By Alison Robertson
Southern rust is a leaf disease of corn that is caused by the fungal pathogen, Puccinia polysora. The disease is recognized as clusters of small, circular (1/16” diameter) spots that produce thousands of bright orange urediniospores on the upper side of the leaf (Fig. 1). With southern rust, there are multiple disease cycles during the growing season. As the crop matures, the pathogen switches from producing orange spores and starts to produce black teliospores, which cannot infect corn.
Puccinia polysora urediniospores cannot survive on corn residue or in soil. Every growing season, spores are blown up to Iowa from Central America via the southern states. Warm (77-85oF), very wet conditions favor infection and disease development. As temperature and leaf wetness duration increase, so does southern rust severity (Hollier and King, 1985). Hybrids vary in their resistance to P. polysora, but since southern rust is rarely a problem in Iowa, seed companies may not have published resistance ratings.
Source : iastate.edu