Characterization of shrimp waste-based compost and its antifungal efficacy against pathogens causing olive tree decline.

Marine waste valorization through composting offers a sustainable strategy to reduce environmental burdens while generating bioactive amendments capable of suppressing soil-borne phytopathogens. This study characterized shrimp waste-based compost and evaluated its in vitro antifungal potential against pathogens causing olive tree decline. Composting for 120 days, a duration selected to encompass the thermophilic, cooling, and maturation phases, produced a mature material with near-neutral pH (7.2 ± 0.3), moderate electrical conductivity (2.1 ± 0.15 mS cm⁻¹), high organic matter (45.2 ± 2.1%), total nitrogen (2.8 ± 0.2%), and a stabilized C/N ratio (18.2), together with humic and fulvic fractions, phenolics, and residual chitinase and protease activities. Culturable antagonistic communities increased markedly during composting, notably Bacillus spp., Pseudomonas spp., Trichoderma spp., and actinomycetes. Compost-derived bacterial isolates (X12, B4, H1, H7, and K) showed broad-spectrum antagonism against major olive decline fungi, including Verticillium dahliae, Fusarium solani, Neofusicoccum sp., Biscogniauxia mediterranea, Macrophomina phaseolina, and Lasiodiplodia theobromae. The strongest inhibition was recorded for isolate X12 against V. dahliae (81.2%) and F. solani (67.8%), and for H7 against V. dahliae (69.86%) and M. phaseolina (61.78%). Molecular identification based on 16 S rRNA gene sequencing assigned the five antagonists to well-known biocontrol taxa: Bacillus subtilis (X12), Bacillus amyloliquefaciens (B4), Pseudomonas putida (H1), Pseudomonas fluorescens (H7), and Bacillus velezensis (K). Compost juice and leachate exhibited strong dose-dependent antifungal activity in vitro, with filter-sterilized preparations reaching up to 99.8% inhibition, while autoclaving reduced activity in a pathogen-dependent manner, indicating contributions from both heat-labile and thermostable compounds. These findings support shrimp waste composting as a promising circular-economy strategy that yields a mature organic amendment and antifungal microbial/chemical fractions; however, greenhouse and field validation are required before disease-management recommendations can be made. This integrated proof-of-concept links chitin-rich marine-waste valorization with olive-decline pathogen suppression and identifies candidate compost-derived microbial fractions for subsequent greenhouse and field validation.