Disclosed are photocatalytic compositions comprising zinc (Zn) doped titanium dioxide (TiO2) nanoparticles, wherein the ratio of titanium dioxide nanoparticles to zinc is from about 5 to about 150. Also disclosed are photocatalytic compositions where the titanium dioxide is further doped with silicon dixoide (SiO2). Further provided are methods for treating or preventing microbial diseases and infestations in plants and methods for increasing crop yield of a plant by applying the photocatalytic compositions taught herein to the surface of a plant. A method for treating microbial diseases on a surface applies the photocatalytic compositions taught herein to a surface illuminated by artificial light.
