ABSTRACTMonilinia gaylussaciae sp. nov.(Discomycetidae: Sclerotiniaceae) is sympatric with several other species such asM. vaccinii-corymbosi, M. polycodii, andM. azaleae, which also parasitize ericaceous hosts endemic to North America. It is generally restricted toGaylussacia(Huckleberry). In contrast to the species onVaccinium, the mummified fruit of the new species remain attached to the host until late summer, and the pseudosclerotium within matures on the bush. The apothecia develop from pseudosclerotia on the ground, formed the previous year, and mature in late March through April. Asci are octosporic and stain blue in Melzer's iodine reagent; ascospores are 13.0-15.1×7.5-8.6μm, ellipsoid, subhyaline and smooth; conidia are formed on very tender foliage of the current year, are ash gray limoniform, 26–31×20–23μm and are separated by fusiform disjunctors 8–10×2–2.5μm. Cultures ofM. gaylussaciaeon potato dextrose and yeast-malt agar media are zonate, without micro-conidia and produce a copious amount of a yellow crystalline material in the agar and black stromata arranged concentrically.