D.J. Lodge provided the following notes on the question of whether Hygrocybe persistens is synonymous with H. acutoconica and which one takes precedence:
“I found with diligent tracking of the release dates of the original descriptions that Hygrocybe acutoconica has priority over H. persistens by two months. Unpublished DNA analyses of nuclear ribosomal DNA – ITS sequences show that H. persistens (described from Europe) only differs by 3.5% from H. acutoconica, which was described from Nebraska. That is on the boarderline for separating species, and one could consider them the same or different, depending on one’s preference. However, H. persistens in the strict sense isn’t in the USA, as far as I know. Morphologically (macro- and micro-) you would be hard pressed to find differences, so I follow Boertmann in considering them synonyms, though in his earlier version (now in revision), he had followed Arnolds in assuming that H. persistens had priority over H. acutoconica, but then Arnolds and Boertmann hadn’t really tracked down the release dates, as I have.”
See Cantrell, S.A. and D.J. Lodge. 2000. Hygrophoraceae of the Greater Antilles: Hygrocybe, subgenus Hygrocybe. Mycological Research 104: 873-878.
H. persistens is considered to take precedence over H. acutoconica in the Index Fungorum (as of January 2009). Hygrophorus persistens is used by Largent in the Agaricales of California (1985) along with Hygrophorus acutoconicus. Largent states “The red color of the pileus is the only reliable way to separate Hygrophorus persistens from Hygrophorus acutoconicus.”
Based on this information, I’ve made both names valid and broken them apart as separate names. I’ve also updated my votes so that collections from the US are considered H. acutoconica.