Here is all I have on it (Alan Weakley, "Flora of the Southeast", Sep 2012 draft):
A genus of about 60 species, herbs, shrubs, and trees, of temperate, subtropical, and tropical regions. Recent molecular studies have indicated the likely polyphyly of Conyza and its close relationship with Erigeron; the ultimate circumscription of these genera is in doubt (Nesom 2000b, Noyes 2000). References: Strother in FNA (2006b); Cronquist (1980)=SE; Nesom (2000b).
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Conyza canadensis (Linnaeus) Cronquist
var. canadensis, Common Horseweed. Old fields, disturbed areas, gardens. July-November. S. Canada south through nearly all of the United States to tropical America. [= C, G, K, Pa, SE, W, WH;
= Erigeron canadensis Linnaeus var. canadensis – RAB; < C. canadensis – FNA
; = Erigeron canadensis – F; = Leptilon canadense (Linnaeus) Britton – S
; < Erigeron canadensis – WV]
Conyza canadensis (Linnaeus) Cronquist
var. pusilla (Nuttall) Cronquist, Southern Horseweed. Dunes, old fields, disturbed areas. (May-) July-December. Se. MA and CT west to s. IN, south to FL and TX, and south into tropical America. [= C, G, K, Pa, SE, W, WH;
= Erigeron canadensis Linnaeus var. pusillus (Nuttall) Boivin – RAB; < C. canadensis – FNA;
= Erigeron pusillus Nuttall – F; = Leptilon pusillum (Nuttall) Britton – S; = Conyza parva Cronquist]
I could try to find out why Dr Weakley has not changed it. It is possible that he already has, but he has not made another draft available in awhile. I think he is finishing the Flora up and is close to publication.