Bunt (stinking smut) in wheat

Note: Also called stinking smut.

Pathogen

Tilletia tritici

Hosts

The disease is specific to wheat.

Symptoms

Bunt symptoms show after ear emergence. Flag leaves show yellow streaks and plants can become stunted, with squat, dark grey-green ears and slightly gaping glumes. In infected ears, grain is replaced by seed-like 'bunt balls', each containing millions of greasy, black, foul-smelling spores. In severe cases, the whole field may smell of rotting fish. In wet conditions, a black ink-like substance may cover the ears. This occurs when released spores run out of the protective glumes onto the ear and stem.

Life cycle

The spores on the seed surface germinate along with the seed. Each produces a short fungal thread, terminating in a cluster of elongated cells. These produce secondary spores that infect the coleoptiles of the young seedlings before the emergence of the first true leaves. The mycelium grows internally within the shoot, eventually infecting the developing ear. Affected plants develop apparently normally until the ear emerges, when bunt balls replace grain sites.

In damp soil, spores usually germinate and then, in the absence of the host plant, die. However, in dry seasons, they may survive in the soil (especially if they are protected within the glumes of shed ears) from the harvesting of one crop to the sowing of the next. Wind-blown spores, particularly from late-harvested crops, can contaminate neighbouring fields. Soilborne spores can invade seedlings very early in germination.

Importance

As each bunt ball contains millions of spores, the capacity for contamination of healthy grain in the same field is enormous. Thus, if seed is continually saved and re-sown without treatment the disease can build up very rapidly. Dry spores can survive for several years. Harvesting or handling equipment contaminated by spores can introduce the pathogen into seed lots harvested in following seasons.

The disease is rare in the UK, as the vast majority of seed receives a fungicidal seed treatment. However, the disease can lead to unsaleable grain, because of discolouring and smell. Cases usually arise from sowing untreated farm-saved seed, although soilborne infections also occur.

High-risk factors

Seedborne infection

Seed sown without a fungicide treatment, especially over many years

Slow emergence, caused by suboptimal seedbed conditions

Soilborne infection

Very short time between harvesting a first wheat and sowing a second wheat

Management

Test seed, with a wash or molecular test (both take 48 hours). The former is quicker and cheaper. Treat infected seed, if infection is greater than one spore per seed.

Good hygiene during harvest, storage, and transport will reduce the chance of the disease spreading to healthy grain. Delaying drilling of a second wheat crop, after harvesting an infected wheat crop, also reduces infection risk. This is because any wet spell causes the soilborne bunt balls to germinate before they can infect the following crop.

Summary

  • Good machinery and grain storage hygiene
  • Delay drilling of a second wheat after harvest of an infected crop or wait until after a wet spell
  • Use a fungicidal seed treatment on infected seed
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Wheat flag leaf showing typical yellow streak symptoms caused by bunt

In infected wheat ears, the grain is replaced by seed-like ‘bunt balls’


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