Antifungal Efficacy of Selenium Microparticles Biosynthesized by Lysinibacillus sphaericus Against Citrus Postharvest Blue and Green Molds: Mechanistic Insights

Citrus postharvest blue mold caused by Penicillium italicum and green mold caused by P. digitatum are major diseases responsible for fruit decay and economic losses. Increasing fungicide resistance and concerns regarding the safety of chemical fungicides have created an urgent need for alternative control strategies. In this study, selenium microparticles (SeMPs) were biosynthesized by Lysinibacillus sphaericus N30 and characterized by scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS), X-ray diffraction (XRD), Raman spectroscopy, and dynamic light scattering (DLS). The control efficacy of N30-derived SeMPs against citrus postharvest diseases was evaluated using a 6-h post-inoculation treatment and a co-inoculation treatment, and their antifungal effects were further investigated by assessing reactive oxygen species (ROS) accumulation, membrane integrity, extracellular conductivity, and leakage of intracellular macromolecules. The N30-derived SeMPs were predominantly spherical and exhibited a Z-average hydrodynamic diameter of 656.5 nm with a PDI of 0.274. EDS identified Se as the predominant element, while XRD and Raman analyses supported the formation of predominantly amorphous elemental selenium. In vivo assays showed that 640 mg·L−1 SeMPs completely prevented blue mold incidence in the P. italicum co-inoculation treatment, whereas 1280 mg·L−1 completely suppressed blue mold under both application modes. Green mold caused by P. digitatum was completely suppressed at 1280–2560 mg·L−1, with co-inoculation treatment generally showing greater efficacy than post-inoculation treatment. SeMP exposure was associated with increased intracellular ROS accumulation, loss of membrane integrity, increased extracellular conductivity, and leakage of intracellular macromolecules in both pathogens. P. italicum showed greater susceptibility to SeMPs-associated membrane perturbation than P. digitatum, consistent with its higher sensitivity in the fruit assays. These findings indicate that biogenic SeMPs produced by L. sphaericus N30 have potential as selenium-based antifungal materials for the management of citrus postharvest blue and green molds.