The ascomycete fungus Zymoseptoria tritici (synonym: Mycosphaerella graminicola) is the causal agent of the major wheat foliar disease Septoria tritici blotch. Despite its importance as a pathogen, very little is known about how Z. tritici senses and responds to light. Light is a major environmental cue for fungi, but also is a major stressor, causing DNA damage and oxidative stress within fungal cells. Fungi use light as a cue to produce photoprotective pigments and proteins, and to control many aspects of growth and reproduction, such as asexual versus sexual sporulation, and the production of mycotoxins. A previous RNA sequencing experiment identified many Z. tritici genes that respond to different wavelengths of light. Multiple putative light-sensing genes in Z. tritici as well as those that showed differential responses to light were knocked out using Agrobacterium tumefaciens-mediated transformation. Phenotypes to be tested include changes in growth morphology and spore production under a variety of conditions, such as the effect of high and low temperatures, rich and nutrient-poor media and changes in pathogenicity to wheat. Environmental stresses to be tested include the effects of light and dark, varying levels of oxidative stress and different osmotic conditions. These experiments should help to reveal the effects of light on basic biological processes and the pathogenicity of Z. tritici to wheat.