<i>Mycosphaerella graminicola</i> (synonym: <i>Zymoseptoria tritici</i>; anamorph: <i>Septoria tritici</i>) causes Septoria tritici blotch (STB) of wheat. STB is an economically important disease that occurs throughout the world wherever wheat is grown. The pathogen typically infects through stomata, colonizes the substomatal cavity and then grows intercellularly for 8-12 days during a biotrophic phase before causing massive cell death and necrosis and then switching to necrotrophic growth. Its exact mode of nutrition during biotrophic growth, what triggers the switch from biotrophy to necrotrophy and which genes are expressed during necrotrophic growth are not known. To better understand these processes, the genome of the pathogen was sequenced completely by the Joint Genome Institute of the U.S. DOE. The 39.7-Mb genome consists of 21 chromosomes, 8 of which can be lost with no obvious phenotypic effects so are dispensable and probably were acquired during the past 10,000 years. The genome contained a reduced number of genes for cell wall-degrading enzymes compared to other plant pathogens and many of those that are present are not expressed during the biotrophic phase of infection, leading to the hypothesis that protein degradation may be more important than carbohydrates for biotrophic growth. The reduced number of genes for cell wall-degrading enzymes and lowered expression during biotrophic growth may be an adaptation to facilitate the stealth pathogenicity of <i>M. graminicola</i>.
