Southern Rust

Puccinia polysora

Southern rust infection on a corn leaf
Southern rust infection on a corn leaf. Photo courtesy of Scot Nelson. Licensed under CC BY-SA 2.0.
Southern rust is a potentially significant fungal disease of corn. It thrives under hot, humid conditions and hence is more common in the southern United States. However, under the right conditions it can develop farther north. It causes the most damage to younger corn and hence is particularly severe in late-planted corn.
Southern rust is characterized by small pinhead size red or orange lesions. Unlike common rust, the lesions are found only the upper surface of the leaves (although there may be some yellow discoloration on the lower surface) and may also be found on the husks.
Southern rust reduces the photosynthetic area of the leaves and also ruptures the leaf surface resulting in water loss. This leads to a weakened plant that may have poor kernel fill and is susceptible to stalk rot and lodging.
Fungicides can be used to treat southern rust. They are most effective if applied at the first signs of disease.