Question re source(s) for variety descriptions for Symphyotrichum lateriflorum

[choess]

No. If you accept the criterion of monophyly (that any given taxon should contain all the descendants of a common ancestor, a clade), DNA reconstruction of phylogeny can make you pretty confident that certain individuals shouldn’t be lumped together. However, clades in a phylogenetic tree can nest to an arbitrary depth. Both lumping together a pair of sister clades as one taxon and treating them as two separate taxa produce a monophyletic classification; which approach to adopt is a matter of taxonomic judgment. There’s no quantitative measurement obtained by DNA sequencing that defines a boundary between “conspecific” and “not conspecific”. (At least not for eukaryotes; with microbes things are somewhat different, see https://mbio.asm.org/content/11/1/e02475-19 for some background.)