Characterization of Citrus tristeza virus Single-Variant Sources in Grapefruit in Greenhouse and Field Trials

Abstract

Citrus tristeza virus (CTV) is endemic to southern Africa and the stem pitting syndrome that it causes was a limiting factor in grapefruit production prior to the introduction of cross-protection in the Citrus Improvement Scheme. This disease mitigation strategy, using various field-derived CTV sources, has significantly extended the productive lifespan of grapefruit orchards in South Africa. CTV commonly occurs as a population of various strains, masking the phenotypic effect of individual strains. Likewise, current South African CTV cross-protection sources are strain mixtures, obscuring an understanding of which strains are influencing cross-protection. The severity of various CTV strains has mostly been assessed on sensitive indicator hosts, but their effect on commercial varieties has seldom been investigated. Single-variant CTV isolates were used to investigate the phenotypic expression of CTV strains in commercial grapefruit varieties as well as CTV indicator hosts. They were biologically characterized for their ability to cause stem pitting and their rate of translocation and titer in the different hosts, monitored by enzyme-linked immunosorbent assay. Complete genome sequences for three CTV strain variants were generated. Isolates of CTV strains VT, T68, RB, and HA16-5 did not induce severe stem pitting in four grapefruit hosts in a glasshouse trial. Viral titers of the strains differed in the grapefruit hosts, but the RB isolate reached a higher titer in the grapefruit hosts compared with the VT, T68, and HA16-5 isolates. Additionally, horticultural assessment of two grapefruit varieties inoculated with the RB isolate in two field trials demonstrated that mild stem pitting did not negatively influence the horticultural performance of the grapefruit trees over an eight-year assessment period. ‘Star Ruby’ trees containing the CTV source GFMS35 showed less stem pitting than trees inoculated with the RB isolate, but had smaller canopy volumes and lower yields than trees containing the RB isolate. This suggests that the influence of CTV sources on tree performance is not limited to the effect of stem pitting.

In silico analysis of the grapefruit sRNAome, transcriptome and gene regulation in response to CTV-CDVd co-infection

“…Grapefruit plants (cultivars ‘Marsh’ and ‘Star Ruby’) on ‘Carrizo’ citrange rootstocks were bark-inoculated from an asymptomatic Citrus sinensis (sweet orange) plant that was confirmed to be infected with CTV (genotype T3) and CDVd by RT-PCR using previously described protocols [ 28 , 29 ]. Briefly, bark-inoculation was performed by patch-grafting two bark chips of the source plant to each grapefruit scion, once the scion was approximately 7 mm thick.…”

Population dynamics of Citrus tristeza virus (CTV) in single aphid-transmitted sub-isolates of the South African GFMS12 isolate

“…The objective of this study was to characterize the complexity of the mild CTV isolate, GFMS12 using the HMA. The field isolates of GFMS12 were studied previously ( van Vuuren et al., 2000 ; Scott et al., 2013 ; Zablocki and Pietersen, 2014 ; Cook et al., 2016 ; Read and Pietersen, 2016 ). The GFMS12 isolate used for the current study was maintained since 1994 in the USDA ARS Exotic Citrus Disease Quarantine Facility, Beltsville, MD, and designated as B389.…”

Citrus Tristeza Virus Genotype Detection Using High-Throughput Sequencing

“…The term genotype is used to group genetically similar genomes together [32,33]. These genotypes are found worldwide as members of mixed populations within a single host plant; therefore, complicating the genotypic and phenotypic associations since different combinations of genotypes affect symptom expression and disease severity [31,34,35]. Depending on the genotype and the citrus scion-rootstock combination, CTV can cause three syndromes named tristeza (quick decline), stem pitting, and seedling yellows [35].…”

“…Depending on the genotype and the citrus scion-rootstock combination, CTV can cause three syndromes named tristeza (quick decline), stem pitting, and seedling yellows [35]. CTV has caused major tree losses resulting in a severe decline in production [35]. Both quick decline and stem pitting can result in tree loss.…”