Abstract: In order to find effective measures to control tobacco black shank, effects of nitrate, ammonium and their different ratios on spore germination and mycelial growth of tobacco black shank as well as the secondary metabolism and disease resistance of tobacco were studied. The results showed that the germination of zoospores of tobacco black shank was accelerated by nitrate, ammonium and their mixed solutions, and the mycelia of tobacco black shank grew better in their mixed solutions. With the increasing of ammonium concentration in solutions, the contents of lignin, cellulose and MDA and the activities of SOD, POD and PAL in tobacco roots increased significantly, while the incidence and the disease index of tobacco black shank decreased significantly. The correlation analysis showed that there was a extremely significantly negative correlation between the incidence/disease index of tobacco black shank and the contents of lignin and MDA and the activities of SOD, POD and PAL(p<0.01), and there was a remarkable negative correlation between the incidence of tobacco black shank and the content of cellulose. So the results revealed that the incidence and the disease index of tobacco black shank are closely related to the secondary metabolites, which can be significantly reduced by ammonium application. However, a large number of field trials need to be carried out to study the optimal ratio of nitrate and ammonium nitrogen under field conditions.