Corn Smut Identification Guide

What You Are Actually Looking At

Set your expectations before you start, because this fungus does not build a mushroom. There is no cap, no stipe, no gills or pores, and no spore print in the usual sense. Ustilago maydis is a smut - a parasitic basidiomycete that lives inside the tissues of a single host plant and produces its spores in a mass rather than on an exposed spore-bearing surface. The whole identification therefore runs on the shape, surface, interior and spore characters of the swelling it induces, and on which host plant it is sitting in.

The setting is a field, allotment or garden row of maize in mid to late summer. Walking the rows, you notice a plant that looks wrong: an ear that bulges irregularly instead of tapering smoothly, or a tassel with a lumpy grey growth where the flowers should be, or a leaf blade with a row of blisters running along it. Move in and the diagnosis is quick.

Reading the Gall Instead of a Cap

The structure to examine is the gall, and its equivalent of the cap surface is the outer membrane. Individual maize kernels swell into fleshy tumours anything from one to fifteen centimetres across, sometimes fusing so that a whole ear becomes one distorted mass the size of a fist or larger. Galls also form on the tassel, on leaf blades and midribs, on stem nodes and even on aerial prop roots, and they are always largest and most numerous on the ear.

The surface is smooth, slightly shining and taut, silvery-white to pale grey or grey-green, sometimes flushed olive, purplish or bronze where sunlight strikes it. This skin is a true peridium - a composite membrane built from host epidermal tissue and fungal hyphae - and it is what makes a smut gall look so unlike a bacterial swelling or an insect gall. Young galls feel firm and heavy for their size, with a smooth rubbery resistance under the thumb. Older ones dull, wrinkle, and eventually split.

The attachment is the equivalent of a stipe base: the gall is continuous with the plant tissue it replaced, sitting on the cob axis where a kernel would have been, or set into a leaf blade with the surrounding leaf tissue running unbroken around it.

The Interior and Its Colour Sequence

Cut a gall in half with a knife, and read the interior as you would read the flesh of an agaric. The colour sequence is the most reliable single character, and it runs one way only.

A very young gall is white to pale cream inside, firm and marbled, with visible streaks and pale veining. Within days it becomes grey and softer, the marbling breaking up. Next the interior turns olive-grey to greenish-brown as spore formation begins. Finally the whole inside converts to a loose, dry, sooty mass of powder in dark olive-brown to black-brown. At that point the outer membrane ruptures, the gall collapses, and dark dust spills over the plant and the ground.

Cut surfaces of young galls also discolour in air, drifting from white toward grey and olive within minutes to hours, so a photograph taken immediately after cutting will not match one taken later the same day.

Reading the Spores Without a Print

There is no way to lay a gall on paper and get a conventional print, but the equivalent test is easy. Hold a mature ruptured gall over a sheet of white paper and tap it. A fine dust falls out, dark olive-brown to blackish in mass and faintly greenish in a thin scatter. Rub a little between finger and thumb and it feels dry and powdery, not sticky.

Under a microscope those teliospores are diagnostic and easy to see even at moderate magnification. They are globose to broadly ellipsoid, roughly 8 to 12 micrometres across, olive-brown to dark yellowish-brown individually, with thick walls that are distinctly and evenly spiny - covered in fine, closely set echinulate warts. That combination of size, colour and clear spininess is what separates this from smut fungi with smooth or merely roughened spores.

Host, Season and Ecology

The host range is extremely narrow: cultivated maize, Zea mays, and its wild relative teosinte. Nothing else. A comparable smut on wheat, oats, barley, sorghum or a wild grass is a different species in a different genus, and host identity is therefore a hard constraint rather than a soft hint.

The fungus overwinters as teliospores in soil and in old plant debris. Those spores germinate to produce sporidia - small yeast-like cells - which are spread by wind and rain splash. When sporidia of compatible mating types meet on a maize plant they fuse, and only the resulting dikaryotic hypha can penetrate the host. The fungus can enter only actively dividing meristematic tissue, which is why galls appear on young ears, tassels, growing points and expanding leaves rather than on mature stem tissue.

Infection is local rather than systemic: each gall represents a separate infection point, and one plant may carry galls in several places while its neighbour has none. Galls become obvious from midsummer through early autumn, roughly from silking onward. Wounding of any kind favours the fungus - hail damage, insect feeding, wind abrasion, mechanical injury - and infection is generally heavier where nitrogen is high and where a dry spell is broken by heavy rain.

Separating It From Other Maize Smuts and Look-alikes

Head smut, Sporisorium reilianum, is the main confusion, and the difference is structural. Head smut is systemic: it replaces the entire tassel or the entire ear with a dusty dark mass held loosely together by a skeleton of stringy vascular strands, and the affected inflorescence is deformed as a whole, often proliferating into leafy shoots. It does not produce discrete, fleshy, silvery-skinned galls in place of individual kernels. If you can see a normal cob axis with sound kernels next to swollen ones, you are looking at Ustilago maydis.

Smuts on other cereals - loose smut and covered smut of wheat, barley and oats - can look broadly similar in the abstract, but they are on the wrong host, and their spore ornamentation and size differ under the microscope.

Ear-rot moulds such as Fusarium, Gibberella and Aspergillus grow as pink, white, grey or greenish felty mould over the surface of otherwise normally shaped kernels. They do not swell the kernels into smooth-skinned tumours, and there is no dry sooty spore powder inside a membrane.

Bacterial soft rots produce collapsing, wet, foul-smelling tissue rather than a firm dry gall. Insect galls and physical damage from hail or birds leave torn or scarred tissue with no internal spore mass at all. In every ambiguous case, the answer is to cut the swelling open and look for the white-to-grey-to-black interior sequence and the powdery spore mass inside a silvery membrane.

The Short Version

Maize only; individual kernels or growing points swollen into smooth silvery-grey galls of one to fifteen centimetres; interior passing from firm white marbling through grey and olive to dry sooty black powder; membrane finally rupturing; spores globose, olive-brown and finely spiny at around 8 to 12 micrometres; midsummer to early autumn, concentrated on young meristematic tissue and on plants that have been injured.