A review of the known unknowns in the early stages of septoria tritici blotch disease of wheat

Septoria tritici blotch (STB) disease of wheat is caused by the fungal pathogen Zymoseptoria tritici. It is the most important foliar disease of wheat in western Europe and affects wheat cultivation worldwide. The combination of intensive fungicide usage, a polycyclic asexual life cycle and an active sexual cycle has led to the emergence of fungal strains resistant/tolerant to all the major classes of fungicides used in its control. The hallmark of this disease is a long, symptomless latent phase that precedes the onset of visible symptoms. Understanding the processes that occur during the symptomless phase of infection is paramount in developing alternative strategies for disease control; however, large gaps in our knowledge of the disease remain. The known unknowns of the latent stage of infection can be summarized in three questions. Does the fungus initiate or manipulate host defences to trigger programmed cell death in order to facilitate nutrient acquisition or is the host acting exclusively? Does the fungus feed during both the latent phase and the necrotrophic phase like a true hemibiotroph? Does the long latent phase serve a beneficial function for the fungus or is it simply an artefact of evolution? This review aims to distil observations made during studies that have directly or indirectly contributed to answering these questions and points towards their most likely answers.