Selenium nanoparticles (SeNPs) are currently receiving attention for controlling plant pathogenic microorganisms, are expected to be especially effective against the genus Phytophthora, and show high anti-oomycete activity. SeNPs synthesized with plant extracts have shown low toxicity, high bioavailability, and mechanisms of action that alter cellular integrity and damage key components of phytopathogen metabolism, causing denaturation and cell death. The aim of this study was to evaluate the inhibitory activity of SeNPs on mycelial growth and the development of reproductive structures in Phytophthora capsici in vitro. Different concentrations of SeNPs (0 to 400 µg/mL) in culture media were used to analyze mycelial growth, sporangium formation, zoospores, and germination of the germ tube. To explain the changes in morphology and development of P. capsici, increased relative conductance and activation of glycerol synthesis were related to osmotic stress and damage to membrane permeability. In addition, SeNPs inhibited the production of exopolysaccharides (EPSs), which are compounds associated with pathogen virulence. A lower accumulation of its biomass evidences alterations in the oomycete growth. The percentage of inhibition of mycelial growth increased with higher SeNP concentrations and incubation time, reaching 100% growth inhibition at 300 and 400 µg/mL. A concentration-dependent reduction in the number of spores, sporangia, and zoospore germination was observed. Concentrations of 50 and 100 µg/mL of SeNPs reduced biomass production by 30%. The increase in glycerol levels indicated an osmoregulatory response to SeNP-induced stress. Also, the increase in electrical conductivity suggested plasma membrane damage, which supports the potential of SeNPs as antifungal agents by inducing cell disruption and structural damage in P. capsici. These results provide new knowledge on the in vitro mechanism of action of SeNPs against P. capsici and offer a new biological alternative for the control of diseases caused by oomycetes.

Selenium nanoparticles (SeNPs) synthesized through green routes have emerged as promising nanobiostimulants in sustainable agriculture due to their ability to enhance plant growth and antioxidant defense. The aim of this study was to evaluate the biostimulant effect of SeNPs on Capsicum annuum at two stages of crop development to characterize the response to SeNP exposure and identify concentration-dependent effects and application methods. Physiological indicators, including growth, photosynthetic pigment content, and antioxidant activity, were evaluated. Different concentrations of SeNPs were tested during germination, and dosage and two types of application were compared during the vegetative phase in a hydroponic experiment. SeNPs at concentrations of 1.25, 2.5, 5, 10, 20, 40, and 80 µM were applied to chili seeds for 20 days. The plants were exposed to SeNPs concentrations ranging from 1.25 to 80 µM, applied through the roots and leaves. Germination parameters were not significantly affected except for the seed vigor index, which increased at all concentrations, particularly at 20 µM. Low to moderate doses (1.25–20 µM) acted as biostimulants, enhancing plant height, root length, biomass accumulation, photosynthetic pigment content, and phenolic and flavonoid compound synthesis. Conversely, high doses (80 µM) induced phytotoxic effects, especially via root exposure, reflected by growth inhibition, and reduced chlorophyll content. Foliar application demonstrated a systemic biostimulant response, improving root growth and photosynthetic activity without toxicity symptoms. Antioxidant assays (DPPH and ABTS) revealed dose-dependent modulation of redox balance, suggesting adaptive responses to SeNP-induced oxidative conditions. These findings highlight the potential of SeNPs as biostimulants that improve physiological performance in chili plants, while emphasizing the importance of an optimal dosing and application method for sustainable nanotechnology-based crop management.