The disease is specific to wheat.
No symptoms can be observed prior to ear emergence. The flag leaves of infected plants show yellow streaks and plants can be stunted, with 'squat', dark grey-green ears and slightly gaping glumes In infected ears the grain is replaced by seed-like 'blunt balls' each containing millions of greasy, black foul smelling spores. In severe cases, the whole field may smell of rotting fish. In wet weather conditions the ears may appear to be covered in a black ink-like substance as the spores are released and run out of the protective glumes onto the ear and stem.
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 then produce secondary spores which infect the coleoptiles of the young seedlings before the emergence of the first true leaves. The mycelium grows internally within the shoot infecting the developing ear. Affected plants develop apparently normally until the ear emerges when it can be seen that grain sites have been replaced by bunt balls.
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 which may present bare soil ready for planting the next crop.
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 from an infected crop can thus serve to introduce the pathogen into seed lots harvested in the following season.
The disease is rare in the UK as the vast majority of seed is treated with a fungicidal seed treatment. However, the disease is potentially very damaging and can lead to complete crop loss due to grain being unsaleable because of discolouring and smell. Cases usually arise from sowing untreated farm-saved seed although soil-borne infections also occur.