Southern rust pustules look similar to common rust. They typically are more abundant than common rust and generally occur only on the upper leaf surface, often giving the upper leaf a dusty appearance. Brown pustules (uredia) are often surrounded by black pustules (telia) on husk leaves, ear shanks, and stalks. Southern rust does not overwinter in Kansas. Spores blow up from southern production areas in mid- to late-July. Warm, humid weather favors infection. Resistant hybrids are the best choice for management. If susceptible hybrids are planted late and disease conditions are favorable, applications of a systemic fungicide may be warranted.
Taken from Corn Production Handbook, KSU Extension
Pest photo source
Diagnosing Corn Problems in Kansas, KSU Extension