The causative organism of maize rust is a fungus. Maize rust is a fungal disease that affects maize crops, and it is caused by the fungus Puccinia polysora. The fungus attacks the leaves of the maize plant, causing yellowish-orange pustules or lesions on the upper surface of the leaves. These pustules eventually turn brown and release spores, which can spread the disease to other plants. Maize rust can significantly reduce crop yield and quality, and it is important for farmers to take measures to control its spread, such as planting resistant maize varieties and practicing good crop management practices.