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Epidemiology of viruses infecting hop (Humulus lupulus L.) in Australia


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Pethybridge, SJ 2000 , 'Epidemiology of viruses infecting hop (Humulus lupulus L.) in Australia', PhD thesis, University of Tasmania.

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Significant differences in virus incidence were consistently demonstrated between
cultivars. 'Victoria' gardens planted with elite (virus-tested) material became almost
totally re-infected with PNRSV within eight years. Mechanical inoculation of PNRSV
into a range of hop cultivars suggested ' Victoria' was more susceptible than traditional
ones. In contrast, the spread rate of HpL V, HpMV, and PNRSV was consistently slower
in 'Opal' gardens, and this was found to be the most field resistant cultivar to infection by
all three viruses.
PNRSV was detected by double antibody sandwich enzyme-linked immunosorbent assay
(DAS-ELISA) in chronically infected ' Victoria' plants throughout the growing season.
Testing of a range of tissues from 'Victoria' plants suggested a symmetrical distribution
ofPNRSV within the plant. Similar testing of 'Nugget', 'Pride of Ringwood', and 'Opal'
plants suggested an asymmetrical distribution of PNRSV within the plant. The longer
period of elevated virus levels in all tissues in 'Victoria' may increase the probability of
virus transmission and be responsible for the accelerated transmission of PNRSV in this
cultivar. The asymmetric virus distribution in 'Nugget', 'Pride of Ringwood', and 'Opal'
suggested that accurate virus testing relies upon sampling from several hines from each
Spatial analysis of PNRSV epidemics by ordinary run and radial correlation analyses in
'Victoria' gardens in Myrtleford, Victoria and Bushy Park, Tasmania associated PNRSV
transmission with mechanical mowing of basal growth. Transmission was reduced in
field trials by preventing basal growth contact between infected and virus-free plants
along rows early in the season. This demonstrated that plant contact early in the season
increases the probability of transmitting PNRSV to virus-free plants by decreasing the
distance infective virions have to travel to infect new plants. Glasshouse trials also
confirmed PNRSV to be transmitted by contact and simulated slashing between infected
and virus-free plants. Root grafting was also successful at transmitting PNRSV between
infected and virus-free plants. The presence of root grafts in Tasmanian hop gardens was
suggested by injection of the translocatable herbicide marker, glyphosate. However,
quantifi cation of the extent to which root grafts contribute to transmission of all three
viruses requires further work.
Spatial analysis of carl avirus epidemics showed different distributions between
'Victoria' gardens in Myrtleford and Bushy Park. Random distributions of both HpL V
and HpMV at Myrtleford suggested transmission by alatae aphid vectors. Autocorrelated
along row distributions of both viruses at Bushy Park suggested transmission by either
mechanical transmission through basal growth mowing, and/or aphid vectors (alatae or
apterous) directed along rows from basal growth bridges formed through basal growth
mowing between rows. A significant positive association between HpLV and HpMV
was consistently demonstrated in several cultivars. This may suggest transmission by
common aphid vector species, transencapsidation, or the possibility that infection by one
virus makes the plant more susceptible to infection by the other.
In most hop cultivars grown in Australia the slow rate of virus transmission and
significant effect of some viruses on yield of cones and levels of brewing organic acids
suggested the continued use of a virus certification scheme for planting stock is
warranted. However, in 'Victoria', the usefulness of certification schemes is uncertain
because of the rapid spread ofviruses in this cultivar and its tolerance to infection.

Item Type: Thesis - PhD
Authors/Creators:Pethybridge, SJ
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