A previously undescribed leaf spot disease of banana has been discovered in Southern and Southeast Asia. The fungus identified as the causal agent of this leaf spot has a Mycosphaerella teleomorph stage and a Septoria anamorph stage. Isolation and reinoculation of the fungus to banana reproduced symptoms and confirmed its pathogenicity. Phylogenic analysis based on sequences of the internal transcribed spacer and 5.8S ribosomal DNA regions from the different leaf spot pathogens of bananas was consistent with the definition of a new species. M. eumusae (anamorph S. eumusae) is the name proposed for the causal agent and Septoria leaf spot as the name for the disease. The presence of the pathogen has been confirmed in leaf specimens from Southern India, Sri Lanka, Thailand, Malaysia, Vietnam, Mauritius, and Nigeria.