Sweet orange is one of the world’s most important fruit crops. However, increased demand and growing pressure on the sector have created an ideal scenario for the emergence and spread of diseases. Specifically, Penicillium digitatum, the causal agent of green mold disease, is responsible for 90% of postharvest losses in the citrus industry. Seaweeds produce numerous bioactive compounds with antimicrobial properties.

This study aims to evaluate the antifungal activity of extracts from two invasive seaweeds from the Portuguese coast, Asparagopsis armata Harvey and Sargassum muticum (Yendo) Fensholt, obtained using green solvents (water and ethanol), against the above-mentioned pathogen. Antifungal activity was assessed in vitro using the poisoned food technique and microdilution method and in vivo using fruit immersion assays. The cell lysate of A. armata (CL) at 1 mg/mL showed strong antifungal activity against P. digitatum in vitro, resulting in morphological changes in mycelial growth and complete inhibition of spore germination. To improve the bioavailability, solubility, and release of the CL bioactive compounds, encapsulation in pyrogenic amorphous silica and maltodextrin–pectin (1:0.12) was explored and successfully achieved. Comparative analyses of the antifungal efficacy of the encapsulated and free CL revealed that the maltodextrin–pectin formulation preserved the inhibitory activity. In vivo assays on sweet orange fruits demonstrated a 36% reduction in mycelial growth area and a 42% reduction in sporulation area following CL application, while maintaining fruit quality.

These findings highlight the potential of invasive seaweeds as eco-friendly alternatives to synthetic pesticides, while also contributing to marine environment restoration through a circular economy approach.