Common Smut is caused by the fungus Ustilago maydis. The fungus infects actively growing corn tissue and produces swollen galls on ears, tassels, leaves and stalks. Young galls are firm and pale or silvery-white, but as they mature they fill with millions of dark spores and eventually rupture. The spores can survive in soil and crop debris and provide infection in future crops. Wounds caused by cultivation, hail or other damage can increase infection, and excessive nitrogen can encourage disease.

How to Fix It
Remove galls early: Cut off and dispose of galls before they rupture and release their black spores.
Don't compost mature galls: Once a gall contains black spores, keep it out of the compost pile to avoid spreading spores around the garden.
Avoid injuring plants: Cultivate carefully around corn and avoid damaging stalks and roots.
Don't overdo nitrogen: Excessive nitrogen can increase smut development.
Choose resistant varieties: If Common Smut repeatedly causes serious damage, try a resistant or tolerant corn variety.
Keep corn growing steadily: Healthy, steadily growing plants are less susceptible to severe smut infection.