S. tritici can attack triticale and rye during disease-favorable conditions and can also be found on barley and certain oat species. S. nodorum can cause serious losses in triticale and barley and infects leaves of rye, but not the heads. The grasses Poa and Agrostis spp. are also susceptible, but to only a limited extent. Lesions will have numerous black pycnidia dispersed throughout. Pycnidia are spore-producing structures of the fungus which, in the case of S. tritici , are visible to the naked eye.
Pycnidia are most noticeable following a period of dew or rain.
Lesions will have pycnidia, but these are difficult to see except by using a 20X lens and looking at lesions by holding diseased leaves up to the light. The difficulty in seeing pycnidia is the result of their light color and the fact that they are buried in the leaf tissue rather than being superficially imbedded, as is the case with S. tritici . Pycnidia of S. nodorum are honey-brown and are almost perfect circles. Individual lesions frequently have pycnidia of both fungi present. Lesions of one or both Septoria fungi can rapidly expand and kill portions of leaves or entire leaves.