Arthur later added a range restriction in his 1934 "Manual of the Rusts of North America and Canada," indicating that the rust only occurred on Ludwigia peploides in southern California. He wrote:

"Aecidium betheli: with pycnia unknown; aecia hypophyllous, in small groups on hypertrophied substratum, cupulate, aeciospores globoid, 13-18 by 16-18 μ, wall colorless, 1 μ or less thick, very finely verrucose; occurs on ONAGRACEAE: Jussiaea californica in southern California."

Andrew Cannon, who pointed this out in a comment to https://www.inaturalist.org/observations/63582194, suggested that the telial host might not be present outside of CA. A rust can't spread in the absence of its telial host, because it can't complete its lifecycle, he explained.