Abstract
Purple loosestrife is an introduced plant that has escaped from cultivation to become a noxious weed and is spreading rapidly throughout North America. A fungus that attacks purple loosestrife and may have potential for its biological control is described and illustrated as a new species in the coelomycetous genus Harknessia. This new species, Harknessia lythri, has dark-brown, unicellular conidia with 2–7 widely spaced, narrow, longitudinal slits. Conidiogenesis in both H. eucalypti, type of the genus, and H. lythri is determined to be holoblastic with 1–2 annellides developing on each conidiogenous cell.