Vitamin C Industrial Byproduct: A Promising Enhancer for Trichoderma harzianum-Driven Biocontrol of Tomato Pathogenic Fungi

Trichoderma harzianum, a prominent biocontrol microorganism, often exhibits restricted colonization efficiency in nutrient-poor soil, thus reducing its biocontrol effectiveness. This study investigated the impact of vitamin C industrial fermentation byproduct (residue after evaporation, RAE), which is recognized for enhancing plant growth and stress tolerance, on the colonization ability and anti-pathogenic fungi activity of T. harzianum through in vitro and pot experiments. In vitro experiments demonstrated that RAE and its main component (2-keto-L-gulonic acid, 2KGA) significantly enhanced biomass and spore production (41.44% and 158.46% on average) of two T. harzianum strains in an oligotrophic medium (1/5 PDA). In a more nutrient-limited medium (1/10 PDA), RAE significantly increased the inhibition rates of T. harzianum S against Fusarium graminearum, Botrytis cinerea, and Alternaria alternata by 6.12–7.77%. Pot experiments further revealed that, compared with T. harzianum application alone, the combined application of RAE and T. harzianum S, (1) significantly elevated T. harzianum S abundance by 23.77% while significantly reducing B. cinerea abundance by 33.78% in rhizosphere soil; (2) significantly improved the content of soil available phosphorus (147.63%), ammonium nitrogen (60.05%), and nitrate nitrogen (32.19%); and (3) significantly improved the superoxide dismutase activity (17.39%) and fresh weight of tomato plants (130.74%). Correlation analysis revealed that there were significant positive correlations between T. harzianum S abundances/plant biomass and RAE, and significantly negative correlations between B. cinerea abundance and T. harzianum S/plant biomass/peroxidase activity. Collectively, RAE effectively promoted the growth of T. harzianum and pathogen suppression ability, while improving soil fertility and tomato biomass. This study offers novel insights into RAE’s agricultural application for plant disease control while supporting the sustainable development of vitamin C production.