Huitlacoche, the disease that became a delicacy

A sweetcorn cob infected with Ustilago maydis stops being sweetcorn. The kernels swell into lumpy grey galls, sometimes as big as a fist, and a British grower pulls the cob off, bins it and counts the loss. An American extension service calls it corn smut and tells you to destroy affected plants.

In Mexico the same cob is worth more than the sweetcorn would have been. It is called huitlacoche, it has been eaten since before the Spanish arrived, and it turns up in quesadillas, soups and restaurant tasting menus. Growers in Mexico and in parts of the United States infect their crops on purpose.

What it is

Ustilago maydis is a fungus that lives on maize and on its wild ancestor teosinte, and on nothing else. It gets in through the silks or through a wound, and the plant responds by inflating the affected kernels into galls.

Inside, the gall starts firm and pale, marbled white and blue-grey, with a texture close to a fresh mushroom. Left on the plant it matures, blackens, dries to a papery shell and splits, releasing spores that survive in the soil for years. Everything about its life cycle is ordinary plant pathology.

Cooked young, it tastes dense and earthy, somewhere between a mushroom and a truffle, with a smokiness and a faint sweetness from the corn sugars the fungus has been living on. It turns inky black in the pan and colours whatever it is cooked with.

Why one country’s disease is another’s crop

The Aztecs ate it, and the name is Nahuatl. The usual gloss is unflattering, along the lines of corn excrement or sleeping filth, which suggests the appreciation was practical rather than romantic. It was food that appeared in a bad year, and it was good.

When maize crossed to Europe and to North America, the fungus went with it and the culinary tradition did not. Nineteenth and twentieth century agriculture classified it as a smut, which is what it is, filed it with the smuts of wheat and barley that ruin grain and are inedible, and set about breeding it out. Resistant varieties, clean seed and crop rotation followed. Nobody in Iowa was weighing up whether it might be nice with epazote.

So the same organism ended up on two different lists, and what put it there was what people in each place were prepared to eat. Nothing changed about the fungus.

The honest version for a British grower

We have written the pathology up properly at Ergot and smut, which covers what to do if you find it and why the spores are worth keeping out of your soil.

Take the galls while they are firm and pale, at around two to three weeks after they appear. A gall that has gone black and papery is a bag of spores and is past eating, whatever you were hoping.

Removing them early is the right thing to do agriculturally regardless of whether you eat them, because a burst gall puts spores in the ground that will be waiting for the next crop of sweetcorn.

And a British summer is not the weather this fungus wants. It does best in heat and drought broken by rain, so most years you will not see it at all. Do not plan a menu around it.

The safety line, which is short but firm

Corn smut is not poisonous. People have eaten it for centuries.

The caution has nothing to do with the smut and everything to do with a different fungus that also replaces grain in an ear. Ergot produces hard, dark, horn-shaped bodies on rye, wheat, barley, oats and wild grasses, it is poisonous, and the ergot and smut page covers its long and grim history. Ergot does not occur on sweetcorn in Britain, so if the plant is sweetcorn and the growth is soft and grey, the two cannot be confused.

Where it goes wrong is enthusiasm crossing crops. Something black in an ear of barley is not a lunch opportunity. Identify the plant first, and if you are not certain what you have, do not eat it.

Why this one is here rather than in the problem pages

Most of what we publish tells you what has gone wrong and how to stop it, and that part of the site now runs to 79 pages. Corn smut has its entry there and the advice on it is sound. What would not fit is everything above, where the diagnosis is right, the control advice is right, and the conclusion still depends on where you happen to live.

We like that it undercuts the category a little. “Pest” and “disease” describe a relationship between a plant, an organism and somebody’s plans, and the third term in that list moves around more than the writing usually admits.