Since 2015, all publications on new fungal species, phylogenetically belonging to the genus Rhizoglomus (e.g. Sudová et al. 2015, Turrini et al. 2018, Błaszkowski et al. 2021) were published as Rhizoglomus species. Thus, we have decided to place the new species in Rhizoglomus instead of Rhizophagus, also due to the fact that it is impossible to know if or where Rhizophagus populinus P.A. Dang., the type species of Rhizophagus P.A. Dang., clusters in Glomeromycota. The second fungal species described in earlier times as a Rhizophagus species, R. tenuis Greenall (Greenall 1963), was recently attributed to the phylum Mucoromycota instead of the Glomeromycota due to phylogenetic analyses (Walker et al. 2018). The morphological taxonomy for the Glomeromycota species is based in the subcellular structures of the spore, and practically there is nothing about the spore of R. populinus in the original description. Furthermore, the author of Rhizophagus (Dangeard 1896) has not designated or deposited type specimens, but just illustrations. There were no holotype, no isotype and no paratype established for P. populinus. There are no sequences for R. populinus, thus it is not possible to place R. populinus in any clade of the Mucoromyceta by phylogenetic analyses. Considering the above mentioned, at the best, Rhizophagus would be a genus “incertae sedis” in the kingdom fungi, in the Mucoromyceta, or, when considering the arbuscular mycorrhizal structures drawn by Dangeard (1896), in the Glomeromycota. Dangeard (1896) decribed Rhizophagus as a ‘deadly’ root pathogen and Saccardo & Trotter (1912) placed Rhizophagus into the Peronosporaceae, based on the description and some of the drawings that were published by Dangeard in 1900. Hence, the genus Rhizophagus and R. populinus are members of the Peronosporales.

Glomus intraradices, described from a citrus plantation in Florida (Schenck & Smith 1982), is an arbuscular mycorrhizal fungus (AMF) that predominantly forms its spores intraradically. After Glomus was shown to be well separated at the generic level from this species, G. intraradices was renamed Rhizophagus intraradices (Schüβler & Walker 2010) following previous use of the genus name for AMF forming their spores in roots (Butler 1939, Gerdemann & Trappe 1974). Sieverding et al. (2014) proposed that Rhizophagus should be replaced with Rhizoglomus, but Walker et al. (2017) challenged this and proposed that the generic name Rhizophagus should be conserved, but with a change of type species to R. intraradices. We follow recommendation 14A.1 of the International Code of Nomenclature for algae, fungi, and plants (ICNafp) (Turland et al. 2018) by retaining ‘existing usage’.