@drdawg I think that when the original Meyer Lemon was brought from China to California early in the 1930-1940s, it had a virus that wasn't noticeable. Killed lots of citrus and sometime in the 1950's someone in California came out with a Meyer Lemon variety that did not carry the virus and wasn't susceptible to it and they name it the Improved Meyer Lemon. I think that was what I was told when I bought mine last year.
