Willows of Russia and Adjacent Countries. Skvortsov 1999
http://172.104.19.75/announcements/skv/skvortsov.pdf

Iridaceae presenti in Italia Iridaceae Present in Italy:
www.editricesapienza.it/sites/default/files/4679_Colasante_Iridaceae.pdf

Phelipanche nana from P. ramosa and P. mutelii is surely osften challenging and they have been sometimes misidentified.
In particular, many observations of P. ramosa from North America seem to be more fitting with P. nana.

This is a key that could turn out to be useful for the users:

1) Corolla upper outline not or only slightly gibbous (lower and upper outlines nearly parallel towards corolla gorge). Calyx teeth more or less equal to calyx tube. Flowers usually erecto-patent, rarely spreading.: P. mutelii.

1a) Corolla upper outline distinctly gibbous. Calyx teeth longer than calyx tube or more or less equal to calyx tube. Flowers erecto-patent or spreading: 2.

2) Upper calyx teeth as long as or shorter than calyx tube, triangular. Flowers erecto-patent. Stem often branched. Corolla light blue to whitish. Parasitizing cultivated plants: P. ramosa.

2a) Upper calyx teeth longer than calyx tube, subulate to narrowly triangular. Flowers spreading, rarely some or all erecto-patent.. Stem rarely branched. Corolla blue, rarely light blue or whitish. Parasitizing wild plants: P. nana.

ALTERNATIVE KEY

1 - Calyx teeth more or less equal to calyx tube. Flowers usually erecto-patent, rarely spreading. Corolla upper outline distinctly gibbous or not or only slightly gibbous: 2

1' - Calyx teeth longer than calyx tube. Flowers usually spreading, rarely somehow erecto-patent. Corolla upper outline distinctly gibbous: P. nana

2 - Corolla upper outline distinctly gibbous. Plants faintly coloured; often profusely branched. Parasites of cultivated plants: P. ramosa
2' - Corolla upper outline not or only slightly gibbous. Plants usually deeply coloured, usually unbranched or with few stems. Parasites of wild plants. P. mutelii

N.B. in order to observe the "calyx teeth" character, it would be useful to detach the flower from the inflorescence, to remove the bract and to photograph the calyx with a macro lens or under a stereomicroscope. It is also important to see many flowers since there can be some calyces with untypical teeth.

Sources:

• Holoparasitic Orobanchaceae (Cistanche, Diphelypaea, Orobanche, Phelipanche) in Armenia: distribution, habitats, host range and taxonomic problems.
  https://biotaxa.org/Phytotaxa/article/view/phytotaxa.386.1.1

• Más, a propósito de algunas Phelipanche Pomel, Boulardia F. W. Schultz y Orobanche L. (Orobanchaceae) del oeste del Paleártico. Documentos Jard Bot Atlántico (Gijón) 6: 1-128
  https://www.researchgate.net/publication/268809711_Mas_a_proposito_de_algunas_Phelipanche_Pomel_Boulardia_F_W_Schultz_y_Orobanche_L_Orobanchaceae_del_oeste_del_Paleartico_Documentos_Jard_Bot_Atlantico_Gijon_61-128

• Phelipanche mutelii in Index of Orobanchaceae
  http://www.farmalierganes.com/Flora/Angiospermae/Orobanchaceae/Phelipanche/Phelipanche_mutelii/Phelipanche_mutelii.htm

• Phelipanche nana in Index of Orobanchaceae
  http://www.farmalierganes.com/Flora/Angiospermae/Orobanchaceae/Phelipanche/Phelipanche_nana/Phelipanche_nana.htm

• Phelipanche ramosa in Index of Orobanchaceae
  http://www.farmalierganes.com/Flora/Angiospermae/Orobanchaceae/Phelipanche/Phelipanche_ramosa/Phelipanche_ramosa.htm