Sensitivity of citrus rootstocks to two citrus viroids and the influence on tree growth
Date
2022-03
Authors
Journal Title
Journal ISSN
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Publisher
Stellenbosch : Stellenbosch University
Abstract
ENGLISH ABSTRACT: Despite the supply of disease-free propagation material to the industry by the South African Citrus
Improvement Scheme, citrus viroids, particularly citrus dwarfing viroid (CDVd) and hop stunt viroid
(HSVd), are occasionally detected in commercial orchards. Locally, these viroids were associated
with stunting of trees on sensitive trifoliate hybrid rootstocks. New rootstock cultivars are
developed and, in addition to their horticultural performance, it is important to evaluate the
effects
of viroid infections.
A field trial was established consisting of ‘Midknight’ Valencia on ten different rootstocks,
including a trifoliate (‘Rich16-6’), two non-trifoliates (Rough Lemon ‘Cairn’ and ‘Rangpur’ lime)
and seven trifoliate hybrids (‘US-812’, ‘MxT’, ‘C-22 Bitters’, ‘C-54 Carpenter’ and ‘C-57 Furr’
citrandarins, as well as ‘Carrizo’ and ‘C-35’ citranges). The scions were graft-inoculated with
CDVd as a single infection, and in combination with HSVd to investigate potential viroid
interactions.
Transmission detection was notably erratic in scions on some rootstocks, particularly MxT and C-22,
indicating that rootstocks may influence viroid distribution within scions. To study the effect of
rootstocks on spatial distribution of CDVd and HSVd within scions on C-35, Carrizo, MxT and Rough
Lemon, a relative RT-qPCR assay was developed, using GAPC2 (Glyceraldehyde-3- phosphate
dehydrogenase C2) and UPL7 (Ubiquitin-protein ligase 7) reference genes for qPCR data
normalization, to determine viroid concentration ratios from inner and outer scion canopy
positions. Large variation in concentration ratios were seen for both viroids, demonstrating uneven
spatial distribution, especially for CDVd in scions on C-35 and Rough Lemon, but also HSVd in
scions on C-35. Despite this variation, the inner canopy proved to be a more reliable sampling
position for detection as higher viroid titres were mostly determined from this position.
Absolute CDVd and HSVd quantification was done from inner canopies of scions on the ten rootstocks.
CDVd copies ranged from 1 to 831, and HSVd copies were determined in a broader range from 8 to
3963. Rootstock cultivar had a significant influence on viroid titres in the scion. Scions on C-22
and Rich16-6 had significantly lower CDVd titres than C-57, Carrizo, ‘Rangpur’ lime, Rough Lemon
and US-812. However, scions on MxT and Rich16-6 accumulated significantly fewer HSVd copies than
C-35, C-57, Carrizo and ‘Rangpur’ lime, indicating that viroid species accumulation differed for
each rootstock. CDVd titres were not significantly influenced by co-infection with HSVd, but HSVd
reached significantly higher copy numbers than CDVd in scions on all ten rootstocks, with as much
as a 24-fold increase of HSVd compared to CDVd copies in
scions on C-35.
CDVd and HSVd genome sequences generated from a scion of each rootstock, 19 months post
inoculation, were determined and mutations were detected within the pathogenic regions of both
viroids. The significance of these mutations requires further investigation. After 2 years of field
growth, no significant influence of viroid infections on either trunk circumference or canopy
volume, were observed. However, impact on growth is expected only after 4 to 5 years in the field.
This study indicated that rootstock cultivar influenced the spatial distribution, accumulation and
genetic variability of two citrus viroids within ‘Midknight’ Valencia scions. The quantitative
real-time RT-PCR assays developed in this study will be used in further analysis of this field
trial to monitor viroid titres in both scions and rootstocks to unravel the complex interactions of
the
viroids with both scion and rootstocks.
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar.
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar.
Description
Thesis (MScAgric)--Stellenbosch University, 2022.