Producers in the southwestern third of Iowa should be watching for southern rust on corn.  On August 13, a field in Taylor County was treated; nearly all plants had southern rust lesions.  Southern rust is more of a concern than common rust on corn, and positive identification is critical.  

Key differences between the two rusts are:

• Southern rust pustules are usually only found on the upper leaf surface, compared with common rust pustules that are found on the upper and lower leaf surfaces.
• Southern rust pustules tend to be round, whereas common rust pustules are typically more elongated.
• Southern rust spores are orange to orange brown and pustules tend to be more densely clustered, while common rust has brick red spores and pustules tend to be more scattered on the leaf.
• Southern rust is favored by warm (77—82°F) and humid conditions while common rust is favored by cooler temperatures.