Sustainable control of Phytophthora capsici in tomato (Solanum lycopersicum L.) using garden waste compost tea enriched with Streptomyces AC-3

• A 1:3.5 compost tea formulation increased nutrients, humic compounds, and microbial abundance. • Compost tea showed a stable core microbiome dominated by plant-beneficial taxa. • A native Streptomyces (AC-3) isolated from compost tea showed strong inhibition of Phytophthora spp. • Compost tea enhanced root growth and its combination with Streptomyces AC-3 achieved the highest disease suppression. The search for sustainable alternatives to chemical pesticides remains a key priority in crop protection. This study evaluates the suppressive potential of a compost tea (CT) derived from pruning and garden residues, together with a native Streptomyces isolate (AC-3) with strong in vitro antagonism against Phytophthora spp. Two CT formulations with compost-to-water ratios of 1:5 and 1:3.5 (v/v) were evaluated to assess whether a more concentrated formulation could maintain chemical and microbiological quality while facilitating product handling and commercialization, and whether its application, alone or in combination with Streptomyces AC-3, could suppress pathogen growth and reduce disease development in plants. The concentrated CT (1:3.5) exhibited higher electrical conductivity, nutrient content, and microbial abundance without major changes in the overall microbial structure, as confirmed by metagenomic analysis. Dominant bacterial genera included Mesorhizobium, Micromonospora, and Streptomyces , taxa associated with nutrient cycling and biocontrol. Streptomyces AC-3 showed strong inhibition of oomycetes, including P. capsici , while not affecting beneficial fungi such as T. harzianum . Greenhouse assays demonstrated that CT enhanced root biomass and significantly reduced disease incidence in tomato plants, with the combined treatment (CT plus AC-3) achieving the highest protection. These results highlight the potential of CT as both a biostimulant source and a reservoir of beneficial microorganisms. The combination of CT with selected Streptomyces strains represents a promising, eco-friendly strategy for the management of oomycete pathogens within sustainable agricultural systems.