Incidence of Citrus Tristeza Virus, the Stem Pitting Isolates in Thailand

Citrus tristeza Closterovirus (CTV) occurs in most citrus producing regions of the world including Thailand. Strains of CTV have been identified and the major groups of strains are mild strain (M), decline-inducing strain (D) and stem pitting strain (SP). Nigh incidence of CTV infected citrus with prominent severe stem pitting symptoms was recorded during 2004-2005 survey. The suspected SP strains of CTV were found in three different cultivars of mandarin (Citrus reticulata), namely, 'Orah', 'Shogun' and 'Somkeawan'. The virus was isolated from naturally infected citrus host by leaf grafting and tested for biological characters in Mexican lime seedling, the indicator plant. Most of SP isolates caused severe stem pitting, corky vein and growth reduction in the indicator host. The growth (height) of indicator plants infected with SP-CTV isolate decreased by 30-68% compared with the healthy plants. The single strand conformation polymorphism (SSCP) profile of p27 (minor coat protein gene) of CMO1 and TS12, Thai SP-CTV isolates showed similar migration pattern of two DNA hands in polyacrylamide gel. Confirmation has been made through p27 nucleotide sequence analysis. Results indicated that CMO1 and TS12 shared 99% identities of their nucleotide sequence. In addition, amino acid sequences deduced from p27 gene of the two SP isolates were different only at amino acid residue position 9 as lysine (K) for CMO1 and asparagine (N) for TS12. Attempt will be made to produce antiserum against p27 minor coat protein to be used to identify SP strain of Thai isolates.