Helotium phormium was named by Cooke in 1879 from a single New Zealand specimen collected on dead Phormium leaves. The original illustrations attached to the type specimen in Kew show more or less clavate, slightly curved ascospores. The illustrations from the same specimen in Dennis (1961) have ascospores that are straighter, more cylindric and tapering more or less evenly to both ends.

This fungus is very common on dead Phormium leaves in New Zealand. It varies somewhat in ascospore shape (some specimens with ascosopres matching exactly the Cooke illustration, others tapering more or less uniformly to each narrow rounded end, often with gel caps or small frills at the ends of the spores).

Based on ITS sequences (see image of ITS gene tree under H. phormium) H. phormium is closely related to Hymenoscyphus caudatus. H. caudatus has been treated as a species complex by authors such as Baral (Sydowia 58: 145-162, 2006) and Dennis (Mycological Papers 62: 1-216, 1956). Baral discussed differences in development of croziers at the ascus base, ascospore shape, and paraphysis morphology between some of the populations within this species complex. H. phormium appears to be Phormium-specialised and has larger ascospores than given for H. caudatus by Dennis 1956. NZFungi has previously incorrectly reported this species as Dicephalosporium rufocornea.

Hymenoscyphus sensu stricto (sensu Baral 2006) is rare in New Zealand. Apart from Helotium phormium, four other Hymenoscyphus sensu stricto isolates have been sequenced. All four are from exotic hosts - PDD 105204 from Rubus fruticosus has an ITS and morphology that matches H. caudatus; PDD 117703 has an ITS sequence close to H. macroguttulatus but differs morphologically; ICMP 14420 and ICMP 17116 have matching ITS sequences are were isolated from nashi fruit rots in orchards.