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Maize streak virus (MSV), an important pathogen of maize in Africa, is the most extensively studied member of the Mastrevirus genus in the family Geminiviridae. Comparatively little is known about other monocot-infecting African mastreviruses, most of which infect uncultivated grasses. Here we determine the complete sequences of 134 full African mastrevirus genomes from predominantly uncultivated Poaceae species. Based on established taxonomic guidelines for the genus Mastrevirus, these genomes could be classified as belonging to the species Maize streak virus, Eragrostis minor streak virus, Maize streak Reunion virus, Panicum streak virus, Sugarcane streak Reunion virus and Sugarcane streak virus. Together with all other publicly available African monocot-infecting mastreviruses, the 134 new isolates extend the known geographical distributions of many of these species, including MSV which we found infecting Digitaria sp. on the island of Grand Canaria: the first definitive discovery of any African monocot-infecting mastreviruses north-west of the Saharan desert. These new isolates also extend the known host ranges of both African mastrevirus species and the strains within these. Most notable was the discovery of MSV-C isolates infecting maize which suggests that this MSV strain, which had previously only ever been found infecting uncultivated species, may be in the process of becoming adapted to this important staple crop.