Symptoms and Signs
Common rust causes pustules that may appear on any aboveground part of the corn plant but they are most abundant on leaves. The pustules, which may erupt and become powdery, occur nearly simultaneously on both leaf surfaces. They are golden brown to cinnamon brown, becoming black as the spores mature.
Comments on the Disease
Mild temperatures (60° to 73°F) and high relative humidity (near 100%) favor rust development. Initial inoculum is probably windblown spores from subtropical and tropical areas where the pathogen persists on living corn plants. The alternate host of the fungus, Oxalis species, probably does not play an important role in the disease cycle in California.
Management
Cultural Control
Use resistant hybrids to prevent common rust. Generally, fungicides are not required under California conditions.