Colletotrichum lagenarium is the causal agent of anthracnose of cucumber. This fungus produces a darkly melanized infection structure, appressoria, to penetrate the host leaves. The C. lagenarium CMK1 gene, a homologue of the Saccharomyces cerevisiae FUS3/KSS1 mitogen-activated protein (MAP) kinase genes, was shown to regulate conidial germination, appressorium formation, and invasive growth. In S. cerevisiae, Ste12p is known to be a transcriptional factor downstream of Fus3p/Kss1p MAP kinases. To evaluate the CMK1 MAP kinase pathway, we isolated the Ste12 homologue CST1 gene from C. lagenarium and characterized. The cst1Δ strains were nonpathogenic on intact host leaves, but could form lesions when inoculated on wounded leaves. Conidia of the cst1Δ strains could germinate and form melanized appressoria on both host leaf surface and artificial cellulose membrane, but could not produce infectious hyphae from appressoria, suggesting that CST1 is essential for appressorium penetration in C. lagenarium. In addition, matured appressoria of the cst1Δ strains contained an extremely low level of lipid droplets compared with that of the wild-type strain. Lipid droplets were abundant in conidia of the cst1Δ strains, but rapidly disappeared during appressorium formation. This misscheduled lipid degradation might be related to the failure of appressorium penetration in the cst1Δ strain.