Agro-industrial compost teas induce metabolic priming and hormonal crosstalk in lettuce diseased by Pythium irregulare

Sustainable lettuce production faces significant challenges due to soilborne pathogens such as Pythium irregulare , necessitating the development of effective biological control strategies. This study evaluates the potential of two agro-industrial waste-derived compost teas (CT1 and CT2) to suppress pathogen pressure. By integrating rhizosphere pathogen quantification (qPCR) with plant biomass assessment, defense-related gene expression, and phytohormones, we characterized the distinct physiological and molecular mechanisms through which these organic amendments enhance systemic resistance. Both compost teas enhanced disease suppressiveness compared to peat, with CT1 showing the highest suppressiveness index, while CT2 more effectively reduced pathogen abundance over time. These effects were associated with distinct physiological and molecular responses. CT1 primarily promoted plant growth, increasing cytokinin and gibberellin levels, whereas CT2 induced higher levels of stress-related hormones, including jasmonic acid (JA) and abscisic acid (ABA). Gene expression and hormone profiling revealed the activation of both induced systemic resistance (ISR) and systemic acquired resistance (SAR) pathways, although ISR-related responses were more pronounced, particularly in CT2-treated plants. Temporal analysis indicated that defense responses peaked at 48–72 h after pathogen inoculation, underscoring the dynamic plant–pathogen interactions. The results suggest that compost teas suppress P. irregulare through a dual mechanism involving direct pathogen suppression and plant defense priming. These findings highlight that compost feedstock composition is a critical factor in determining both soil suppressiveness and the architecture of the plant immune response.