My bible on Plant Systematics gives the ICBN recommendations, which are that both the generic and specific epithets should be italicized. The authority is not italicized. If you are referring to a variety or subspecies, the “var.” or “subsp.” is not italicized, but the following epithet is, e.g., Lyonia ligustrina (L.) DC. var. filiosiflora (Michx.) Furlow Higher taxa are always capitalized; they are generally italicized by Europeans, but not by Americans. Cultivar names should have a capital initial and either be included in single quotes or preceded by “cv.” They are never italicized. Hybrid names are preceded by ´, and are italicized. David Ehrlich