The phylogenetic position of Uncinula forestalis within the Erysiphales has been inferred from 5.8S, 18S, and 28S rDNA sequences. Although the appendages of the ascomata are Uncinula-like, i.e. unbranched with curved-coiled apices, U. forestalis is situated at the very base of the large Erysiphales cluster, far from the ‘Pseudoidium’ clade (Erysiphe, including Microsphaera and Uncinula) and separate from the recently introduced basal genus Parauncinula. U. forestalis differs morphologically from the species of Erysiphe sect. Uncinula in having terminal, fasciculate (as in Podosphaera tridactyla), septate ascoma appendages and a Euoidium-like anamorph (conidia catenate). In Parauncinula, the appendages are also terminal but not fasciculate, the ascospores are curved, and the anamorph is lacking. Uncinula forestalis is a basal, tree-inhabiting powdery mildew with some additional ancestral characteristics, including Uncinula-like appendages and 6–8-spored asci. The new genus Caespitotheca gen. nov. is described with C. forestalis comb. nov. (syn. Uncinula forestalis) as the type species. We calculated the timing of the divergence of U. forestalis and P. septata using a molecular clock of the Erysiphales (6.5×10−10 changes per site per year in domains D1 and D2 of 28S rDNA) and a 28S rDNA data set. The results suggest that the divergence of U. forestalis and P. septata from other Erysiphales occurred between 90 and 80 Myr ago; i.e. the divergence of the two ancestral species may have occurred in the late Cretaceous.