English abstract
Citrus Tristeza is the most serious viral disease of citrus in the world. Due to the lack of information about Citrus Tristeza Virus (CTV) in Oman, this study was conducted to characterize distribution, host range, strains and potential sources of CTV in the country.
A survey over 2009 and 2010 that covered 22 different districts and 577 citrus trees in
Oman showed that CTV occurs in most (20) of the surveyed districts in Oman, from
Salalah in the South to Musandam in the far North. A higher percentage of infection
with CTV in lime was detected in the Northern part of Oman (15-47%) compared to the South (0-12%). Citrus species which tested positive for CTV included acid lime, sweet lime, sweet lemon, citron, mandarin and sweet orange. Symptoms of CTV which were detected in different farms during the survey included stem pitting, honeycombing, stunting, leaf curling and cupping and vein clearing.
Further analysis was conducted for 21 randomly selected CTV positive lime samples
using bi-directional reverse transcription-polymerase chain reaction (bi-directional RT
PCR) of the coat protein (CP) gene with the use of HCP1, HCP2, CP3, and CP4 primers.
The analysis showed that 16 samples have severe strains of CTV while the rest (5
samples) were co-infected with severe and mild strains. Cloning and sequencing the CP gene of 4 severe isolates showed that the isolates share over 99.6% nucleotide similarity with each other. Isolates from Oman were found to have up to 99.1% nucleotide similarity with isolate C9E (AY896570) from New Zealand. Phylogenetic analysis of CTV isolates from Oman showed that they belong to CTV Group 4, a group which is known to contain worldwide isolates causing stem pitting and sudden decline in different citrus species.
The study also investigated whether nurseries act as a source of CTV into Oman. > 0 ST .
Investigating the occurrence of CTV in 119 nursery seedlings imported from abroad or propagated locally showed that 53 (45%) seedlings were infected with CTV. CTV was detected in citrus seedlings originating from Syria, Lebanon, India, Pakistan, Egypt and Oman.
Due to the wide distribution of CTV in Oman and the continuous introduction of CTV
from different countries, the study suggests a number of management options tocombat the problem. This includes avoiding the use of susceptible rootstocks (e.g. sour orange) as well as implementing strict quarantine measures in order to reduce the introduction of sever CTV strains into farms in Oman.
