Although these plants were initially identified as the more western O. ludoviciana Nutt., they have now been described as a distinct species by Collins et al. (2009; see also H. Uhlich at www.orowiki.org). Myzorrhiza differs from Orobanche s.s. plus Phelipanche as follows (W): calyx 5-lobed, the lobes subequal, all well-developed (versus 2-4 lobed or rarely with a minute 5th). M. riparia has flowers with distinctly acute calyx lobes and strongly curved corolla, produced in Aug-Oct (versus Apr-Aug). It is known along larger streams and rivers in or near the southern Great Plains and (somewhat disjunct) in east-central states from Ill. and Tenn. to Va. The global status of riparia remains uncertain, but in the eastern half of its reported range it appears generally imperilled, with only about 30-40 records and mostly from before 1950. In eastern states, it is known only along or near the following rivers: central Mississippi, Sangamon, Wabash, lower Ohio, New, James, Shenandoah and Potomac. It grows on roots of tall annuals: usually Ambrosia trifida or Xanthium strumarium, occasionally A. artemisiifolia, Nicotiana tabacum or other species. D. Boone (pers. comm.) has recently rediscovered this species in Hamilton Co., Ohio, but he could locate it here during Aug 2020. There are only a few old records from Ky. or elsewhere along the Ohio Rv. Collins al. cited a coll. from PEND: Chicoine #4745, 27 Nov 1942, Ohio Rv. banks at Ivor. B's report of ludoviciana from PEND must also belong here, but there is no coll. at US. An old coll. from DAVI (KY ex. Agr. Sch. with duplicates at BEREA and KNK) also appears to be this species: Geo. M. Taylor, 24 Sep and 1 Oct 1902, at "Maceo" [near Ky. 231 bridge across Ohio Rv.] "on tobacco."