A powdery mildew fungus belonging to the genus Setoidium (anamorph of Cystotheca) was found on Castanopsis javanica in Cibodas Botanical Garden and mount Tangkuban Perahu, West Java, Indonesia. The fungus is considered as a new species, namely, Setoidium castanopsidis. Phylogenetic analyses of the 28S and ITS rDNA regions showed that S. castanopsidis formed a distinct lineage separated from Cy. tjibodensis, Cy. lanestris, and Cy. wrightii. Setoidium castanopsidis also differs morphologically from Cy. tjibodensis in having distinct appressoria (nipple-shaped), longer conidiophores with longer foot-cells, larger conidia, and being found on Ca. javanica. The teleomorphic state has not been found during the collection.