(a) In order to prevent the introduction into the United States of the citrus canker disease(Xanthomonas citri (Hasse) Dowson) and other citrus diseases, the importation into the United States of plants or any plant part, except fruit and seeds, of all genera, species, and varietiesof the subfamilies Aurantioideae, Rutoideae, and Toddalioideae of the botanical family Rutaceae is prohibited, except as provided in paragraphs (b), (c), and (d) of this section.