Species in the genus Gnomoniopsis occur in plant tissues in the families Fagaceae, Onagraceae and Rosaceae as saprobes, endophytes or pathogens. These fungi are identified and defined in relation to their specific hosts and through morphological and genetic analyses and are generally presumed to be host specific. This presumption does not always hold true, however, and Gnomoniopsis castaneae, a recently described member of the genus, may be an exception. It is an emerging pathogen known to cause branch and stem cankers and nut rot primarily in Castanea sp. commonly cultivated in Europe, Oceania and North America. However, the fungus has also been isolated from the cankers of Corylus avellana L. and Buxus sempervirens and leaf tissue of Fraxinus ornus, and the mechanisms that turn the fungus from endophyte to pathogen within plant tissue remain unknown. A nut rot and canker inducing pathogen, which may not be host specific, has profound ecological and economic implications for mast producing species. The G. castaneae genome sequence will provide the data needed to illuminate the fungal origins, support population studies of a global pathogen and reveal factors that mediate tissue degradation associated with cankers and nut rot.