A study on avocado sunblotch viroid (asbvd) with a focus on symptomless carrier trees.
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Abstract
The avocado (Persea americana Mill.) is an important subtropical fruit worldwide
(Blakey et al, 2014). South Africa is among the top five avocado exporters and the
exports are primarily aimed at the European market (DAFF, 2019). According to the
profile of the South African avocado market value chain by DAFF (2019),
approximately 54% of total avocados produced in South Africa are exported, 14% are
sold through the National Fresh Produce Markets (NFPMs), 12% are sold to the
informal markets (bakkies and hawkers), 10% are processed, while the remaining 9%
are delivered directly to retailers. Avocado sunblotch disease (ASBD), caused by
avocado sunblotch viroid (ASBVd), is one of the smallest pathogens inciting
economical losses in the avocado crop worldwide (Palukaitis et al. 1979). The infected
trees can either express ASBD symptoms phenotypically or can show no symptoms
or signs of infection but still be carriers of ASBVd; these trees are known as
symptomless carrier trees (Acheampong et al., 2008). Characteristic ASBD symptoms
include the appearance of irregular, sunken areas of white, yellow, or reddish colour
in infected fruit; white/yellow sunken streaks on green stems and variegated or
bleached symptoms on the leaves (Vallejo-Perez et al., 2014).
The ASBVd symptomless carrier trees have been described as the primary sources
of infection by spreading the disease through budding and grafting practices thus
playing an essential role in the epidemiology of ASBVd (Saucedo Carabez et al.,
2019). Therefore, the main aim of the study was to investigate ASBVd in avocado
symptomless carrier trees. The specific objectives of the current study were to (1)
identify and monitor ASBVd-infected trees from the flowering stage until harvest for
any visible physical indications of the disease compared with healthy trees, (2)
determine the maturity of ASBVd - infected and healthy fruits over time from early
development stages until harvest, (3) determine the productivity of ASBVd - infected
fruits of the symptomless carrier trees to healthy ones by counting the number of fruit
per tree, (4) investigate direct ripening, storage effects and internal quality of ASBVd
- infected fruits, (5) investigate honeybees as possible ASBVd vectors for tree-to-tree
transmission during pollination, (6) determine the occurrence of root graft transmission
of ASBVd from root systems of infected trees to healthy trees, (7) investigate the
impact of cutting tools on the plant to plant transmission, (8) study the movement of
ASBVd from initially infected cells to the rest of the tree through the grafting of infected
scions on healthy rootstocks and (9) investigate t 85 he genetic differences between
ASBVd variants within a clonal symptomless carrier ‘Fuerte’ tree population.
Differences were observed in the orchard between infected and healthy trees; medium
and highly infected trees excessively produced flowers, lost leaves during flowering
and ultimately bore few to no fruit at the end of the season. The dry matter content
results showed that ASBVd did not affect the maturity of the fruit; fruit from infected
and healthy trees matured at the same time. Yield measurements indicated that
medium and highly infected trees produced between 83% and 96% lower yields
compared to healthy trees. Postharvest studies showed that medium and highly
infected fruit significantly lost firmness and developed colour more rapidly than healthy
fruit. Infected fruit also developed external rots and shrivels for non-stored fruit,
however, these injuries were reduced for fruit stored at 5°C for 28 days. Therefore,
flower overbearing with the shedding of leaves and lower yields can be used as an
indication of ASBVd infection in ‘Hass’ orchards; however, confirmation with molecular
testing is required. These observations can be incorporated as an ASBD management
strategy in 'Hass' orchards.
Graft transmission (top-work) was demonstrated by grafting 30 healthy and 30 infected
‘Hass’ scions onto a healthy ‘Bounty’ rootstock. There was a 53% infection success
rate for the infected symptomless carrier trees compared to the 43.3% for the healthy
trees. The statistical analysis showed that there were no significant differences
between grafting the healthy or infected scions to the rootstocks (p≤0.05), implying
that ASBVd spread through the infected scions is unavoidable unless viroid-free
scions are used. Two separate root graft experiments were conducted, one in the
tunnels where a single infected tree was planted in the 10L plastic bag to force root
contact with four healthy trees; the healthy trees outcompeted the infected trees in all
six experiments and they died. In the field, the healthy young ‘Fuerte’ trees were
planted one meter closer to the old infected orchard trees with confirmed ASBVd
positive roots to force root contact. Eventually, the trees died from the lack of sunlight
and water stress and the experiment was discontinued. Seventy-five ‘Fuerte’ clonal
trees were used in the mechanical transmission experiment using pruning shears
(secateurs). The trees were infected by cutting the infected branch and then using the
same shear to prune the healthy trees. The shears were treated with four different
percentages of sodium hypochlorite (3.5% M/V) and the untreated shears were used
as controls. ASBVd was not successfully transmitted 118 mechanically using pruning
shears, however, when the stability of ASBVd was tested on different surfaces, it
showed to survive up to 24 hours. Therefore, if equipment used on an infected tree is
used for the next tree, there is a chance that ASBVd will be transmitted to the next tree
and lead to ASBVd spread in an orchard. Pollen and bees were sampled from the
beehives at four different sites in KwaZulu-Natal. ASBVd was successfully detected in
the pollen from all four site sites, however, only detected in the bees from three sites The samples were sequenced and the blast results confirmed the sequences to be
ASBVd and the phylogenetic analysis gave a 93% identity between the detected
sequences and the existing ASBVd variants retrieved from the GenBank® database.
From the current study, it was confirmed that graft transmission is the most prevalent
mode of transmission for ASBVd and that honey bees carry pollen from infected trees
to healthy trees in the field and can play an important role in pollen transmission.
A total of 103 positive symptomless carrier trees were detected with a real-time qRT132
PCR assay from a population of 453 young ‘Fuerte’ trees. Results showed that 22%
of this population was infected with ASBVd without showing signs of infection. In a
further investigation, complete ASBVd genomes were obtained by using a
conventional PCR with primer sequences that yielded a 250 bp product, here only 76
samples tested positive for ASBVd, the remaining 27 samples tested negative. The 76
samples were sequenced and the sequences obtained had lengths that variedbetween 248 and 253 nucleotides. From the original 76 sequences, 42 ASBVd
variants were identified and several sequences were repeatedly detected in the
population. The variants were deposited to the NCBI GenBank® and were assigned
ON135462 to ON135503. The phylogenetic analysis of the variants obtained from this
study showed a high sequence identity of 97% with the reference ASBVd variants
obtained from GenBank®. The current study is crucial for the development of accurate
detection techniques for ASBVd and contributes to the ASBVd action plan goals that
promote the removal of all infected material from avocado orchards to prevent the
further spread of the viroid in avocado orchards.
Description
Doctoral Degree. University of KwaZulu-Natal, Pietermaritzburg.