The extract from Hedyotis diffusae was used to treat tobacco plants infected by TMV before and after incidence. The effect of the changes of some physiological and biochemical indices related with the virus-resistance in tobacco leaves on tobacco quality ware determined. The resuits showed that H. diffusae extract could remarkably increase the total phenol content in tobacco leaves ,strength the activity of phenylalanine ammonialyase （ PAL） , improve the membrane permeability and tobacco quality. Prevention treatment was better than therapy treatment. In conclusion, H. diffusae extract could induce plants to produce disease resistance and strength tha antiviral capability of tobacco against TMV.
