Fusarium head blight

What you are looking at

An ear of wheat in which some spikelets have gone bleached white or pale straw while the rest is still green. The bleaching often takes everything above one point on the ear, so a green ear carries a white tip, and the stem below stays green.

In damp weather there is a pink or salmon-orange mould at the base of the affected spikelets, which is the clearest confirmation there is.

Open a bleached spikelet and the grain inside is shrunken, chalky white or faintly pink, and light. Those are the grains that blow out with the chaff.

The damage is patchy across a field rather than even, and none of it shows until about a fortnight after flowering.

The fungus dies and the toxin stays

Most crop diseases are a yield problem. This one is a food safety problem that also costs yield, and the reason is chemistry rather than biology.

Fusarium graminearum and its relatives produce mycotoxins as they grow in the developing grain. Deoxynivalenol is the one usually measured, with zearalenone and others alongside it depending on the species.

These compounds are stable. They survive harvest, drying, storage, milling and baking. The fungus is long dead by the time flour is made and the toxin is still in it, which is why most jurisdictions set legal maximum levels for grain and grain products, and why a merchant tests rather than looks.

Deoxynivalenol is sometimes called vomitoxin, and pigs are the animal it affects most. They refuse feed and lose condition at concentrations that cattle, sheep and poultry tolerate, so where contaminated grain has to be used on farm, which stock it goes to is a genuine decision.

The practical consequence for a grower is that visible damage is a poor guide. Grain that looks acceptable in the hand can be over the limit, and grain that looks poor can be within it. After a wet flowering, the sample gets tested.

Everything happens in the flowering window

Infection needs the flowers open and the weather warm and wet at the same time. Spores land on the exposed anthers, which are unusually easy tissue for a fungus to colonise, and grow from there into the developing grain.

That window is short, often only a few days for any one crop. A wet spell falling in it produces a bad year, and the same wet spell a fortnight later or earlier produces almost nothing.

Two things follow. Watching the weather during flowering tells you more than inspecting the crop, because the crop shows nothing for another fortnight. And splitting the flowering dates across a farm, with different varieties and drilling dates, means one unlucky week cannot catch everything at once.

Maize in the rotation is the thing to change

Fusarium graminearum survives and produces spores abundantly on crop residue, and maize stubble is by a distance the best material it has.

Wheat drilled into or immediately beside last year’s maize takes far more infection than wheat following a break crop, and the rise of maize in arable rotations has tracked the rise of this disease across Europe and North America.

The choices that matter are the rotation and the cultivation. Put a non-cereal break between maize and wheat. Plough the residue in rather than leaving it on the surface, since minimum tillage after a susceptible crop leaves the inoculum exactly where spores can reach the next ear.

Variety resistance is partial and worth having on top of both, particularly on land where maize is in the rotation.

What to do about it

Keep wheat out of maize stubble, and put a break crop between them.

Plough residue in rather than leaving it on the surface.

Pick a variety with a decent resistance rating where the land has a history.

Spread flowering dates across varieties and drilling dates.

Record the weather during flowering, since that is when the year is decided.

Harvest promptly rather than leaving a ripe crop out in the wet.

Turn the combine fan up to blow the light shrivelled grains out.

Dry the grain down promptly and watch the store for damp patches.

Have the sample tested after a wet flowering, before it is sold, milled or fed.

Keep suspect grain away from pigs.

What it gets mistaken for

Ergot replaces a grain with a hard black horn projecting from the ear.

Barley yellow dwarf stunts plants in circular patches and colours the leaves in spring.

Fusarium wilt is the same genus blocking the plumbing of tomatoes and melons.

Frost at flowering bleaches spikelets on the night it happens, across the exposed part of the field.

Rust raises pustules that rub off onto your fingers.