ABSTRACT


Citrus tristeza virus (CTV) has caused substantial economical losses to the citrus industry worldwide. In Chile, the real situation of the disease was not clearly understood until this study was conducted, although the virus had been identified and reported at the end of the 1960 decade, affecting Meyer lemon trees which were eradicated. Considering the importance of CTV and the disease that causes, the basic objective of this thesis was to analyze the present CTV situation in Chile, a broad survey was conducted to estimate its incidence, studying its epidemiology, and carrying out a complete biological, serological and molecular characterization of the CTV Chilean representative isolates. The incidence was estimated sampling 1% of the trees from the citrus tree regions: I, III, IV, V, VI, VII and Metropolitan Region. CTV was detected by ELISA-DAS between the years 1999-2001 and by Tissue Print-ELISA between the years 2001-2004. Both techniques were carried out combining the monoclonal antibodies 3DF1 and 3CA5. Furthermore, symptoms were recorded, established aphid species colonies monitored and identified. One hundred CTV isolates collected from different hosts and ecological zones were characterized. The biological characterization was conducted on Mexican lime, sweet and sour orange, grapefruit and the sweet/sour and sour/sweet combinations. The serological characterization was done by DASI-ELISA with twelve different CTV specific monoclonal antibodies. The molecular characterization was done employing 12 different pairs of RT-PCR primers. Additionally RT-PCR and real-time PCR were carried out with CTV universal primers. Also, an epidemiological study was conducted to evaluate the spatial-temporal evolution of CTV in six different experimental plots, and two transmissions tests were done with ten representative Chilean CTV isolates.

CTV was detected in all the citrus regions in Chile, with the exception of Region VII, with an average media of 0.38% and associated to all the citrus species. Decline symptoms were not found on trees grafted on sour orange. In the Pica Oasis (Region I), stem pitting was found in grapefruit and Mexican lime trees. The CTV vector aphid species detected were: Aphis gossypii, A. spiraecola and Toxoptera aurantii. According to the results obtained in the CTV Chilean isolate characterization, most isolates present in the central zone of Chile were considered mild (MCA 13 negative), while in the Region I, the presence of severe isolates was established, some capable of causing stem pitting on grapefruit and sweet orange, all resulting MCA13 positive and with molecular characteristics associated to VT type. In the spatial-temporal study, it was possible to demonstrate an increase in the CTV incidence in the experimental plots over time, with the exception of one lemon plot. In the transmission trials only one CTV isolate was transmitted with low efficiency.

In view of the results obtained, a series of recommendations are given to the Chilean citrus industry which include the exclusive use of certified plant material and the convenience of eradicating the source of severe isolates.