Citrus tristeza virus strain interactions impact stem pitting in grapefruit.

Cross-protection is the application of mild variants of viruses to mitigate viral diseases in crops. Superinfection exclusion (SIE) was demonstrated for citrus tristeza virus (CTV) in citrus whereby a homologous strain to a primary infection was excluded from infection, but not a heterologous strain. SIE was proposed as the mechanism facilitating cross-protection in citrus. A glasshouse trial was conducted to assess the principle of SIE and the pathogenesis of CTV strain interactions. 'Duncan' grapefruit (Citrus × paradisi [Macfad.]) was used to evaluate stem pitting induced by single-strain isolates T68, RB and HA16-5, but also their influence as primary infections on stem pitting post challenge with a severe stem pitting T68 isolate. The trial was duplicated at two temperature regimes. Stem pitting was associated with three of four isolates with varied severity, which was enhanced at the cooler temperature range. Stem pitting severity of the challenged plants depended on the primary inoculation, indicating strain interactions. Less severe stem pitting was observed post challenge with HA16-5 and the mild T68 isolates as primary infections, but not with the RB strain. SIE was not a durable defence, and the severe T68 challenge variant was detected over time in plants with the mild T68 variant as the primary infection. The severe T68 variant dominated in plants dually inoculated with mild and severe stem pitting T68 variants. Quantitative RT-PCR indicated no significant concentration difference between these T68 variants as single infections implying that viral concentration alone did not account for the dominance of the severe variant.