Alexander Ziskind Mintz, an Ashkenazi merchant, started claiming in 1846 that only those growing in Parga are not grafted, and therefore kosher for the ritual. He claimed that under the previous Ottoman system, citrons could only be planted in Parga under the control of Ali Pasha, who collected all proceeds for the Ottoman empire, and those were thus the only citrons known as kosher and un-grafted through the years. Since the rules have been changed, and farmers are allowed to earn their income and pay only a tax, that's how the plantations have expanded to Agia and Preveza. And since those places are new, no one could not be sure whether those picked from newly planted trees, were grafted or not, at least without careful tree checking upon picking.