Septoria tritici is the most significant foliar disease of winter wheat in the UK, with reported yield losses from the disease as high as 50%. Whilst wheat is the most common host, it can also be found on rye, triticale, and some grass species.

Symptoms can be seen throughout the season, often being predominant on the lowest leaves by early December. In young plants, water-soaked patches turn brown and necrotic, and the black pycnidia are often visible. On more mature plants, the lesions are brown, and often have a rectangular appearance. As symptoms develop, the pycnidia become more visible.

The ideal conditions for Septoria tritici are temperatures around 15-20 degrees Celsius, with long periods of high humidity. Most of the disease spread to upper leaves comes from periods of heavy rainfall in the spring, causing rain-splash, where the lower leaves have been actively infected throughout the winter and early spring.