Concept:Maize rust is a fungal disease caused by a pathogen from the genus Puccinia.Explanation:Rust diseases are common plant infections caused by fungi.Maize rust appears as reddish-brown pustules on the leaves of the plant.These pustules are produced by the fungus Puccinia sorghi.Since the causative organism is a fungus, it cannot be a bacterium, protozoon, or virus.Answer:D. fungus