For those who have been scouting wheat this spring and also read the Wisconsin Crop Manager and Wisconsin Pest Bulletin will note that there has been reference to observations for both Septoria leaf blotch and Tan spot this spring. We have also noted these two diseases but feel it is important to discuss the differences in symptoms.
Septoria leaf blotch: symptoms start as a light green to yellow spot that is between leaf veins on the lower leaves. This is due to contact with the soil. As these symptoms elongate, they become irregularly shaped and become more tan to red-brown. A key diagnostic sign to different Septoria leaf blotch with Tan spot are black speckles on the lesions. These are called pycnidia, which is formally defined as asexual, globose or flask-shaped fruiting body of certain imperfect fungi producing conidia (Source: Illustrated Glossary of Plant Pathology, http://www.apsnet.org/education/IllustratedGlossary/default.htm).
Tan spot: symptoms are typically small tan spots that are lens-shaped, however, symptoms may be tan to brown, round to slightly elongate spots that are surrounded by a yellow halo. Often, the center spot will appear diamond-shaped. The variability in symptom type is dependent on the wheat variety.
Image Credits: Craig Grau, UW-Madison, and American Phytopathological Society Image Gallery