Corn smut

Ustilago maydis

also called common smut, huitlacoche, maize smut

Grotesque silvery-gray galls swelling from the cobs, tassels and stems of sweetcorn, bursting to release a mass of sooty black spores.

Fungus Serious Airborne sporesSoil-borneRain splash Spreads to neighbors

What you'll see

Swollen galls, at first covered in a smooth silvery-white or gray skin, on cobs, kernels, tassels and stems.
The galls grow large and fleshy, then rupture to release masses of powdery black spores.
Kernels on the cob are replaced by these bloated gray growths.
Young plants may be stunted or distorted where infection strikes early.
The exposed black spore dust stains everything it touches.

Why it happens

A fungus of maize and sweetcorn. Its spores are tough and survive for years in the soil and on debris; they are blown or splashed onto the plant and enter through wounds and through the silks, especially where growth is soft and lush.

Warm weather, and plants stressed by drought then heavy watering, or by wounding, mechanical damage and nitrogen-rich feeding, are most prone. Hail and cultivation injuries open the door.

Treating it

Cut out and bin the galls while their skin is still intact, before they burst and shed spores.
Never compost infected material — the spores persist for years.
Avoid wounding plants during cultivation and hoeing.
Remove badly affected plants entirely to protect the rest of the block.
Where prized as a delicacy, the young firm galls (huitlacoche) are gathered and eaten before they turn to spore dust.

Keeping it away

Rotate sweetcorn onto fresh ground and clear all debris after harvest.
Grow resistant varieties where smut is a known problem.
Water evenly to avoid the stress that invites infection, and don’t over-feed with nitrogen.
Handle plants carefully to avoid the wounds the fungus needs.
Destroy infected plants rather than let galls mature in the plot.

Who gets it

Sweetcorn and maize only; it does not spread to other crops.