x Reyllopia Holub (Reynoutria x Fallopia)
This nothogenus was created by Holub (1998) in order to accommodate hybrids between species of Reynoutria and Fallopia. These two genera are often amalgamated but molecular studies have shown them to be distinct (Galasso & al. 2009). The occurrence of intergeneric hybridization, though, suggests at least a close relationship.
So far, a single hybrid has been found in the wild, x Reyllopia conollyana (J.P. Baily) Galasso, initially described as Fallopia x conollyana J.P. Bailey. It has been recorded in Belgium as well.
References
Galasso G., Banfi E., De Mattia F., Grassi F., Sgorbati S. & Labra M. (2009) Molecular phylogeny of Polygonum L. s.l. (Polygonoideae, Polygonaceae), focusing on European taxa: preliminary results and systematic considerations based on rbcL plastidial sequence data. Atti Soc. It. Sci. Nat. Museo Civ. Stor. Nat. Milano 150(1): 113-148. [available online at: https://www.researchgate.net/publication/257029473_Molecular_phylogeny_o...
Holub J. (1998) Reclassifications and new names in vascular plants. 1. Preslia 70(2): 97-122.