Bunt

Bunt, fungal disease of wheat, rye, and other grasses. Infection by Tilletia tritici (formerly T. caries) or T. laevis (formerly T. foetida) causes normal kernels to be replaced by “smut balls” containing powdery masses of brownish-black spores characterized by a dead-fish odor. Smut balls break open and contaminate healthy kernels during harvest, and the spores may remain viable in dry soil for several years. Seedling infection occurs shortly after kernels germinate in cool, moist soil. Temperatures between 5 °C and 15 °C (41 °F and 59 °F) are optimal for spore germination. Similar infection is caused in rye by T. secalis and T. contraversa.

Since the domestication of wheat, bunt has been a major cause of yield and quality losses in the cereal around the world. Infected plants cannot be readily identified, but they may be slightly to moderately stunted compared with uninfected plants. Wheat heads infected by the fungus may remain green longer than healthy ones and are slender by comparison. The glumes (leaf-like structures at the base of each spikelet) appear splayed because of the presence of the smut balls. Usually all ovaries on a spike are infected. The disease occurs frequently in wheat-producing areas of Asia, Australia, North and South America, and Europe. It can, however, be controlled and the damage minimized with proper management. In the United States the average economic loss incurred from the disease in 2024 was $0.07 per acre. Bunt, however, continues to be a major threat to organically farmed wheat and in developing countries, where farmers have less access to agricultural chemicals. Considerable yield losses are also reported in the rye-growing regions of the world, particularly Europe and Russia.

Disease management

Bunt is controlled by growing resistant varieties from fungicide-treated seeds. Planting winter wheat early and spring and summer wheat late to keep the soil temperature above 15 °C (59 °F) can reduce the chances of infection. Crop rotation with non-host plants—any plant besides wheat, rye, and other affected grasses—can disrupt the disease cycle. Thorough cleaning and sterilization of contaminated farm equipment is necessary to remove residual spores.

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