The initial symptoms of septoria tritici blotch are small chlorotic spots on the leaves that appear soon after seedlings emerge in the fall or spring. As they enlarge, the lesions become light tan and develop darker-colored fruiting bodies. Lesions on mature leaves most often are long, narrow, and delimited by leaf veins but also can be shaped irregularly or can be elliptical, particularly on seedlings or leaves that were young when infected. Mature lesions contain black or brown fruiting structures, the asexual pycnidia or sexual pseudothecia. The pycnidia or pseudothecia develop in the substomatal cavities of the host so are spaced regularly within the lesions.