It is recognized by its characteristic orange-red pustules that erupt from the upper surface of leaves. Rust spots are very small, circular or elongated, and are vivid orange red in colour. They may occur on stems but are most common on the upper surface of the leaves. The spores often form the core of rain drops and initially infect and produce pustules where the raindrops have collected at leaf axils and tips. When heavily infected, the whole leaf will die.
Winds can carry rust spores for great distances.
As the plant matures, the rust pustules darken and may appear black. Scouting Techniques Look for small, round, orange pustules on the leaves and leaf sheaths. As the plant matures, the pustules darken. In some states in the USA, fungicide application is recommended if there is an average of one leaf rust pustule per leaf. Spraying is recommended if the threshold is met in three of the five spots sampled in the field.