Nanoparticle compositions for treating citrus greening disease and other plant diseases include a liquid or gel carrier and metal nanoparticles dispersed therein. The metal nanoparticles can be spherical-shaped and/or coral-shaped. Methods of treating plant diseases include applying a nanoparticle composition to an infected plant part to kill the microbe causing the disease. The method may further include removing an infected plant part, such as a branch, treating the infected plant part with a nanoparticle composition, and grafting the plant part (branch) back onto the plant. The plant may particularly be a citrus tree.