Favored by cool to warm, wet, humid weather, continuous corn with reduced tillage. Cleanly plow under infected residue. Initial symptoms are chlorotic flecks on leaf surfaces. Flecks develop into oval to elongate reddish brown powdery pustules on upper and lower leaf surfaces. Reddish brown spores break through the leaf epidermis.
Pustules become brownish-black as they mature.
Usually not a serious disease in hybrids. Spores blown into the Midwest from the South. Does not survive winter in Indiana, except possibly in rare years along the Ohio River. Foliar fungicides may be useful in seed production fields. Similar to common rust except pustules occur almost exclusively on the upper leaf surface, rarely on lower.