The tan spot fungus can infect growing wheat plants at any time during their development. Lesion type and symptoms can vary from a tan blotch with a dark brown centre and no surrounding yellow zone to a tan blotch surrounded by a yellow zone without a dark brown centre. Tan spot lesions usually remain small on actively growing young leaves, however, when spots are abundant, the leaves turn yellow which gives the field an overall yellow cast. Severely diseased leaves eventually wilt and die prematurely. Red smudge on wheat kernels appear when conditions favouring seed infection occur during kernel development.
Infected kernels appear red or pink and occasionally slightly smaller and wrinkled.
Planting seed with large numbers of red smudge kernels can result in reduced stands and lower grain yield particularly in durum wheat. Seedling emergence is further reduced when red smudge seed is planted below 3 inches. The tan spot fungus survives and reproduces on infected wheat straw or stubble and spreads by two types of spores. In the spring, large numbers of ascospores produced in slightly raised black pinhead size fruiting structures called pseudothecia are the primary source of inoculum. These distinctive fruiting structures are visible on weathered wheat stubble and straw on the soil surface and feel like rough sandpaper to the touch.