The invention discloses application of 
guaiacol to preventing and treating tobacco diseases and a method for applying the 
guaiacol. The application and the method have the advantages that the 
guaiacol can be applied to the field of 
agriculture, the tobacco diseases can be prevented and treated by the aid of the guaiacol, and accordingly novel application of the guaiacol is developed; the guaiacol, 
chitosan and other auxiliaries can be sprayed in a matched manner when the guaiacol is applied, and accordingly the tobacco diseases can be effectively prevented and treated; the guaiacol is low in production cost as compared with existing other 
disease-preventive and 
disease-resistant pesticides, and accordingly the application and the method are worthy of being popularized.