The wind-blown spores of this fungus fall on the corn plant, germinate and infect the plant. These pustules erupt, revealing the orange-red spores that will be dispersed by the wind to cause another cycle of infection. As the level of infection increases, the leaves are essentially covered by these pustules and the leaves are killed. Often the plants look like it should be time to harvest the crop just because the rust has killed the plant tissue prematurely. The pustules of common rust are elongated and spores are a darker brick-red color.
Common rust is much more prevalent in the spring and early summer.
In the tests grown at the Mississippi State location, the development of southern corn rust from natural inoculation was extensive by the time ratings were taken on August 9, 1994. Natural inoculation usually is not as uniform over an entire test area as when all plants are manually inoculated with spores at a given time. All plants had susceptible-type pustules, but not all plants were equally infected. Each two-row plot of corn was rated for relative severity of southern corn rust using a rather arbitrary visual scale of 1 to 5, with a 1 indicating the least and a 5 indicating the most severe infection and damage level. On this scale, primary considerations were for the amount of leaf tissue killed and the extent of pustule coverage especially on the upper leaves of the plant.