Molecular Characterization of FloridaCitrus Tristeza Virus Isolates with PotentialUse in Mild Strain Cross Protection

Abstract

Mild strain cross protection (MSCP) is an important strategy for maintaining citrus production in Florida. Recent surveys indicate that severe strains of citrus tristeza virus (CTV) are spreading following the arrival of the brown citrus aphid in 1995, thus, increasing the threat of CTV to the Florida industry. It is desirable to be able to detect and select mild strains for further evaluation as protective isolates with potential for MSCP. Recent progress on the characterization of the CTV genome has led to the development of several methods to differentiate strains, which previously could only be identified by time-consuming bioassays. In addition to the serological differentiation using MCA-13 monoclonal antibody for the selective detection of severe strains, recently developed methods are based on the major and minor coat protein gene sequences: strain specific probes and single stranded conformational polymorphism assays. These methods were applied to characterize isolates of CTV and aphid transmitted sub-isolates to identify the presence of mixed infections containing both mild and severe strains. We report here the screening of 20 CTV isolates collected from surviving trees in areas where CTV was causing decline on sour orange rootstock. The results indicate that 10 isolates were mild strains (T30like), five were decline strains (T36-like), and five isolates were mixtures containing both mild and decline strains or mild and stem pitting strains of CTV.