Southern rust of corn
Southern rust is caused by the fungus Puccinia polysora. Although generally considered a tropical disease, southern rust can occur in important corn production areas of the United States and Canada. 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.
The disease is favored by high humidity and temperatures around 80°F. Southern rust pustules on corn leaf. Image: A. Sisson
Southern rust pustules on leaf sheath. Image: A. Robertson
Southern rust disease cycle.
Resistant hybrids and inbreds are available. Foliar fungicides labeled for southern rust are available. The Integrated Pest Information Platform for Extension and Education (iPiPE) helps track the movement of southern rust throughout the season. It is available online at ext.ipipe.org.