Once established, the fungus grows systemically in the plant. At flowering, entire flowering heads may be infected, and the fungus converts the developing kernels into bunt balls. Healthy seeds are contaminated by the release of the spores at harvest. Common bunt of wheat Loose smut, caused by the fungus Ustilago tritici , has little effect on seed quality. However, it can result in substantial yield losses if left unmanaged.
Symptoms are most noticeable between heading and maturity of the crop.
Diseased flowering heads are conspicuously darker when compared to healthy, green heads. Also, diseased heads typically emerge earlier. Diseased spikelets may be entirely transformed into dark fungal spore masses. As the head emerges, floral tissue is torn and the spores are released. Eventually only a spike is left where a normally healthy head should be.