In Vitro Evaluation of Agro-Industrial By-Product Extracts Against Pseudomonas savastanoi and Fruit Bioassay Evaluation of Their Protective Effect Against Colletotrichum fioriniae

The increasing demand for sustainable agricultural practices has intensified the search for natural alternatives for the control of plant pathogens. This study aimed to evaluate the in vitro inhibitory activity of plant extracts obtained from agro-industrial byproducts, including pomegranate peel (Punica granatum), chestnut peel (Castanea sativa), and grape pomace (Vitis vinifera), against the bacterium Pseudomonas savastanoi and the fungus Colletotrichum fioriniae. Antibacterial activity was assessed using the disk diffusion and Minimum Inhibitory Concentration (MIC) methods. Antifungal activity was evaluated for all extracts through mycelial growth inhibition, spore quantification, and viability assays. Based on its overall antimicrobial performance and extraction yield, the P. granatum extract was selected for evaluation in a detached olive fruit bioassay. The results showed that the pomegranate peel extract exhibited an inhibition halo of 9.80±0.10 mm against Pseudomonas savastanoi, with a MIC of 5.0 mg/mL and a predominantly bacteriostatic effect. Regarding antifungal activity, the Castanea sativa extract showed the highest inhibition of mycelial growth, reaching 27.27% after 3 days and reducing sporulation by up to 44.36%, whereas the Punica granatum and Vitis vinifera extracts exhibited only moderate inhibitory effects. In the sporulation assay, all extracts reduced spore production and increased early conidial germination, a transient response not accompanied by enhanced fungal development. In the detached olive fruit bioassay, pre-treatment with the P. granatum extract delayed disease progression and reduced final disease severity from 100% in the positive control to 64%, whereas simultaneous application of the extract and the pathogen did not provide a protective effect. Overall, the findings highlight the potential of plant-derived agro-industrial byproducts as sources of bioactive compounds for sustainable plant disease management strategies.