Corn Smut

Corn Smut (Ustilago maydis) is a basidiomycete in the Ustilaginales, the smut fungi, which parasitise grasses and produce dark, powdery masses of thick-walled spores in place of normal plant tissue. This species is specific to maize (Zea mays) and its wild relative teosinte, and it is one of the most intensively studied fungi in biology because its life cycle can be completed in the laboratory.

Instead of forming a mushroom, the fungus induces the plant to build the fruiting structure for it. Infected tissue swells into a gall covered by a tough, silvery-white to greenish-grey membrane derived from the host. Inside, the tissue is replaced by a mass that begins white and firm and darkens to a bluish-grey and finally to a sooty black spore powder.

Galls can form on any actively growing above-ground part: ears, individual kernels, tassels, leaves, stalks and even aerial nodes. Ear galls are the largest, sometimes reaching the size of a fist, while leaf galls stay small, hard and blister-like.

Galls

Irregular, rounded to lobed or finger-like swellings, from a few millimetres on leaves to 10-15 cm on ears. The outer membrane is smooth, tough and silvery-white, sometimes flushed green, grey or purple where exposed to sun, and it stretches tight as the gall expands.

Interior and spore mass

Cut open when young, a gall is firm, moist and whitish to pale grey with a fleshy texture. As it matures the interior darkens through blue-grey to a dry, sooty black mass of powdery spores that puffs out when the membrane splits.

Where galls form

Most conspicuous on ears, where individual kernels swell enormously and distort the cob, and on tassels, where whole florets are replaced. Leaf and stalk galls are smaller, harder and often crowded in rows along the veins.

Spores

There is no spore print. Teliospores are dark brown to black, roughly spherical and covered in tiny spines, visible under a lens as a fine dust. They are dispersed by wind, rain splash and machinery, and survive in soil and crop residue for years.

Corn Smut occurs on maize wherever it is cultivated, from the Americas through Europe, Africa and Asia, as well as on teosinte in its native range in Mexico and Central America. It is a fungus of farm fields, garden plots and roadside volunteer maize rather than of woodland.

Infection favours warm weather and moderate moisture, and is heaviest where plants have been injured by hail, insects, cultivation or detasselling, since wounds provide entry points. Galls develop during the warm growing season, typically from mid summer into early autumn, and rupture as the crop matures and dries.

Ustilago maydis is a biotrophic parasite that requires living maize tissue. Its life cycle begins with wind-blown teliospores that germinate to produce haploid, yeast-like sporidia. Two compatible sporidia must fuse to form the infectious filamentous dikaryon, which penetrates young meristematic tissue and grows between and within host cells.

The fungus then manipulates the plant's growth regulators, causing the local tissue to swell and divide into the gall that houses the developing spores. Only inside this gall does the fungus produce teliospores. These overwinter in soil and residue and are carried by wind and rain to new plants the following season, so the fungus is dispersed but does not spread systemically through a plant from a single infection.

Head smut of maize, caused by Sphacelotheca reiliana, replaces the whole ear or tassel with a mass of black spores held together by dark thread-like vascular strands, and lacks the smooth silvery membrane and swollen individual kernels of common smut. False smut and various ear moulds produce coloured fuzzy growths on the surface of kernels rather than galls that replace them.

Other Ustilago species attack different grasses, so host is a strong character: loose smut of wheat and barley destroys the developing head into a bare dusty spike, quite unlike a fat membranous gall. Physical injuries and insect galls on maize can swell tissue in a similar way but never contain a powdery black spore mass when cut open.