The fungus Pyrenophora tritici-repentis. Tan spot lesions on leaves characteristically have a small, tan to brown center, which is surrounded by a yellow circular border. As leaves mature, lesions expand, kill tissue, and can impart a tannish hue to leaves. Lesions, which initially are found in late winter or early spring on lower leaves, result from infection by spores released from fruiting bodies that formed on wheat residue left in the field after harvest. Clean tillage and crop rotation facilitate destruction of residue, which reduces inoculum present to start the disease.
Some varieties are more resistant to tan spot than others.
Major Foliar Fungal Diseases of Wheat in Oklahoma .