Spent Mushroom Substrate Adsorbs Fusaric Acid to Protect the Cucumber Rhizosphere Microbiome and Alleviates Fusarium Wilt

Caused by Fusarium oxysporum f. sp. cucumerinum, cucumber Fusarium wilt threatens cucumber production worldwide. In addition to pesticides, using spent mushroom substrate (SMS) has a protective effect against Fusarium infection in cucumber, partly through effects on the microbiome. Fusaric acid (FA), a phytotoxic virulence factor secreted by Fusarium, disrupts both plant physiology and the rhizosphere microbial community. Herein, we investigated the relationship between SMS and this virulence factor. Bioassays demonstrated that SMS adsorbed FA both in culture supernatants and in soil. Adsorption behaviour conformed to the Langmuir-Freundlich isotherm model, with a maximum adsorption capacity of 52.72 g/g. Kinetics followed a pseudo-second-order model, indicating physical and chemical adsorption. Scanning electron microscopy showed that SMS had a porous surface, facilitating FA capture. Fourier transform infrared spectroscopy and chemical blocking assays were used to identify secondary amide groups as the key binding sites. In a dual-compartment pot system designed to isolate adsorption from other direct effects, SMS treatment significantly reduced the FA concentration in the cucumber rhizosphere and mitigated FA-induced disease aggravation. This lowered the disease index by up to 25%. Bacterial 16S metabarcoding showed that FA disrupted the rhizosphere bacterial diversity and community structure. However, when FA was adsorbed by SMS, the microbial diversity and community stability were restored. FA reduced the abundance of sensitive taxa such as Bacillus. Meanwhile, SMS-adsorbed FA preserved its relative abundance, suggesting a selective protective effect for FA-sensitive rhizobacteria. These findings indicate that SMS protects cucumber against Fusarium by alleviating FA toxicity toward beneficial microbes. Through FA adsorption, SMS amendment stabilised the rhizosphere microbial community and reduced disease incidence. This highlights the potential of SMS as a sustainable and microbiome-friendly strategy for managing soil-borne diseases.