Evaluation of Graft-Transmissible Isolates from Dwarfed Citrus Trees as Dwarfing Agents

The dwarfing characteristics of four isolates (CD 4, CD 8, CD 9, and CD 10), derived from healthy-looking dwarfed field citrus trees, were evaluated. Each was bud inoculated to cv. Delta Valencia trees on cv. Yuma citrange rootstock prior to planting in the field. At 5 years after planting, isolates CD 4 and CD 9 reduced canopy volumes by 60%, and CD 10 by 30%, without any detrimental effects. No citrus viroids (CVds) were detected biologically or by sequential polyacrylamide gel electrophoresis in these three isolates. Isolate CD 8, however, contained two viroids, citrus exocortis viroid and a Group-III CVd, but had no deleterious effects on the Yuma citrange rootstock. Citrus tristeza virus (CTV) was the only other pathogen detected in all of the isolates. Indexing for cachexia, psorosis, impietratura, and tatter leaf were negative. The dwarfing abilities of the isolates are therefore attributed to CTV. Fruit yield was according to tree size and the yield efficiency of the inoculated trees was equal to that of the uninoculated control trees. External and internal fruit quality was not affected. The trees became naturally infected with huanglongbing (greening) 5 years after planting, but the disease incidence remained low for several years in trees with isolate CD 4.