Elimination of Citrus Tristeza Virus from Triploid Pummelo Cultivar [Citrus maxima (Burm.) Merr.] by In Vivo Micrografting on Seedlings of Three Citrus Cultivars

Seedlessness in Citrus cultivars is one of the important characters accepted by consumers. Triploid pummelo contains no or a few seeds even though they are planted in a mixed block. Four triploid pummelo hybrids bred in Kyushu University are in the course of registration for new seedless cultivars release. Citrus tristeza virus (CTV) may be the catastrophic cause of decline of the production when the new pummelo cultivars release and expand. Therefore, virus test for the original trees and production of virus–free clones before the release are necessary. Micrografting with shoot apices of scions on the epicotyls of decapitated seedlings is the best ways to remove viruses in Citrus. In this study, shoot tips with 2, 4 or 6 leaf primordia of CTV–infected triploid pummelo 112A were grafted in vivo on 2-week-old seedlings of ‘Hirado–buntan’ pummelo, ‘Natsudaidai’ tangelo and ‘Variegated Daidai’ sour orange rootstocks to seek suitable rootstock seedlings and produce CTV–free clones. It was verified from this study that the larger the shoot tip size, the higher the rate for success in in vivo micrografting. All of the shoots derived through in vivo micrografting of shoot tips with 2, 4 or 6 leaf primordia were determined to be CTV–negative through immunochromographic assay 9–12 months after micrografting; however, 26.3 % of the shoots showed positive for CTV through RT–PCR test. The resulting virus free shoots (73.7 %) will be vegetatively propagated and proliferated for cultivar release