Citrus tristeza virus transmission by the Toxoptera citricida vector: in vitro acquisition and transmission and infectivity immunoneutralization experiments.

Citrus tristeza virus (CTV) is transmitted by several aphid species in a semi-persistent manner with Toxoptera citricida, the brown citrus aphid (BrCA), being the most efficient. As yet, the molecular interactions between the virus and its aphid vectors have not been determined. This is the first report of aphids acquiring CTV from preparations through an artificial membrane and then transmitting it to receptor plants. The BrCA fed across artificial membranes on crude tissue preparations made from CTV-infected bark tissue were able to transmit CTV to virus-free receptor plants at low rates. CTV p20, p27 and p25 proteins, detected by Western blots, were present in all crude tissue preparations from CTV-infected plants. Partially purified CTV preparations were not transmitted by the BrCA in this manner. Infectivity immunoneutralization experiments were conducted where aphids were forced to feed in vitro on three CTV-specific antibodies (p25, p27 and p20) before being placed on receptor plants following a 48h acquisition feed on CTV-infected source plants. There were no differences in transmission rates among the majority of treatments and the control treatments. However, in one infectivity immunoneutralization experiment, the CTV p20 antibodies significantly enhanced CTV transmission compared to buffer only, pre-immune antiserum or no antibody control treatments. This suggests the inactivity of CTV p20 aids BrCA transmission of virions.

Severity of Citrus tristeza virus Isolates from Texas.

Citrus tristeza virus (CTV) isolates collected from the Lower Rio Grande Valley in south Texas and east Texas were characterized using citrus indicators and molecular methods. The citrus indicators were Mexican lime (Citrus aurantifolia), sour orange (C. aurantium), sweet orange (C. sinensis) grafted to sour orange, Duncan grapefruit (C. × paradisi), and Madam Vinous sweet orange, with some CTV isolates additionally indexed using the Texas commercial grapefruit cvs. Rio Red and Star Ruby, and Marrs and N-33 sweet orange. Severity ratings used 11 biotype groups or cumulative mean relative indices. Molecular characterization was carried out using poly- and monoclonal antibodies, seven strain-specific probes and single-stranded conformational polymorphism, and all were based on the CTV major coat protein or gene. All Texas CTV isolates produced vein clearing symptoms on inoculated Mexican lime plants. Over half of the CTV isolates tested were placed in biotype groups IX and X (causing decline of sweet orange on sour orange, seedling yellows on sour orange and grapefruit seedlings, and stem pitting of grapefruit or sweet orange), and one isolate was in biotype I (mild).

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.