Provided is a photocatalytic composition comprising zinc (Zn) doped titanium dioxide (TiO2) nanoparticles, wherein the ratio of titanium dioxide nanoparticles to zinc is from about 5 to about 150. The photocatalytic composition absorbs electromagnetic radiation in a wavelength range from about 200 nm to about 500 nm, and the absorbance of light of wavelengths longer than about 450 nm is less than 50% the absorbance of light of wavelengths shorter than about 350 nm. Further provided is a method for treating or preventing microbial diseases and infestations in a plant and a method for increasing crop yield of a plant by applying the photocatalytic compositions taught herein to the surface of a plant. Also provided is a method for treating microbial diseases on a surface by applying the photocatalytic compositions taught herein to a surface illuminated by artificial light.