Molecular and biological evidence for a severe seedling yellows strain of Citrus tristeza virus spreading in Calabria (Southern Italy)

Citrus tristeza virus (CTV) outbreaks have been reported in the main citrus-growing regions of Italy in the past 10 years. In some areas where eradication efforts failed to suppress spread, high CTV incidence is now observed. Recently, potentially severe CTV strains were detected in Calabria (southern Italy), one of the major citrusgrowing area. As a result, investigations of the virulence and molecular features of CTV populations spreading in this region were undertaken. Virus was detected by enzyme-linked immunosorbent assay (ELISA) using a broad spectrum polyclonal antiserum, and was differentiated into potential virulent categories with the severe-strain discriminating monoclonal antibody MCA13. Isolate genotyping was conducted using reverse-transcription polymerase chain reaction (RT-PCR) with multiple molecular markers (MMM), single-strand conformation polymorphism (SSCP) analysis of the amplicons from the genes coding for the coat protein (CP) p25 and the non-structural p20 protein as well as sequence analysis. Based on the serological reactivity, the isolates were differentiated in two distinct serogroups: MCA13-reactive and MCA13 non-reactive. Similarly, based on the molecular profile, the isolates were grouped in two genetically distinct phylogenetic clusters, and associated either with a T30-like or with a T3-like genotype. These data were related to the results of biological indexing on standard indicator plants, which distinguished isolates causing mild or severe seedling yellow reactions. The study has demonstrated the presence of MCA13-reactive isolates associated with a T3-like genotype and causing severe seedling yellows in sour orange, grapefruit and lemon seedlings, and stem pitting in Mexican lime.