First described in Australia in 1868, flag smut is known to reduce wheat yield by an average of 1 per cent annually. In most areas, the disease is not significant as resistant varieties, crop rotations and seed treatments are used. However, in the northern Western Australian wheatbelt, flag smut is a significant disease and can reduce yield by 10-15 per cent. It is found more commonly on lighter soils, and where susceptible varieties are grown.
Flag smut attacks the leaves and stems of wheat plants. The fungus produces grey-black lines of spores that run parallel to the leaf veins. Infected plants are often stunted and the flag leaf twisted. The head, if produced, is often small and contains screenings. At harvest, spores on infected leaves and straw are spread onto the grain and soil surface. If sown, infected seed will produce infected plants. If uninfected seed is exposed to flag smut spores in the soil after sowing, infected plants will be produced. Therefore flag smut has both a soil and seed borne phase.