Many plant pathogenic fungi are ascomycetes that produce more than one type of spores.The rice blast fungus Magnaporthe oryzae, one of the most destructive diseases of rice and a model for studying fungal—plant interactions, produces both microconidia and macroconidia.However, previous infection studies exclusively dealt with macroconidia.Germination of microconidia has not been observed, and their role in plant infection is not defined.Nevertheless, outbreaks of neck blast and wheat blast in fields without foliar diseases indicate that inoculum other than macroconidia may exist.Here we showed that approximately 10% of microconidia could germinate on plant surfaces.Fungal colonies derived from germinated microconidia were normal in growth, conidiation, and pathogenesis.In infection assays with rice and barley seedling, microconidia failed to infect intact plants but they could colonize and develop necrotic lesions on wounded leaves and stems.Microconidia also caused disease symptoms on inoculated spikelets without wounding in infection assays with barley and Brachypodium heads.Furthermore, microconidia were detected inside rice plants that developed blast lesions under laboratory or field conditions.Two genes encoding hypothetical proteins were identified by RNA-seq analysis to be specifically expressed in microconidia and may be suitable for microconidium detection.Our results demonstrated that microconidiacan germinate and are infectious.Due to their size, microconidia produced in planta allow the pathogen to spread readily via the vascular system, which may be an important but neglected factor in the disease cycle and outbreaks of rice blast, wheat blast, and other important diseases caused by Magnaporthe species.