Common Bunt
Tilletia caries
Also called stinking smut, covered smut of wheat, hill bunt
A smut fungus that hollows out wheat kernels and refills them with a mass of dark brown spores, producing fragile grey bunt balls with a distinctive fishy odour.
Overview
Common bunt is caused by Tilletia caries (syn. T. tritici), with the very similar Tilletia laevis (T. foetida) often found alongside it. These are smut fungi in the Tilletiales, basidiomycetes that live inside their grass hosts and convert developing seed into spore masses.
Unlike the loose smuts, which expose their spore powder openly on the ear, bunt keeps its spores enclosed. The kernel is replaced by a bunt ball, a structure the size and rough shape of a normal grain but shorter, plumper and greyish, with the seed coat intact until it is broken. Crushing a bunt ball releases a puff of greasy, sooty dark brown spore powder and a strong odour usually likened to rotting fish, from trimethylamine, which gives the fungus its other name of stinking smut.
Bunt is one of the oldest recognised plant fungi, described in the eighteenth century, and its study by Tillet and later Prevost provided some of the earliest experimental proof that a microscopic fungus could cause plant disease.
How to identify it
The ear
Affected ears often stand slightly more upright than normal and may look greyish or bluish-green during grain fill. The glumes are frequently spread apart by the swollen bunt balls, so the ear appears slightly gaping and bristly, and the whole head may be shorter than uninfected ones.
Bunt balls
In place of each kernel sits a bunt ball: broader and shorter than a normal grain, dull grey to greyish-brown, with a smooth to slightly wrinkled skin. It is light for its size and crushes easily between the fingers, collapsing into a dark chocolate-brown to almost black greasy powder. A sound kernel by contrast is firm, hard and cannot be crushed by hand.
Plant habit
Infected plants may be somewhat shorter than their neighbours, with the difference in stem height visible as a slight dip in the canopy, though this is inconsistent and not a reliable sole character.
Spores
No spore print applies. Teliospores of Tilletia caries are globose, dark brown and covered in a raised net-like reticulation visible at high magnification; those of T. laevis are smooth-walled, which is the main way the two are separated. Both are accompanied by the characteristic fishy odour.
Where and when it grows
The fungus occurs on wheat (Triticum) principally, and also on rye, triticale and a range of wild grasses in genera such as Aegilops, Agropyron and Elymus. It is found in wheat-growing regions across Europe, Asia, North Africa, North and South America and Australasia, and is especially persistent in cool-climate and organic or saved-seed systems.
Spores rest on the seed surface or in the soil and germinate when the seed germinates, at cool soil temperatures of roughly 5 to 15 degrees Celsius and moderate moisture. Slow emergence in cold soil lengthens the window in which the seedling can be colonised, so autumn-sown crops in cool soils are where bunt is most often seen. Spores can remain viable in dry soil for several years.
Ecological role
Tilletia caries is a systemic parasite with a single annual cycle. Teliospores germinate to produce basidia bearing thread-like basidiospores that fuse in pairs to form infectious hyphae. These penetrate the coleoptile of a very young seedling, then grow upward inside the plant, keeping pace with the shoot apex without producing any visible outward sign.
When the ear differentiates, the fungus colonises the developing ovary and converts its contents into a spore mass, so the plant's reproductive output is entirely diverted into fungal reproduction. At threshing the fragile bunt balls break, dusting spores across sound grain and onto the soil, where they wait for the next germinating seedling. The fungus therefore has one infection opportunity per season, at seedling emergence, and one dispersal event, at threshing.
Similar species
Loose smut of wheat (Ustilago tritici) also replaces the grain, but its spore mass is exposed and blows away soon after ear emergence, leaving a bare rachis, whereas bunt balls stay enclosed inside intact glumes until threshing. Karnal bunt (Tilletia indica) converts only part of a kernel, typically along the groove, leaving a partially sound grain rather than a complete ball. Dwarf bunt (Tilletia controversa) makes very similar bunt balls but severely stunts the plant and is tied to prolonged snow cover.
Ergot (Claviceps purpurea) also replaces kernels, but with a hard, dark, horn-shaped sclerotium that projects beyond the glumes and does not crush into powder, an easy distinction in the hand. Blackened or shrivelled grain from sooty moulds shows surface growth on an otherwise intact kernel.