Submitted to: Conference of International Organization of Citrus Virologists
Publication Type: Proceedings
Publication Acceptance Date: 5/22/1997
Publication Date: N/A
Interpretive Summary: Characterization of different isolates of citrus tristeza virus (CTV) on an international level and evaluation of different sources of citrus germplasm for CTV resistance indicates that previous assumptions about the susceptibility of different citrus cultivars and hybrids to infection by citrus tristeza virus need modification. These experiments confirm field observations that some pummelo cultivars can be severely affected by some isolates of CTV and be nearly immune to others. A pummelo which has performed well in one area may not perform as well in another if exposed to different isolates, and conversely, introduction of new isolates into an established production area can introduce an unrecognized disease hazard when efficient vectors are present. Swingle citrumelo, one of the most popular new citrus rootstocks, also shows a differential susceptibility to CTV infection. So far, the differential reaction to CTV infection in Swingle has not been associated with any disease condition, but suggests that evaluation of new citrus hybrids and selections for CTV resistance and tolerance will require use of a broad panel of isolates. Development of a diverse international collection of CTV isolates at USDA facilities at Beltsville has proven valuable for determining isolates and host specific responses and for screening new selections. It illustrates a unique ARS contribution to the ongoing multilab CTV research effort.
Technical Abstract: In Taiwan some pummelos are severely affected by the dwarf strain of citrus tristeza virus (CTV-D), but not by other isolates. A Hawaiian selection of pink pummelo (HA-Pink), which resisted CTV infection under natural challenge in Hawaii, was infected when graft- inoculated with a hassaku dwarf isolate of CTV from Japan (CTV-HD). Susceptibility of Swingle citrumelo seedlings to CTV infection in Florida also varied with different isolates. Glasshouse experiments confirmed the differential susceptibility of pummelos and Swingle citrumelo to different isolates of CTV. Peiyu (Taiwan), pink (Hawaii), and Thong Dee (Florida) pummelos and three seedling clones of HA-Pink were graft-inoculated with CTV-D (B280), CTV-HD (B31), and Florida decline (T36) and mild (T4) isolates. All pummelos were infected by B31 and B280, and all except the HA-Pink showed strong stem pitting or stunting symptoms. T36 infected all but the HA-Pink while T4 infected only Peiyu and Thong Dee. Swingle citrumelo seedlings and clonal propagations of Swingle on rough lemon were infected when graft-inoculated with the Florida isolates T36 and T68, but were not infected by graft-inoculation with T4, T30, or T55-1. The isolate-specific resistance in pummelo and Swingle is apparently distinct from the more general CTV resistance factor in trifoliate orange.