Why Do My Corn Cobs Have Grey-Black Growths?

Large, irregular, silvery-white to grey-black swellings on the cob, tassels, leaves or stalk of a corn plant are the unmistakable sign of corn smut (Ustilago maydis), a fungal disease that hijacks the plant's tissue and replaces it with masses of spores. It is alarming to look at but fascinating once you understand what it is — and in some cultures it is considered a culinary delicacy.

What corn smut is

Corn smut is caused by the fungus Ustilago maydis. When its spores land on a wound or young tissue of a corn plant — typically entering through insect feeding damage, hail damage, mechanical wounds or natural openings — the fungus colonises the tissue and stimulates the plant cells to produce the characteristic galls. These galls start as white, firm, silvery structures and mature into grey-black masses of powdery spores that burst open and disperse, infecting other plants. The fungus can affect any above-ground part of the plant — the cob, tassels, leaves and stem.

Is corn smut edible?

Young smut galls harvested while they are still white and firm inside are the Mexican delicacy huitlacoche (also spelled cuitlacoche). They have an earthy, mushroom-like flavour and are valued in Mexican cuisine. Once the galls have turned grey-black and begun to release spores, the culinary window has passed and the galls should be removed and disposed of. Eating mature, sporulating smut is not recommended.

Removing galls before they sporulate

The single most important management action is removing galls before they burst and release spores. Once they turn grey and begin to crack open, spores disperse into the soil where they can survive for several years. Bag and dispose of galls in household waste — do not compost them. Remove infected cobs or plant parts early when the galls are still white and firm.

Reducing smut incidence

Smut risk is higher in dry, hot years; in fields with heavy insect damage that creates entry wounds; and in soils with excessive nitrogen that promotes lush, soft tissue. Rotate corn on a three- to four-year cycle, as smut spores persist in soil. Avoid over-feeding with nitrogen. Minimise mechanical damage to plants. Some varieties have partial resistance — check catalogue descriptions if smut has been a recurring problem.