Spent mushroom substrate adsorbs fusaric acid to protect the cucumber rhizosphere microbiome and suppress fusarium wilt caused by Fusarium oxysporum f. sp. cucumerinum

BACKGROUND Cucumber Fusarium wilt, caused by Fusarium oxysporum f. sp. cucumerinum , is aggravated by fusaric acid, a phytotoxic virulence factor that can disrupt rhizosphere microbial communities. Spent mushroom substrate can suppress Fusarium wilt, but whether this protection is linked to reduced fusaric acid stress in the rhizosphere remains unclear. RESULTS Spent mushroom substrate adsorbed fusaric acid in vitro and reduced its transfer through natural soil. Adsorption fitted the Langmuir–Freundlich isotherm, with a maximum adsorption capacity of 52.72 μg g –1 , and followed pseudo‐second‐order kinetics. Fourier transform infrared spectroscopy and chemical blocking assays implicated secondary amide groups as key binding sites. In a dual‐chamber pot system, 1% spent mushroom substrate reduced fusaric acid transfer to the cucumber rhizosphere from 15.75, 31.50 and 62.29 μg g –1 soil to 3.25, 8.31 and 15.49 μg g –1 soil under low, medium and high fusaric acid inputs, respectively. In nonsterile soil, spent mushroom substrate reduced the disease index by 19–26%, whereas this suppressive effect was not observed in sterile soil. Bacterial 16S rRNA gene metabarcoding showed that medium and high fusaric acid inputs reduced bacterial richness and diversity and shifted community structure. Under high fusaric acid exposure, the relative abundance of Bacillus declined from 3.18 to 2.19%, but was maintained at 2.96% with spent mushroom substrate. CONCLUSION Spent mushroom substrate protects cucumber against Fusarium wilt partly by adsorbing fusaric acid and reducing its inhibitory effects on beneficial rhizobacteria. This adsorption‐mediated mechanism supports spent mushroom substrate as a sustainable, microbiome‐friendly amendment for managing soil‐borne Fusarium diseases.