Incidence of Fusarium wilt in Cucumis sativus L. is promoted by cinnamic acid, an autotoxin in root exudates

The effects of Fusarium oxysporum f. sp. cucumerinum, the pathogen causing Fusarium wilt in cucumber and cinnamic acid, a principal autotoxic component in the root exudates of cucumber (Cucumis sativus L.), on plant growth, Photosynthesis and incidence of Fusarium wilt in cucumber were studied in order to elucidate the interaction of autotoxins and soil-borne pathogens in the soil sickness. E oxysporum. f. sp. cucumerinum (FO) and cinnamic acid (CA) at 0.1 or 0.25 mM significantly decreased net photosynthetic rate, stomatal conductance and the quantum yield of Photosystem II photochemistry (Phi(PSII)), followed by a reduction of plant biomass production, but did not induce photoinhibition. Pretreatment with CA before inoculation with FO increased the effectiveness of FO, together with a slight photoinhibition. CA pretreatment significantly increased percentage of plants affected by Fusarium wilt, browning index of vascular bundle and Fusarium population in the nutrient solution. All these results indicate that CA enhanced Fusarium wilt by predisposing cucumber roots to infection by FO through a direct biochemical and physiological effect. It is likely that soil sickness results from an interaction of many factors.