Twenty-eight species in six genera are recognized, described, illustrated, and their distribution in the province of Ontario and adjacent Quebec and Manitoba recorded. Keys based on fungus morphology provide access to the species treated and an alternate access is given by a generic host index. New hosts and new distribution records are noted within species treatments. Microsphaera on Vaccinium, long regarded in North America as part of the complex species M. penicillata, is given species recognition as M. vaccinii.