Tilletia tritici, Tilletia laevis
Bunt is common throughout wheat growing areas of the world. It is difficult to detect in a crop because infected plants appear similar to healthy ones. Plants may be slightly shorter, and the heads are usually darker green than normal and remain green for a longer period. Bunt infections result in the complete replacement of the seed contents with a mass of smut spores. When crushed, the infected grain releases a fine black-brown powder with an odor like rotten fish. Bunt is potentially the most devastating smut disease.
Bunt spores are carried on the outside of the seed and can remain viable for up to three years. When infected seed begins to germinate, the spores also germinate and penetrate the shoot of the seedling just before emergence.
The fungus then grows in between the plant cells just behind the growing point of the plant. Plants become resistant to bunt infection after emergence and therefore, if an uninfected plant has emerged, it cannot become infected. The fungus enters the tissues of the head, once formed, and black spore masses are produced in the head instead of normal grain.
Bunt infection is spread during harvest and other seed handling operations, when bunt balls (infected grains) are mechanically broken and spread amongst healthy grain.