When apparently healthy leaves of mulberry were irradiated with ultraviolet light (UV) for 3-5 min and kept in a moisture box, brownish gray-color and orbicular-shaped spots often appeared on the leaves after 3-4 days. Then a lot of acervuli, conidiospores and setae were formed on the spots. The shape of the conidiospores was cymbi-form or falcate-form, and the fungus was identified as being Colletotrichum morifolium Hara, the pathogen of anthracnose disease of mulberry. Thus, it was shown that the UV-induced spots appeared at the sites where the pathogen had infected latently. The number of latent infection sites can be estimated by UV-irradition.