Within the remaining 44 genotypes,
a self-seedling of Cabernet Franc and
first generation hybrids of V. vinifera
were identified We were also able
to identify the parent cultivar of six
hybrids. Species designations were
assigned to the 38 remaining unique
genotypes by calculating the probability
of a sample being V. vinifera
rather than V. californica because there
is no database of V. californica profiles.
In this manner, a decreasing V.
vinifera contribution was illustrated
(Figure 1).
Results III: RT-PCR assays
The Vitis and non-Vitis samples collected
in Fall 2008 were assayed using
both conventional RT-PCR and realtime
RT-PCR.
175 Vitis samples had positive CT
values between 17.1 and 35.6, which
for all samples indicated a nucleic
acid amount within a 105-fold difference,
relative to positive controls.
Results IV: Virus incidence
In six out of nine sites adjacent to
vineyards (Sites 1, 5, 6, 7, 8, 9), all wild
Vitis tested negative for the viruses
included in this study. At three sites,
(Sites 2, 3, 4), which were within 2 km
of each other along the Napa River,
100% of the wild Vitis collected in
2008 tested positive for GVA and/or
GLRaV-3 (samples circled in yellow
in Figure 3). We returned to these
three sites in 2010 and collected 164
additional samples; the incidence of
GLRaV-3 was 50% to 80% in wild Vitis
and 28% to 80% in V. vinifera within
vineyards (Figure 3).
In the two sites not adjacent to
vineyards (Sites 10 and 11), 4 out of
21 (19%) of the wild Vitis tested positive
for virus. Three samples tested
positive for all four viruses — GLRaV-
2, -3, GVA, and GVB — and one
tested positive for GVA only (data not
shown).
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All Vitis samples tested negative for
GLRaV-1, -4, -5, -9, and GVD. All
non-Vitis tested negative for all viruses
investigated.
Leafroll symptoms in wild vines
were observed sometimes, but often
not.
Results V: Viral sequences
Partial CP gene sequence data was
collected for five GLRaV-2, 17 GLRaV-
3, five GVA, and five GVB isolates
from non-cultivated Vitis and adjacent
V. vinifera. The GLRaV-2 RT-PCR
products were 88% to 96% identical
at the nucleotide level to GLRaV-2
gene sequences in GenBank; GLRaV-3
products were 88% to 100% identical
to GLRaV-3 gene sequences; GVA
products were 91% to 93% identical to
GVA gene sequences; and GVB products
were 91% to 97% identical to GVB
gene sequences.
Conclusions
A high incidence of GLRaV-3 and/
or GVA in V. californica and V. californica
x V. vinifera hybrids was identified
in one localized area of the Napa
Valley, establishing that these wild
Vitis are GLRaV-3 and GVA hosts.
Wild grapes at six other sites were
not positive for the viruses included
in this study, despite the fact that most
sites had many wild vines within
several meters of virus-infected vineyards.
This suggests that GLRaV-3
and GVA incidence is not widespread
in V. californica and hybrids in Napa
Valley.
Analysis of GLRaV-3 gene sequences
indicated that the GLRaV-3 isolates
from wild and cultivated grapes were
closely related. This suggests that
these wild isolates probably do not
represent a major source of diversity
that could lead to major epidemics in
V. vinifera.
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GLRaV-2 and GVB were detected
in wild grape samples collected in
riparian areas not adjacent to vineyards.
Analysis of the GLRaV-2 gene
sequences suggested that GLRaV-2
was probably introduced into riparian
areas relatively recently, via V
vinifera infected with GLRaV-2.
GLRaV-2 is the only leafroll-associated
virus included in the genus
Closterovirus, whose other members
are transmitted semi-persistently
by aphids. No GLRaV-2 vectors
have been identified, however, and
spread within vineyards has not been
observed.
Aphids are not common on grapes
in California, but have been periodically
reported. It is possible that they
could be much more abundant in
riparian areas and serve as a GLRaV-2
vector.
Heartfelt thanks to the American
Vineyard Foundation and the North
Coast Viticulture Research Group for
funding this project; numerous growers
in Napa County; K. Lowe and P. Saldivar
for assistance in locating study sites; J.
Roncoroni, J. DiTomaso, and E. Dean for
plant identification assistance; J. Yang,
FPS, for assistance in DNA identification;
and numerous FPS staff for assistance
in sample collection and testing.
For more information:
Klaassen, V.A., S.T. Sim, G.S.
Dangl, F. Osman, M. Al Rwahnih,
A. Rowhani, and D. Golino. 2011
Vitis californica and Vitis californica
x Vitis vinifera hybrids are hosts for
Grapevine leafroll-associated virus-2
and -3 and Grapevine virus A and B.
Plant Dis. 95:657-665.
online here.
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