In this study, we observed the interaction between maize root and zoospores of Phytophthora nicotianae, the inhibitory activity of maize root exudates against P. nicotianae and the antifungal compounds in the maize root exudates were tested and analysed using HPLC-MS. The results indicated that maize root could not only attract zoospores into the rhizosphere, but also secret some substances to inhibit the swimming of zoospores or cause the rupture of cystospores. Further studies indicated that maize root exudates showed inhibitory activity against the release of zoospores from sporangia, the germination of cystospores, and the hyphal growth. The inhibition rate was 73.8%, 87.2% and 55.2%, respectively, when the root exudates concentration was 0.90 mg/mL. DIMBOA, 2,4-dihydroxy-7-methoxy-2H-1,4-benzoxazin-3(4H)-one and its degradation substance MBOA (6-methoxy-2-benzoxazolinone) were identified in root exudates by HPLC-MS. MBOA also showed antifungal activity against the hyphal growth of P. nicotianae. The inhibition ratio reached 90.94% when the concentration of MBOA in media was 0.30 mg/mL. In summary, maize root can not only attract the zoospores of P. nicotianae, but also secret the antifungal compounds DIMBOA and MBOA. These results support maize and tobacco crop rotation as a theoretical control method for tobacco black shank in tobacco growing.