide nanoparticles (ZnO NPs) of two particle sizes (~ 50 nm and ~ 100 nm) against P. capsici isolates from Sarawak, Malaysia, and assessed their phytotoxicity on black pepper leaves. Both nanoparticle sizes significantly inhibited mycelial growth in a concentration-dependent manner, with smaller particles exhibiting greater efficacy. The minimum effective concentration ranged from 0.4 to 0.5 mg/mL for ~ 50 nm ZnO NPs and 0.5–0.6 mg/mL for ~ 100 nm ZnO NPs. At 1.0 mg/mL, growth inhibition reached 72.2% for ~ 50 nm ZnO NPs and 47.9% for ~ 100 nm ZnO NPs. Nanoparticle treatment also delayed colony expansion and altered growth characteristics. Recovery assays indicated that the antifungal effect was primarily fungistatic. No visible structural damage to hyphae was observed; however, reduced aerial mycelial formation suggested interference with fungal development. No phytotoxic effects were detected on detached black pepper leaves under the tested conditions. These findings demonstrate the in vitro inhibitory potential of ZnO NPs against P. capsici and support their further evaluation under whole-plant and environmentally relevant conditions.