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. The presence of these fruiting structures indicates tan spot is at least partially responsible for the leaf spot diseases in that field. The ascospores are discharged from the fruiting structures to the lower leaves of tillering plants. At least 6 hours of leaf wetness either due to rain showers, fog or heavy dew is required for infection.
Lesions develop within five to seven days if conditions are favourable after infection.
Mature lesions eventually produce asexual spores called conidia that spread by wind from the lower leaves to the upper leaves and head. Leaves of seedlings and tillering wheat plants can become severely diseased during warm wet springs, however, farmers may not notice tan spot until flag leaf emergence or early heading. Seed infection is also favoured by wet weather during grain filling if large numbers of air-borne spores are present. Conditions favourable for seed infection and red smudge development do not occur every year. Tan spot can infect crested and intermediate wheatgrass, Russian and Altai ryegrass, bromegrass, and other grass hosts.