These conidia are blown to other plants and to other fields and are responsible for spread and infection of upper leaves during late May and June. Fields distant from infested stubble fields are less severely infected but there is evidence that long distance spread by wind does occur. By June fields some distance from old infested wheat stubble may show severe tan spot development on upper leaves because of secondary conidial spread. The disease progresses rapidly when many spores are present and rainy, misty or foggy weather lasting more than 24 hours allows spores to germinate and infect plants. New releases of spores occur with each wet period.
Symptoms appear within five to seven days after infection.
Free moisture on the leaf surface is the main limiting environmental factor associated with infection. Due to the importance of crop residue in reducing soil erosion a discussion on crop residue management is included. Although wheat stubble is a primary source of tan spot inoculum, the threat it presents does not offset the value of crop residue. Also, tan spot can be effectively controlled with fungicides or crop rotation as is practiced in the ecofarming system. To reduce soil erosion from wind and water, crop residue needs to be maintained on the surface.