Objectives Phenolic acid compounds are common autotoxin in rhizosphere soils of crops, and the allelopathic autotoxins play a key role in soil sickness and soil-borne disease. The aim of this study was to evaluate the role of ferulic acid (FA) in Fusarium oxysporum f. sp. fabae (FOF)-infected faba bean roots, reveal the mechanism of continuous autotoxins promoting the occurrence of soil-borne diseases.

Methods Effects of ferulic acid on the growth of faba bean seeding and Fusarium wilt occurrence were studied using hydroponics method. Faba bean seedlings of 4–6 open leaves were cultured in 2 L Hoagland nutrient solution with ferulic acid concentrations of 0, 50, 100 and 200 mg/L, respectively. After 2 days’ culture, 25 mL of FOF suspension (1 × 10⁶ cfu/mL) were incubated into the solution. Continuously cultured for another 40 days, faba bean samples were collected for investigation of faba bean growth and Fusarium wilt incidence. Faba bean roots tissue structure changes were observed using microscope. The effects of FA on growth and pathogenicity of FOF in vivo were also examined.

Results Compared to the 0 mg/L, the FA treatment significantly reduced the growth of faba bean seedlings, the higher the treatment concentration, the stronger the inhibition; in addition, FA treatments increased the incidences of Fusarium wilt by 300.0%–500.0% and the disease index by 113.3%–1666.7%, with the maxima at FA concentration of 200 mg/L. The FA inhibited the mycelium growth of FOF, but significantly increased the activities of pectinase, cellulase, amylase and protease secreted by FOF, with increase ranges of 44.8%–59.0%, 78.2%–145.6%, 975.6%–2435.4% and 165.1%–622.9%, respectively, and significantly stimulated fusaric acid secretion of FOF by 107.6%–236.2%. Under FA stress, the epidermal cells of faba bean roots were distorted, the xylem vessels became thinner, and the duct wall thickened. The whole cells were fully filled with jelly and inclusions, hindering the normal transportation of nutrients and water, thus accelerating the death of the faba beans.

Conclusions FA inhibits the growth of FOF, but stimulates the mycotoxin production and the activities of pathogenesis-related hydrolytic enzymes (pectinase, cellulase, amylase and protease), which is related to the damage of root tissue structure and benefit the invasion of FOF, and accelerate the incidence of Fusarium wilt consequently. Therefore, FA could be involved in plant–pathogen allelopathy as a stimulator.