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Crop rotations are widely used because they can significantly reduce the incidence of pests and diseases.The interactions between non-host roots and pathogens may be key in the inhibition of soilborne pathogens in crop rotations.Interactions between fennel (Foeniculum vulgare) roots/root exudates and Phytophthora nicotianae were investigated because of the known allelopathy between fennel and tobacco (Nicotiana tabacum).The effects of the key compounds in the fennel rhizosphere on the mycelial growth and zoospore behavior of P.nicotianae were assessed.The roots of fennel attracted P.nicotianae zoospores and inhibited their motility and the germination of cystospores,with some cystospores rupturing.4-ethylacetophenone,vanillin and N-formylpiperidine were consistently identified in the fennel rhizosphere and were found to interfere with the infection of P.nicotionoe,especially vanillin.Hyphae treated with these compounds produced more abnormal branches and accumulated reactive oxygen species.These interspecific interactions between non-host roots and pathogens were found to be an important factor in the inhibition by fennel of infection by P.nicotianae.