Description and Symptoms
Southern rust (Puccinia polysora) often occurs in Iowa towards the end of the growing season and is seldom severe enough to cause yield loss. Symptoms are similar to common rust, but pustules are smaller and occur almost exclusively on the upper leaf surface. Pustules are usually circular or oval, very numerous, and densely scattered over the leaf surface. Spores are orange when they erupt from the pustule. As pustules age, they become chocolate brown to black, often forming dark circles around the original pustule. Spores do not continue to erupt through the epidermis. The disease is favored by high humidity and temperatures around 80°F. Resistant hybrids and inbreds are available. Crop rotation and tillage reduce survival of the fungus. Foliar fungicides labeled for southern rust are available.
The best time to scout for southern rust is VT through R4; earlier in seed production fields. Southern Rust is easily confused with common rust. Orange pustules predominantly on upper leaf surface are diagnostic for southern rust.
The best time for southern rust to appear is in high humidity, temperatures around 80 degrees Fahrenheit, and leaf wetness for six or more hours.
Photo by Adam Sisson