Loose smut is probably the most obvious of all wheat diseases. At flowering, all parts of the head and grain except the rachis (backbone) are replaced by black spore masses. This disease occurs throughout the world and reduces yields proportionate to the incidence of smutted heads. The disease does not cause complete yield loss, but losses as high as 27% have been reported, and more commonly, losses of 15% are recorded.
This disease is more prevalent in Western Australia than the rest of Australia and especially in barley. In contrast to seed borne bunts, loose smut has little effect on seed quality.
The loose smut fungus survives as a dormant fungal thread inside the embryo of wheat seed. The pathogen is activated when the infected seed germinates, and it extends toward the growing point of the plant. Evident from flowering onwards when the plant begins to form the head, the fungus invades all of the young head tissue except for that of the rachis (backbone). Production of plant growth hormones by the fungus results in infected plant heads reaching flowering earlier than healthy heads.
The head produced by the infected plant contains black spore masses in place of the grain. The spores are loosely held and are easily spread by wind onto neighboring healthy plants. Because flowering of infected heads occurs earlier than healthy heads, production and release of spores occurs when the rest of the crop is flowering. Spores are blown by the wind into the flowers of the healthy plants. The spores enter the ovaries and become part of the developing grain. In this way, seed for the following year becomes infected.