Characterization and assessment of seasonal variation of Citrus tristeza closterovirus (CTV) in citrus in Pakistan

Mandarin (Citrus reticulata) and sweet orange (Citrus sinensis) trees in three major citrus producing regions in Punjab, Pakistan, were assessed to detect and characterize Citrus tristeza closterovirus (CTV). Leaf, twig bark, and stem bark tissues of the trees were tested for the presence of CTV using enzyme-linked immunosorbent assay (ELISA). Plant samples showed positive reaction with CTV specific antisera graft-inoculated on different indicator hosts for characterization of CTV biological properties. ELISA test indicated the presence of CTV in 115 out of 450 citrus trees (25.6%). Biological indexing of virus isolates recovered from CTV infected trees induced only vein clearing symptoms on acid lime (Citrus aurantifolia), but none of these isolates produced any symptoms on other indicator hosts. Assessment of seasonal changes in virus titer showed that it was significantly reduced during the months of June and July in leaf samples. The twig and stem bark samples collected in December also showed a decrease in virus titer. Analysis of the relationship between environmental factors and virus titer showed that temperature and the amount of rainfall significantly affected the CTV titer. This study indicated that citrus trees in Punjab, Pakistan, were infected by mild biotype CTV isolates, and selection of appropriate plant tissues appeared to be important in diagnosing CTV in citrus groves.