Aspergillus ear rot
Infected kernels have masses of yellow green spores on them. The tip of the ear is most often infected but colonies may appear anywhere on the ear. Symptoms are most severe in continuous corn. Infection is favored by hot, droughty conditions during grain fill. Insect damaged ears and stressed plants are most susceptible. Supplemental irrigation. Tillage to eliminate corn debris. Caution: Aspergillus colonies can produce aflatoxin.
Taken from Corn Production Handbook, KSU Extension
Pest photo source
Diagnosing Corn Problems in Kansas, KSU Extension