Managing viruses of horticulture: Citrus tristeza virus mild strain cross-protection

Abstract

Citrus tristeza virus (CTV) is the most destructive virus-associated disease of citrus worldwide. This graft- and aphid-transmissible Closterovirus can cause severe disease of which stem pitting (SP) represents its most economically important symptom in New Zealand. CTV possesses six phylogenetically distinct strains, named RB, T68, VT, T30, T3 and T36. Mild strain cross-protection (MSCP) is a method of protecting plants from infection by a severe virus isolate by deliberate infection with a mild isolate of the virus. Two putative CTV mild isolates, termed ‘Miho isolate’ and ‘Parent isolate’, were obtained by Hort-Research prior to 1996. The Miho isolate was obtained from a Miho Satsuma mandarin ‘donor’ tree and the Parent isolate was maintained in two Parent navel orange ‘donor’ trees termed ‘Parent 1’ and ‘Parent 2’. In MSCP trials set up in two commercial citrus orchards in 2008 (Taipa) and 2009 (Gisborne), three navel orange ‘receptor’ cultivars (Fukumoto, Navalina and Newhall) were inoculated with either the Miho or the Parent isolate. In this project CTV infecting ‘donor’ and ‘receptor’ trees as well as mandarin and navel orange trees surrounding the MSCP trial plots (‘neighbouring’ trees) was characterised by quantifying SP and by determining their CTV strain identity via next generation sequencing and quantitative reverse transcription polymerase chain reaction. The Miho ‘donor’ tree was found to contain RB, Parent 1 contained a mixture of RB, T68 and VT and Parent 2 contained RB and VT; none of the donor trees exhibited SP symptoms. Miho ‘receptor’ trees contained RB or T30 and remained largely symptomless. Parent ‘receptor’ trees contained either VT or a mixture of T68 and VT and exhibited symptoms ranging from mild to severe SP. ‘Neighbouring’ trees contained predominantly T3 and remained largely symptomless. CTV symptomology was primarily dependent upon infecting CTV strain not upon the ‘receptor’ navel orange cultivar. Correlations between symptom and CTV strain identity data suggest that the Parent isolate is not a suitable candidate to be used for MSCP in New Zealand while the Miho isolate could potentially be used to cross-protect commercial citrus orchards against infection with severe strains of CTV in New Zealand.