Of all the tree and fruit crops of the world, citrus is affected by the greatest number of graft transmissible pathogens. Citrus propagative material is highly regulated as compared with propagative material of most other crops. Germplasm is usually acquired as either budwood or seed, and recently as nucellar embryogenic callus cultured in vitro. Citrus germplasm require import permits that spell out the requirements for accepting new material. Phytosanitary certifications are required or preferred. In the USA, introductions of budwood even through approved channels for scientific research are given post-entry inspections, and it is good that they are because budwood shipped through approved channels are routinely contaminated. During the period 1954-1966, 70% of all budwood entering California was infected, and from 1968 to 1978, 100% of the budwood entering the USA was infected. Over all from 1958 through 1993, 65% of the budwood entering the USA was infected. Citrus seeds are not quarantine items in the USA. Although not strictly required from a regulatory standpoint accessions sent as seed are in some cases also routinely pathogen tested before release. - Millet
(Adapted from Citrus Genetics, Breeding & Biotechnology)