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Grape rootstocks were first developed to address the phylloxera crisis during the late 1800s, and many of these rootstocks continue to be used. However, changes in the climate, water availability, pest pressure, and pest control practices require the development of new and better-adapted rootstocks. Many of the traits we need to address these issues are from Vitis species that have not been widely used in the past, either because they rooted poorly or had marginal phylloxera resistance.
Breeding Grape Rootstocks for Resistance to Phylloxera and Nematodes
– It’s Not Always Easy
M.A. Walker, K. Lund, C. Agüero, S. Riaz, K. Fort, C. Heinitz and N. Romero
Department of Viticulture and Enology
University of California, Davis
California 95695
Keywords: fanleaf virus, phylloxera diversity, Vitis, Muscadinia, ring nematode
Grape rootstocks were first developed to address the phylloxera crisis during
the late 1800s, and many of these rootstocks continue to be used. However, changes
in the climate, water availability, pest pressure, and pest control practices require
the development of new and better-adapted rootstocks. Many of the traits we need to
address these issues are from Vitis species that have not been widely used in the past,
either because they rooted poorly or had marginal phylloxera resistance.
The UC Davis grape rootstock breeding program has been studying
phylloxera resistance for many years. Recent studies have examined the
development of phylloxera strains capable of aggressive nodosity-based feeding on
resistant rootstocks and the occurrence of foliar feeding strains, once rare in
California. In order to better understand the origin of these strains, more than 500
leaf gall phylloxera samples were collected from 19 States along a meandering 5,000
km transect across phylloxera’s native range. Analysis of SSR data from 22 markers
found that phylloxera populations primarily grouped by host.
Efforts to utilize Muscadinia rotundifolia’s exceptional pest resistance
continue with an emphasis on resistance to nematodes, phylloxera and fanleaf
degeneration. Large hybrid populations between various rootstocks and Vitis species
× Muscadinia rotundifolia have been created. Many of these have strong resistance
and some root moderately well, however no fertile progeny have been found. We are
testing the breadth and durability of phylloxera resistance of a few fertile V. vinifera
× M. rotundifolia hybrids in hopes of using these to introgress M. rotundifolia’s pest
and disease resistance into commercial rootstocks. We are also using V. arizonica
and a number of related species from the southwestern United States that possess
strong resistance to Xiphinema index, Pierce’s disease, drought and salinity. These
breeding efforts include developing strongly linked markers from SSR-based genetic
maps to expedite traditional breeding and the physical mapping of resistance genes.
Five rootstocks have been released from the program with resistance to aggressive
root-knot nematode strains; X. index; these nematodes in a combined inoculum and
at high soil temperature; and resistance to lesion, citrus and, in one case, ring
This review of the University of California, Davis grape rootstock breeding
program will focus on current and recent breeding efforts. The breeding of rootstocks
began with the inadvertent importation of the grape phylloxera (Daktulosphaira vitifoliae
Fitch) into France in the 1860s. This American insect pest spread rapidly through the
vineyards of Europe, which were planted with own-rooted vines of the highly susceptible
Vitis vinifera, the world’s predominant wine, table and raisin grape. There were three
approaches to controlling this insect pest: killing the insect primarily with carbon
bisulfide; breeding new hybrid grape varieties with resistance from American grape
species – the Hybrid Direct Producers; and developing grape rootstocks with resistance
from the American Vitis species. This last approach was the most successful and led to
the development of 100s of rootstocks developed between 90 and 120 years ago. About
Proc. VIth Intl. Phylloxera Symp.
Eds.: N. Ollat and D. Papura
Acta Hort. 1045, ISHS 2014
20 of these rootstocks are commonly used to control phylloxera. This brief history of
phylloxera is surprising in two aspects: 1) that this resistance has remained effective over
this long period of time; and 2) that there has been relatively little rootstock breeding until
recently. It is also interesting that the vast majority of rootstocks are hybrids among three
species – V. riparia, V. rupestris and V. berlandieri. The first two are the only Vitis
species that root from dormant cuttings as easily as V. vinifera, and V. berlandieri
provides excellent lime tolerance for the often calcareous European soils. Grape breeding
efforts at the University of California, Davis (UC Davis) began with H.P. Olmo who
released many table and wine grape varieties. He also produced the “K” and “L” series
rootstock selections, which have good root-knot nematode resistance and included crosses
of V. champinii with V. riparia. Lider exported several of these selections to Australia –
where K51-32 and K51-40 have been used to a limited extent.
Fanleaf Degeneration
Olmo also produced a series of V. vinifera × Muscadinia rotundifolia (VR hybrid)
rootstocks as part of his efforts to utilize M. rotundifolia’s tremendous pest resistance.
L.A. Lider and A.C. Goheen tested six of these selections in several fanleaf degeneration
sites in the late 1970s and 1980s. These trials led to the patenting and release of two VR
hybrid rootstocks O39-16 and O43-43 (Walker et al., 1991). O43-43 collapsed to
phylloxera within a few years of commercial use and it was pulled from distribution.
O39-16 was only recommended for use in fanleaf degeneration sites and growers were
advised that its V. vinifera parentage might result in eventual decline to phylloxera
feeding as phylloxera strains adapted to it. However, O39-16 has survived without signs
of phylloxera infestation for over 20 years in California vineyards. Both of these
rootstocks have the unusual ability to induce tolerance to fanleaf infection (Walker et al.,
1994a, b). Although both resist the feeding of grapevine fanleaf virus’ (GFLV) nematode
vector (Xiphinema index), the nematode’s probing into young xylem tissue allows GFLV
to spread up the rootstock and into the scion, where it attains high titers, but does not
cause the typical disrupted set and very low crop yields associated with fanleaf
degeneration. This response is likely related to O39-16’s M. rotundifolia parentage and its
root system; and may be the result of the production of more or less of a phytohormone
capable of compensating for GFLV’s impact on flowering.
We have studied this phenomenon in an effort to discover what is responsible for
root-induced tolerance to fanleaf infection. We grafted GFLV-infected and non-infected
buds to O39-16 and the highly susceptible Rupestris St. George (Rupestris du Lot) and
evaluated xylem constituents at bud-break and anthesis. These sap samples were
evaluated at the UC Davis Metabolomics Facility and we analyzed the data searching for
compounds correlated with fanleaf tolerance. There is some evidence that cytokinins are
involved but we have not linked this conclusively and the biochemistry of cytokinin
production and presence of multiple precursors and forms has complicated the evaluation.
We hope to subject this data to a systems biology analysis with Dr. Dario Cantu, a new
member of the UC Davis Department of Viticulture and Enology faculty.
Release of New Rootstocks
Five rootstocks were recently released from UC Davis. They were selected for
their ability to resist a wide range of nematodes including Xiphinema index, Meloidogyne
incognita Race 3; two strains of Meloidogyne capable of damaging the root-knot
nematode resistant rootstocks Harmony and Freedom – HarmA and HarmC; the 3 strains
in a mixed inoculum; the three Meloidogyne strains at 32°C soil temperatures where root-
knot resistance from V. champinii often collapses; and a combined inoculum of HarmA,
HarmC and X. index. They were then tested for their ability to resist grape phylloxera
feeding on root tips (nodosities), citrus nematode (Tylenchulus semipenetrans), lesion
nematode (Pratyclenchus vulnus), and ring nematode (Mesocriconema xenoplax) – see
Table 1. They were named GRN-1 through GRN-5, have been patented and are available
from California nurseries. The University of California, Davis Technology Transfer
Service is in charge of international licensing and thus far they have been released to
Chile under test agreement. These rootstocks have been planted in a number of field trials
across California and more information on their viticultural characteristics will soon be
available. GRN-1 has the broadest and strongest resistance, but is more difficult to
propagate due to its V. rupestris × M. rotundifolia parentage. It is hoped it will have the
same ability to induce tolerance to fanleaf degeneration that O39-16 has without the
potential phylloxera susceptibility (due to the V. vinifera in O39-16’s parentage). GRN-2,
-3 and -4 have strong resistance based on Dog Ridge, a selection of V. champinii, ‘Riparia
Gloire’, and V. rufotomentosa. GRN-5 has resistance from ‘Ramsey’, ‘Riparia Gloire’ and
a different selection of V. champinii.
Ring Nematode
Current nematode resistance breeding efforts are focused on strengthening
resistance to ring nematode (Mesocriconema xenoplax), which is lacking in all but the
GRN-1 and O39-16 rootstocks. This nematode builds to high numbers in vineyard soils
and is a major cause of replant disorder. This disorder is found in many California
vineyards where high value vineyard sites are pulled in the Fall and replanted in the
Spring without a fallow period. We have discovered strong ring nematode resistance in
Vitis × Muscadinia hybrids, and have also found strong resistance in V. acerifolia and V.
doaniana and are advancing selections to field testing (Table 2). These two species are
also being examined for their salt and drought resistance.
Ring nematode resistance from M. rotundifolia is also being pursued. Breeding
populations were created using the rootstocks 101-14 Mgt, Kober 5BB and pollen from
M. rotundifolia Trayshed. These hybrids must first be screened for their ability to root
from dormant cuttings, since M. rotundifolia will not root from dormant cuttings. Some of
these progeny have rooted well in preliminary screens and selections have strong
resistance to phylloxera, X. index, HarmA, HarmC and ring nematodes. Selections with
the broadest resistance to these pests will be re-evaluated for their ability to root from
dormant cuttings. In addition, about 300 of these selections have been evaluated for
fertility. Crosses between Vitis and Muscadinia normally result in sterile progeny with
2n=39. There have been a few rare examples of fertility in V. vinifera × M. rotundifolia
hybrid selections and these have been used in efforts to exploit M. rotundifolia’s excellent
resistance to powdery mildew, Erysiphe necator (Riaz et al., 2011). We have examined
over 300 of the Vitis rootstock × M. rotundifolia Trayshed hybrids for both ovule and
pollen fertility, but have not yet discovered any fertility. Such fertility would be very
valuable and allow the integration of M. rotundifolia’s unequaled pest and disease
resistance with rootstock traits like better rooting, better nutrient uptake, or resistance to
drought and salinity. We are also testing 20 VR hybrids, with varying levels of fertility, to
see if they have strong phylloxera resistance prior to integrating them into the rootstock
breeding effort.
Grape phylloxera was introduced into California about the same time it was
introduced into Europe. It spread rapidly through own-rooted plantings of V. vinifera
cultivars. By 1890 replanting had occurred with rootstocks imported from Europe, and
large-scale rootstock trials were initiated across California by United States Department
of Agriculture (USDA) and UC Davis scientists to determine which were best adapted.
Lider summarized these trials in 1958, which preceded large scale replanting and
expansion of vineyards in the late 1960s and 1970s. The evaluation of data from scores of
trials over northern and central California viticultural regions determined that many
rootstocks performed well, but AXR#1 did well in almost all regions and vineyards.
These trial sites were assumed to have phylloxera, as they were often replant sites. Other
V. vinifera × V. rupestris rootstocks (1202C, 93-5C) also performed well. Although some
California researcher’s knew of the French, Italian and South African experiences with
AXR#1 and its failure after 15 years of use, they felt California’s phylloxera were less
aggressive. AXR#1 was not used in large-scale monoculture until the planting boom of
the late 1960s and early 1970s when many vineyard were replanted and new acreage was
added to northern California. AXR#1 was used in more than 75% of those plantings. In
1985, 15 years later, AXR#1 was discovered to be failing to phylloxera. AXR#1’s failure
was initially attributed to the evolution of a single more aggressive strain – named biotype
B, but it was soon clear that many genotypes shared the ability to overcome AXR#1’s
weak resistance. In fact, the genetic diversity of phylloxera was unexpectedly high in
California (Fong et al., 1995), and there are still strains including one in the UC Davis
vineyards that do not feed on AXR#1. There are still a few acres of AXR#1 remaining in
cultivation, but they are in remote isolated sites.
Karl Lund has been working on the biology and genetics of phylloxera in my lab
and is now completing his PhD dissertation. He evaluated the feeding ability of
phylloxera strains collected from resistant rootstocks on select rootstocks and grape
species accessions that had previously shown to be resistant or susceptible within a given
species (Grzegorczyk and Walker, 1998). He identified new sources of hypersensitivity,
but found that this type of resistance is phylloxera strain specific. He also used 22 newly
developed SSR markers to analyze over 500 phylloxera samples collected from 19 States
across the US. Preliminary results suggest that there are 5 populations of phylloxera in the
US and that they are associated with Vitis species hosts (Fig. 1). Analysis also suggested
that the sexual cycle is common across the native range, but that asexual reproduction is
the most common reproductive mode in the introduced range.
Foliar phylloxera have been rare in California, until a recent outbreak in rootstock
mothervine nursery plantings. This infestation has spread to the USDA National
Germplasm Repository near Davis and UC Davis’ Foundation Plant Services vineyards.
Karl Lund examined over 280 foliar phylloxera samples collected over an 80 km range
along the east side of the Coast Range. Spread there was rapid and apparently due to
prevailing winds along this corridor. Genetic analysis with the above markers found that
this new foliar strain may be unique from what has been found in California and may
have a central US origin (Fig. 2).
We have also established mapping populations to explore the genetics of
resistance in V. berlandieri 9031, which shows a strong hypersensitivity to most strains of
phylloxera. This accession has also performed very well in salt resistance studies.
Southwestern U.S. Vitis
In 1988, Olmo made a series of crosses between two V. rupestris and 6 M.
rotundifolia genotypes, for which I provided pollen from forced muscadine cultivars in
the greenhouse. When I was hired in 1989 Olmo gave me the seeds from the 12
populations and we began screening them for resistance to phylloxera, M. incognita, X.
index and later Pierce’s disease (PD). There was resistance to all these pests, but the
segregation ratios did not reflect any particular genetic control. The populations began
flowering and set viable seed, there were also individuals with petioles and internodes
with sparse cobwebby hair. The progeny should have been sterile and glabrous. We made
sibling crosses to create several F2 generations that were tested for X. index resistance and
PD – the F2 populations segregated as though resistance was controlled by a single
dominant gene in both cases. We began genetic mapping with AFLP markers for both of
these traits (Doucleff et al., 2004) and later with SSR markers (Krivanek et al., 2006; Riaz
et al., 2006). The SSR based mapping effort clearly showed that these populations were
not created with M. rotundifolia parents, although there were 5 true V. rupestris x M.
rotundifolia progeny. At this point, M. rotundifolia was thought to be one of the few
grape species with strong resistance to phylloxera, M. incognita, X. index and PD, thus it
was important to determine what the true pollen parent of these crosses were. Vitis ×
Muscadina crosses are difficult due to poor pollen germination and pollen tube growth,
differences in chromosome number and incomplete homology between the genomes
(Riaz et al., 2012). Thus it was likely the original pollinations were contaminated by
pollen in the area and that this pollen was the source of the strong and broad resistance to
pests. We walked the vineyard where the crosses were made and found that the likely
sources of contaminating pollen were Vitis species Olmo collected in northern and central
Mexico in 1961. SSR-based fingerprinting of these potential pollen parents found them to
be forms of V. arizonica (Riaz et al., 2007). The X. index resistance in one of these
parents, V. arizonica/girdiana b42-26, has been genetically (Xu et al., 2008) and
physically (Hwang et al., 2010) mapped and Cecilia Agüero has engineered 5 gene
candidates (all R genes) into the highly susceptible Rupestris St. George to confirm their
The next phase of the UC Davis rootstock breeding program is focusing on
resistance to drought and salinity to address California’s arid viticultural areas. These two
traits are closely joined as dry areas are saline and water is needed to flush salts through
the soil profile. Much of the work to date has been characterizing Vitis species from the
southwestern US and developing reliable and rapid greenhouse-based testing (Fort et al.
2013). There is strong resistance to nematodes in species from this area, but there are
concerns that some species have inadequate resistance to phylloxera. Thus, careful testing
for phylloxera resistance will be required. We have created a V. champinii ‘Ramsey’ × V.
riparia ‘Riparia Gloire’ genetic map (Lowe et al., 2008) and are characterizing rooting
angle and distribution in the F1 and in a more recently created F2 population. We are
characterizing chloride resistance in these generations, and in southwestern Vitis
including V. berlandieri 9031, forms of V. acerifolia, V. arizonica, V. doaniana, V.
girdiana and V. champinii-like hybrids. The southwestern Vitis species can have deep
plunging roots that help them avoid drought, but we are also searching for other
mechanisms that allow grape roots to function with less water (Gambetta et al., 2012).
We gratefully acknowledge research funding from the California Grape Rootstock
Improvement Commission, the California Department of Food and Agriculture
Improvement Advisory Board, the California Table Grape Commission, the American
Vineyard Foundation, and the Louis P. Martini Endowed Chair funds.
Literature Cited
Doucleff, M., Jin, Y., Gao, F., Riaz, S., Krivanek, A.F. and Walker, M.A. 2004. A genetic
linkage map of grape utilizing Vitis rupestris and Vitis arizonica. Theor. Appl. Genet.
Fong, G., Walker, M.A. and Granett, J. 1995. RAPD assessment of California phylloxera
diversity. Molec. Ecol. 4:459-464.
Fort, K.P., Lowe, K.M., Thomas, W.A. and Walker, M.A.. 2013. Cultural conditions and
propagule type influence relative chloride exclusion in grapevine rootstocks. Amer. J.
Enol. Vitic. 64:241-250.
Gambetta, G.A., Manuck, C.M., Drucker, S.T., Shanghasi, T., Fort, K.P., Matthews,
M.A., Walker, M.A. and McElrone, A.J. 2012. The relationship between root
hydraulics and scion vigour across Vitis rootstocks: what role do root aquaporins play?
J. Exp. Bot. 63:6445-6455.
Grzegorczyk, W. and Walker, M.A. 1998. Evaluating resistance to grape phylloxera in
Vitis species with an in vitro dual culture assay. Amer. J. Enol. Vitic. 49:17-22.
Hwang, C.-F., Xu, K., Hu, R., Zhou, R., Riaz, S. and Walker, M.A. 2010. Cloning and
characterization of XiR1, a locus responsible for dagger nematode resistance in grape.
Theor. Appl. Genet. 121:789-799.
Krivanek, A.F., Riaz, S. and Walker, M.A. 2006. The identification of PdR1, a primary
resistance gene to Pierce’s disease in Vitis. Theor. Appl. Genet. 112:1125-1131.
Lider, L.A. 1958. Phylloxera-resistant grape rootstocks for the coastal valleys of
California. Hilgardia 27:287-318.
Lowe, K.M., Riaz, S. and Walker, M.A. 2008. Variation in recombination rates across
Vitis species. Tree Genet. Genom. 5:71-80.
Riaz, S., Krivanek, A.F., Xu, K. And Walker, M.A. 2006. Refined mapping of the
Pierce’s disease resistance locus, PdR1, and Sex on an extended genetic linkage map
of Vitis rupestris × V. arizonica. Theor. Appl. Genet. 113:1317-1329.
Riaz, S., Tenscher, A.C., Ramming, D.W. and Walker, M.A. 2011. Using a limited
mapping strategy to identify major QTLs for resistance to grapevine powdery mildew
(Erysiphe necator) and their use in marker-assisted breeding. Theor. Appl. Genet.
Riaz, S., Vezzulli, S., Harbertson, E.S. and Walker, M.A. 2007. Use of molecular markers
to correct grape breeding errors and determine the identity of novel sources of
resistance to Xiphinema index and Pierce’s disease. Amer. J. Enol. Vitic. 58:494-498.
Walker, M.A., Wolpert, J.A. and Weber, E. 1994. Field screening of grape rootstock
selections for resistance to fanleaf degeneration. Plant Dis. 78:134-136.
Walker, M.A., Wolpert, J.A. and Weber, E. 1994. Viticultural characteristics of VR
hybrid rootstocks in a vineyard site infected with grapevine fanleaf virus. Vitis 33:19-
Walker, M.A., Lider, L.A., Goheen, A.C. and Olmo, H.P. 1991. VR O39-16. HortScience
Xu, K., Riaz, S., Roncoroni, N.C., Jin, Y., Hu, R., Zhou, R. and Walker, M.A. 2008.
Genetic and QTL analysis of resistance to Xiphinema index in a grapevine cross.
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Table 1. Characteristics of the newly released GRN rootstocks. All these rootstocks resist
two aggressive strains of root-knot nematode (Meloidogyne spp.), these nematodes at
32°C soil temperatures, Xiphinema index, and X. index combined with the aggressive
root-knot strains. They also resist lesion, citrus and ring nematodes, and phylloxera as
noted. R = resistant; MR = moderately resistant; MS = moderately susceptible; S =
Selection/parentage Lesion Citrus Ring Phylloxera Characteristics
8909-05 (UCD GRN-1)
V. rupestris ×
M. rotundifolia
R R R R Can be hard to propagate;
Studies underway to determine
fanleaf tolerance; Relatively
deep rooting profile; Leaves are
shiny and intermediate between
V. rupestris and M. rotundifolia;
and Sterile flowers.
9363-16 (UCD GRN-2)
(V. rufotomentosa ×
(Dog Ridge × Riparia
Gloire)) ×
Riparia Gloire
R MS S R Good mothervine with long
canes and internodes, and limited
lateral production; Relatively
shallow rooting depth; Mature
leaves are lobed; and Male
9365-43 (UCD GRN-3)
(V. rufotomentosa ×
(Dog Ridge × Riparia
Gloire)) × V. champinii
c9038 (probably
V. candicans ×
V. monticola)
R R S R Mothervine has moderate vigor,
but long canes with good
internode length, moderate
number of laterals; Moderate
rooting depth; Mature leaves
resemble V. champinii; and
Female flowers.
9365-85 (UCD GRN-4)
(V. rufotomentosa ×
(Dog Ridge × Riparia
Gloire)) ×
V. champinii c9038
(probably V. candicans ×
V. monticola)
R R MR R Good mothervine with long
canes and internodes and few
laterals; Moderately deep rooting
profile; Mature leaves resemble
V. riparia; and Male flowers.
9407-14 (UCD GRN-5)
(Ramsey × Riparia
Gloire) ×
V. champinii c9021
(probably V. candicans ×
V. monticola/
V. berlandieri)
R R R MR Weak mothervine, but long
internodes, good canes; Deep
rooting profile; Mature leaves
resemble glossy V. champinii/
monticola; and Male flowers.
Table 2. New selections that propagate well, resist X. index, ring and aggressive root-knot
strains. Ring nematode resistance was judged after inoculation with 100 nematodes in
125 cm3 pots.
Selection Parents Ring nemas/
g root
HarmA & C egg
masses/g root
11137-01 161-49C × doaniana T9 0 2.5
11138-02 5BB × rotundifolia Tray 1 0
11137-19 161-49C × doaniana T9 4 1.5
11138-01 5BB × rotundifolia Tray 7 0
GRN1 rupestris × rotundifolia Tray 7 0
11133-13 acerifolia OKC1S03 × St. George 17 3.4
11115-13 161-49C
× rotundifolia Tray 24 0
Harmony 1613C op
× Dog Ridge op 679 27.1
St. George rupestris 1209 38.9
Colombard vinifera 1891 28.6
Fig. 1. Principle coordinate analysis of grape phylloxera collected across the US native
range. Data based on 22 SSR markers and over 500 samples.
Fig. 2. Genetic relationships of root (R) and leaf (L) forms of grape phylloxera collected
in northern California. The 294 samples collected from leaf galls in nurseries, the
USDA, and UC Davis FPS were genetically identical.
... DNA-markers linked to these phylloxera resistance loci can now be used to breed durable resistant rootstocks harbouring two or more resistant loci [26]. Aside from phylloxera resistance, rootstocks are also used to counteract other soilborne pests, such as nematodes, as well as maintain vine productivity in response to abiotic stress, soil pH, and porosity [27]. ...
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Background and Aims. Grape phylloxera in Australia comprises diverse genetic strains that feed on roots and leaves of Vitis spp. The G38 phylloxera strain was detected on roots of Vitis spp., for the first time in North East Victoria in 2015. Prior to 2015, G38 phylloxera was only known to feed on leaves. The aim of this study was to evaluate the survival and development of G38 phylloxera on roots of diverse Vitis spp. under field, controlled laboratory, and greenhouse conditions. Methods and Results. In the field, emergence traps quantified first instars and alates emerging from roots of diverse rootstocks and Vitis vinifera L. High numbers of phylloxera were collected in traps placed at vines of rootstocks 101-14, 3309 Courderc and Schwarzmann. Nodosity were also observed on roots of 101-14, 3309 Courderc and Schwarzmann in the field and in-pot vines experiments. The better performance of G38 phylloxera on these three rootstocks compared to V. vinifera in the field and in potted vines parallelled the excised roots experiments. Conclusions. The relatively high performance of G38 phylloxera on the 101-14, 3309 Courderc and Schwarzmann rootstocks suggest a susceptible response and could be associated with rootstock parentage. Further investigation is warranted to determine implications for rootstocks development. Significance of the Study. These findings are fundamental for decision-making in phylloxera risk assessment and rootstock selection. The study reaffirms the need for triphasic (in vitro, in planta, and in-field) rootstock screening protocols for phylloxera.
... The grape phylloxera epidemic of the late 1800s in Europe spurred the development of phylloxera resistant rootstocks and hybrid ownrooted grapevines. A large portion of the resulting rootstock cultivars are closely related and selected for phylloxera resistance and graft compatibility rather than stress tolerance and there has been limited genetic analysis of grapevine root systems (Walker et al., 2014;Ollat et al., 2016;Dalbóand Souza, 2019;Riaz et al., 2019). In contrast, genetic analysis of grapevine scion cultivars for improved fruit quality, abiotic and biotic resistance, cold tolerance, seedlessness, and other enological and phenological traits have proceeded rapidly (Correa et al., 2014;Delrot et al., 2020;Gautier et al., 2020;Zinelabidine et al., 2021;Vezzulli et al., 2022). ...
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Own-rooted grapevines and grapevine rootstocks are vegetatively propagated from cuttings and have an adventitious root system. Unraveling the genetic underpinnings of the adventitious root system architecture (RSA) is important for improving own-rooted and grafted grapevine sustainability for a changing climate. Grapevine RSA genetic analysis was conducted in an Vitis sp. ‘VRS-F2’ population. Nine root morphology, three total root system morphology, and two biomass traits that contribute to root anchorage and water and nutrient uptake were phenotyped. Quantitative trait loci (QTL) analysis was performed using a high density integrated GBS and rhAmpSeq genetic map. Thirty-one QTL were detected for eleven of the RSA traits (surface area, root volume, total root length, fresh weight, number of tips, forks or links, longest root and average root diameter, link length, and link surface area) revealing many small effects. Several QTL were colocated on chromosomes 1, 9, 13, 18, and 19. QTL with identical peak positions on chromosomes 1 or 13 were enriched for AP2-EREBP , AS2 , C2C2-CO , HMG , and MYB transcription factors, and QTL on chromosomes 9 or 13 were enriched for the ALFIN-LIKE transcription factor and regulation of autophagy pathways. QTL modeling for individual root traits identified eight models explaining 13.2 to 31.8% of the phenotypic variation. ‘Seyval blanc’ was the grandparent contributing to the allele models that included a greater surface area, total root length, and branching (number of forks and links) traits promoting a greater root density. In contrast, V. riparia ‘Manitoba 37’ contributed the allele for greater average branch length (link length) and diameter, promoting a less dense elongated root system with thicker roots. LATERAL ORGAN BOUNDARY DOMAIN (LBD or AS2/LOB) and the PROTODERMAL FACTOR (PFD2 and ANL2) were identified as important candidate genes in the enriched pathways underlying the hotspots for grapevine adventitious RSA. The combined QTL hotspot and trait modeling identified transcription factors, cell cycle and circadian rhythm genes with a known role in root cell and epidermal layer differentiation, lateral root development and cortex thickness. These genes are candidates for tailoring grapevine root system texture, density and length in breeding programs.
... As a result of the studies, rootstocks resistant to aggressive root knot nematodes have been developed. [161]. ...
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Many grape varieties or genotypes of Vitis species are grown for different purposes in various parts of the world. However, despite a large number of cultivars, there is a demand for different grape cultivars due to changing consumer expectations. Grapevine breeding programs are carried out by scientists in different countries in order to meet these expectations. Breeding studies, which used to take a long time with traditional crossbreeding methods, have become studies that achieve the desired results in a much shorter time with the development of molecular methods and biotechnology. One of the most important developments in grapevine breeding is that the relevant gene regions in hybrid populations developed from breeding programs can be identified in a very short time. In recent years, the demand for cultivars that are more resistant or tolerant to biotic and abiotic stress conditions has increased, and for this purpose, there has been a significant increase in breeding studies on cultivars and rootstocks that are resistant or tolerant to different stress conditions. Considering the current breeding programs, genetically manipulated new cultivars with desired characteristics and interspecies hybrid cultivars will soon become the main study subjects of grapevine breeding programs.
... A fundamental question in plant biology is how root systems influence phenomic variation in above-ground shoot systems including leaves, flowers, and fruits. Grafting, a common horticultural manipulation that joins the shoot system of one individual (the scion) with the root system of another individual (the rootstock), is commonly used in crop species to confer favorable phenotypes to commercial scions [6], including enhanced disease resistance [7,8], fruit quality, plant form [9], response to water stress [10], and growth on particular soils [11,12]. Because grafting often uses clonally propagated materials, it is possible to manipulate and replicate different combinations of root systems and shoot systems, offering a valuable experimental system in which root system effects on shoot system phenotypes can be evaluated. ...
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Background Modern biological approaches generate volumes of multi-dimensional data, offering unprecedented opportunities to address biological questions previously beyond reach owing to small or subtle effects. A fundamental question in plant biology is the extent to which below-ground activity in the root system influences above-ground phenotypes expressed in the shoot system. Grafting, an ancient horticultural practice that fuses the root system of one individual (the rootstock) with the shoot system of a second, genetically distinct individual (the scion), is a powerful experimental system to understand below-ground effects on above-ground phenotypes. Previous studies on grafted grapevines have detected rootstock influence on scion phenotypes including physiology and berry chemistry. However, the extent of the rootstock's influence on leaves, the photosynthetic engines of the vine, and how those effects change over the course of a growing season, are still largely unknown. Results Here, we investigate associations between rootstock genotype and shoot system phenotypes using 5 multi-dimensional leaf phenotyping modalities measured in a common grafted scion: ionomics, metabolomics, transcriptomics, morphometrics, and physiology. Rootstock influence is ubiquitous but subtle across modalities, with the strongest signature of rootstock observed in the leaf ionome. Moreover, we find that the extent of rootstock influence on scion phenotypes and patterns of phenomic covariation are highly dynamic across the season. Conclusions These findings substantially expand previously identified patterns to demonstrate that rootstock influence on scion phenotypes is complex and dynamic and underscore that broad understanding necessitates volumes of multi-dimensional data previously unmet.
... Vitis is also an important study system because cultivated V. vinifera (hereafter vinifera) is the most valuable horticultural crop in the world [24] and also because it is a model for the study of perennial fruit crops [25]. It is not always appreciated, however, that the cultivation, sustainability, and security of grapevine cultivation relies on North American (NA) Vitis species as rootstocks that provide resistance to abiotic and biotic stress [21,26,27]. There is a need to identify additional sources of resistance to biotic and abiotic stress, however, because the major rootstock cultivars currently utilized represent a narrow genetic foundation [28]. ...
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Background Introgressive hybridization can reassort genetic variants into beneficial combinations, permitting adaptation to new ecological niches. To evaluate evolutionary patterns and dynamics that contribute to introgression, we investigate six wild Vitis species that are native to the Southwestern United States and useful for breeding grapevine (V. vinifera) rootstocks. Results By creating a reference genome assembly from one wild species, V. arizonica, and by resequencing 130 accessions, we focus on identifying putatively introgressed regions (pIRs) between species. We find six species pairs with signals of introgression between them, comprising up to ~ 8% of the extant genome for some pairs. The pIRs tend to be gene poor, located in regions of high recombination and enriched for genes implicated in disease resistance functions. To assess potential pIR function, we explore SNP associations to bioclimatic variables and to bacterial levels after infection with the causative agent of Pierce’s disease (Xylella fastidiosa). pIRs are enriched for SNPs associated with both climate and bacterial levels, suggesting that introgression is driven by adaptation to biotic and abiotic stressors. Conclusions Altogether, this study yields insights into the genomic extent of introgression, potential pressures that shape adaptive introgression, and the evolutionary history of economically important wild relatives of a critical crop.
... M. rotundifolia is resistant to Pierce's disease (Xylella fastidiosa) (Ruel and Walker 2006), phylloxera (Daktulosphaira vitifolia) (Ravaz 1902;Davidis and Olmo 1964;Firoozabady and Olmo 1982), downy mildew (Plasmopara viticola) (Olmo 1971;Staudt and Kassemeyer 1995), powdery mildew (Erysiphe necator syn. Uncinula necator) (Olmo 1986;Merdinoglu et al. 2018), and other diseases and pests (Ravaz 1902;Olmo 1971;Walker et al. 2014). Several loci associated with resistance to pathogens affecting V. vinifera were identified in M. rotundifolia, including Resistance to Uncinula necator 1 (RUN1) (Pauquet et al. 2001), RUN2 (Riaz et al. 2011), Resistance to Erysiphe Necator 5 (REN5) (Blanc et al. 2012), Resistance to Plasmopara viticola 1 (RPV1) (Merdinoglu et al. 2003), and RPV2 (Merdinoglu et al. 2018). ...
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Muscadinia rotundifolia, the muscadine grape, has been cultivated for centuries in the southeastern United States. M. rotundifolia is resistant to many of the pathogens that detrimentally affect Vitis vinifera, the grape species commonly used for winemaking. For this reason, M. rotundifolia is a valuable genetic resource for breeding. Single-molecule real-time reads were combined with optical maps to reconstruct the two haplotypes of each of the 20 M. rotundifolia cv. Trayshed chromosomes. The completeness and accuracy of the assembly were confirmed using a high-density linkage map of M. rotundifolia. Protein-coding genes were annotated using an integrated and comprehensive approach. This included using Full-length cDNA sequencing (Iso-Seq) to improve gene structure and hypothetical spliced variant predictions. Our data strongly support that Muscadinia chromosomes 7 and 20 are fused in Vitis and pinpoint the location of the fusion in Cabernet Sauvignon and PN40024 chromosome 7. Disease-related gene numbers in Trayshed and Cabernet Sauvignon were similar, but their clustering locations were different. A dramatic expansion of the Toll/Interleukin-1 Receptor-like Nucleotide-Binding Site Leucine-Rich Repeat (TIR-NBS-LRR) class was detected on Trayshed chromosome 12 at the Resistance to Uncinula necator 1 (RUN1)/ Resistance to Plasmopara viticola 1 (RPV1) locus, which confers strong dominant resistance to powdery and downy mildews. A genome browser for Trayshed, its annotation, and an associated Blast tool are available at
... Moore (syn. Vitis berlandieri), which is adapted to chalky soils 13 . Despite the global diversity of soils, climates, and grape varieties, only a handful of rootstock cultivars derived from these three species are in widespread use 3 . ...
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Understanding how root systems modulate shoot system phenotypes is a fundamental question in plant biology and will be useful in developing resilient agricultural crops. Grafting is a common horticultural practice that joins the roots (rootstock) of one plant to the shoot (scion) of another, providing an excellent method for investigating how these two organ systems affect each other. In this study, we used the French-American hybrid grapevine ‘Chambourcin’ (Vitis L.) as a model to explore the rootstock–scion relationship. We examined leaf shape, ion concentrations, and gene expression in ‘Chambourcin’ grown ungrafted as well as grafted to three different rootstocks (‘SO4’, ‘1103P’ and ‘3309C’) across 2 years and three different irrigation treatments. We found that a significant amount of the variation in leaf shape could be explained by the interaction between rootstock and irrigation. For ion concentrations, the primary source of variation identified was the position of a leaf in a shoot, although rootstock and rootstock by irrigation interaction also explained a significant amount of variation for most ions. Lastly, we found rootstock-specific patterns of gene expression in grafted plants when compared to ungrafted vines. Thus, our work reveals the subtle and complex effect of grafting on ‘Chambourcin’ leaf morphology, ionomics, and gene expression.
The phylloxera Daktulosphaira vitifoliae (Fitch) is considered the main pest in vine crops in the world. One of the alternatives for pest management is the use of resistant rootstocks. In the present study, 14 vine genotypes comprised of 6 canopy cultivars (‘Bordô,’ ‘Isabel,’ ‘BRS Lorena,’ ‘Cabernet Sauvignon,’ ‘Magnólia,’ and ‘Chardonnay’), 4 commercial rootstocks (‘Paulsen 1103,’ ‘SO4,’ ‘IAC 766,’ and ‘IAC 572’), and 4 promising rootstocks for pest management (‘1111-21,’ ‘548-44,’ ‘548-15,’ and ‘IBCA-125’) were evaluated for resistance to infestation the of root form of pest. For each genotype, the number of eggs, nymphs, and adults present in the roots were evaluated at 15, 20, 25, 30, and 35 days after egg infestation. In addition, the feeding place (lignified or non-lignified root), the presence or absence of tuberosities and nodosities, and the total fecundity of females were evaluated. The highest survival rates of nymphs and adults were observed in Cabernet Sauvignon, BRS Lorena, Chardonnay, and IBCA-25 in lignified roots, with the formation of tuberosities characterizing the materials as susceptible. In contrast, SO4, Paulsen 1103, IAC 572, IAC 766, 548-44, 548-15, Magnólia, and 1111-21 provided the least nymph and adult survival over time in non-lignified roots present in the nodosities, characterizing the materials as resistant. In addition, the lowest fecundity was observed in the roots of Magnólia (16 eggs). For having negative effects on phylloxera, the genotypes S04, Paulsen 1103, IAC 572, IAC 766, 548-44, 548-15, Magnólia, and 1111-21 can be used as a source of resistance for the management of phylloxera.
Introgressive hybridization can introduce adaptive genetic variation into a species or population. To evaluate the evolutionary forces that contribute to introgression, we studied six Vitis species that are native to the Southwestern United States and potentially useful for breeding grapevine ( V. vinifera ) rootstocks. By creating a reference genome from one wild species, V. arizonica , and by resequencing 130 accessions, we focused on identifying putatively introgressed regions (pIRs) between species. We found that up to ~8% of extant genome is attributable to introgression between species. The pIRs tended to be gene poor, located in regions of high recombination and enriched for genes implicated in disease resistance functions. To assess potential pIR function, we explored SNP associations to bioclimatic variables and to bacterial levels after infection with the causative agent of Pierce's Disease. pIRs were enriched for SNPs associated with both climate and bacterial levels, suggesting potential drivers of adaptive events. Altogether, this study yields insights into the genomic extent of introgression, potential pressures that shape adaptive introgression, and the history of economically important wild relatives of a critical crop.
Muscadinia rotundifolia, the muscadine grape, has been cultivated for centuries in the southeastern United States. M. rotundifolia is resistant to many of the pathogens that detrimentally affect Vitis vinifera, the grape species commonly used for winemaking throughout Europe and in New World wine regions. For this reason, M. rotundifolia is a valuable genetic resource for breeding. Single molecule real-time reads were combined with optical maps to reconstruct the two haplotypes of each of the 20 M. rotundifolia cv. Trayshed (Trayshed, henceforth) chromosomes. Completeness and accuracy of the assembly was confirmed using a high-density linkage map of M. rotundifolia. Protein-coding genes were annotated using an integrated comprehensive approach that included full-length cDNA sequencing (Iso-Seq) to improve gene structure and hypothetical spliced variant predictions. Our data confirmed the fusion of chromosomes 7 and 20, which reduced the number of chromosomes in Vitis versus Muscadinia and pinpointed the location of the fusion in Cabernet Sauvignon and PN40024 chromosome 7. The numbers of nucleotide binding site leucine-rich repeats (NBS-LRR) in Trayshed and Cabernet Sauvignon were similar, but their locations were different. A dramatic expansion of the Toll/Interleukin-1 Receptor-like-X (TIR-X) class was detected on Trayshed chromosome 12 at the Resistance to Uncinula necator 1 (RUN1)/ Resistance to Plasmopara viticola 1 (RPV1) locus, which confers strong dominant resistance to powdery and downy mildew. A genome browser for Trayshed, its annotation, and an associated Blast tool are available at
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Breeding for salt resistance in grapevines and other crops has made slow progress despite decades of research. One factor contributing to this problem in grapevines is the weak or nonexistent correlation of field and greenhouse performance observed in some studies when salt resistance is assessed by chloride accumulation in leaf tissue. To develop a rapid chloride exclusion assay for use in rootstock breeding, multiple systems were tested. Results were obtained wherein the well-established field performance of specific genotypes was augmented, equalized, or reversed when in containerized culture. One assay using fritted clay media and herbaceous cuttings yielded a rank order and relative chloride uptake among the tested genotypes that was similar to published values from long-term studies in experimental vineyards. This assay used only 14 days of high salt exposure, was inexpensive, required relatively little space and maintenance, and has continued to provide reliable data in subsequent experiments. The results demonstrate the potential for considerable plasticity in chloride exclusion exhibited by ungrafted grapevines when assayed in containers. This underscores the importance of system design wherein genotypes of known capacity for chloride exclusion are accurately calibrated to their established field performance. This study describes an empirically derived assay that replicates these conditions closely enough to be used in a rootstock breeding program for improved chloride exclusion.
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Vitis vinifera scions are commonly grafted onto rootstocks of other grape species to influence scion vigour and provide resistance to soil-borne pests and abiotic stress; however, the mechanisms by which rootstocks affect scion physiology remain unknown. This study characterized the hydraulic physiology of Vitis rootstocks that vary in vigour classification by investigating aquaporin (VvPIP) gene expression, fine-root hydraulic conductivity (Lp(r)), % aquaporin contribution to Lp(r), scion transpiration, and the size of root systems. Expression of several VvPIP genes was consistently greater in higher-vigour rootstocks under favourable growing conditions in a variety of media and in root tips compared to mature fine roots. Similar to VvPIP expression patterns, fine-root Lp(r) and % aquaporin contribution to Lp(r) determined under both osmotic (Lp(r)(Osm)) and hydrostatic (Lp(r)(Hyd)) pressure gradients were consistently greater in high-vigour rootstocks. Interestingly, the % aquaporin contribution was nearly identical for Lp(r)(Osm) and Lp(r)(Hyd) even though a hydrostatic gradient would induce a predominant flow across the apoplastic pathway. In common scion greenhouse experiments, leaf area-specific transpiration (E) and total leaf area increased with rootstock vigour and were positively correlated with fine-root Lp(r). These results suggest that increased canopy water demands for scion grafted onto high-vigour rootstocks are matched by adjustments in root-system hydraulic conductivity through the combination of fine-root Lp(r) and increased root surface area.
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Over the past 15 years, the grape breeding program at the University of California, Davis, has been evaluating Vitis rupestris x Muscadinia rotundifolia selections for resistance to the dagger nematode, Xiphinema index, and to Pierce's disease (PD). Selections from these crosses exhibit very strong resistance to X. index and PD. In addition to breeding efforts, populations from these crosses have been used to develop genetic maps and locate resistance loci. Genetic mapping efforts recently began incorporating SSR markers to refine and expand existing maps. The use of SSR markers revealed that the mapping population parents were not crosses of V. rupestris x M. rotundifolia. This discovery led to testing of the entire group of 161 V. rupestris x M. rotundifolia progeny. All possible male parents surrounding the V. rupestris female parents in the vineyard where the crosses were made were genetically fingerprinted with up to 15 SSR markers to determine the true male parents. Results indicated that most of the male parents were from collections of forms of V. arizonica gathered in Mexico in 1961. These now correctly identified selections represent novel sources of very strong resistance to X. index and PD. Copyright © 2007 by the American Society for Enology and Viticulture. All rights reserved.
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Recombination rate data are presented for three populations of grape based on framework genetic linkage maps developed with simple-sequence repeat markers. These linkage maps were constructed from different Vitis species and represent three genetic backgrounds. The first population is pure Vitis vinifera, derived from a cross of the European cultivars Riesling and Cabernet Sauvignon. The second is an interspecific cross between two commercially used rootstock cultivars of different North American Vitis species parentage, Ramsey (Vitis champinii) and Riparia Gloire (Vitis riparia). The third population, D8909-15 (Vitis rupestris × (Vitis arizonica/Vitis girdiana form)) × F8909-17 (V. rupestris × (V. arizonica/Vitis candicans form)), is an F1 from two half-sibs. Genome-wide and chromosome-wide recombination rates varied across the three populations and among the six Vitis parents. Global recombination rates in the parents of the third F1 population, with a complex Vitis background, were significantly reduced. In the first and third populations, the recombination rate was significantly greater in the male parent. Specific genome locations with frequent heterogeneity in recombination were identified, suggesting that recombination rates are not equal across the Vitis genome. The identification of regions with suppressed or high recombination will aid grape breeders and geneticists who rely on recombination events to introgress disease resistance genes from the genomes of wild Vitis species, develop fine-scale genetic maps, and clone disease resistance genes.
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The prone position is required for posterior spinal fusion surgery and may be associated with cardiovascular changes, including a decrease in venous return and cardiac index. We report a case of a patient who developed cardiovascular collapse, increased central venous pressure (CVP), and massive bleeding during posterior spinal fusion surgery. A transesophageal echocardiography examination (TEE) documented a right ventricular outflow tract (RVOT) obstruction associated with the use of transverse bolsters. We describe a case of a healthy 14-yr-old male with idiopathic scoliosis who developed severe intraoperative cardiovascular instability and massive bleeding. The surgery was suspended, and the patient was transferred to the intensive care unit. The patient subsequently underwent TEE in the supine and prone positions. The echocardiogram appeared normal in the supine position; however, in the prone position with transverse bolsters, we identified a significant decrease in the diameter of the RVOT that worsened with pressure applied against the thoracic spine. The central venous pressure increased from 10-24 mmHg simultaneously. We found appreciably less impact to the RVOT, RV size and flow, and CVP (10 to 14 mmHg) using longitudinal bolsters both with and without pressure to the back. This position was recommended for the patient's reoperation, which was uneventful. A TEE confirmed a RVOT obstruction in the prone position that was associated, in this case, with the use of transverse bolsters. The RVOT obstruction was explained by the chest deformity, compliant chest cage, bolstering, and pressure applied to the patient's back by the surgeon. This positional RVOT obstruction may explain the increase in the CVP and the secondary massive bleeding during the first operation. The TEE was useful to diagnose the patient's condition and to guide his positioning for the second operation.
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A limited genetic mapping strategy based on simple sequence repeat (SSR) marker data was used with five grape populations segregating for powdery mildew (Erysiphe necator) resistance in an effort to develop genetic markers from multiple sources and enable the pyramiding of resistance loci. Three populations derived their resistance from Muscadinia rotundifolia ‘Magnolia’. The first population (06708) had 97 progeny and was screened with 137 SSR markers from seven chromosomes (4, 7, 9, 12, 13, 15, and 18) that have been reported to be associated with powdery or downy mildew resistance. A genetic map was constructed using the pseudo-testcross strategy and QTL analysis was carried out. Only markers from chromosome 13 and 18 were mapped in the second (04327) and third (06712) populations, which had 47 and 80 progeny, respectively. Significant QTLs for powdery mildew resistance with overlapping genomic regions were identified for different tissue types (leaf, stem, rachis, and berry) on chromosome 18, which distinguishes the resistance in ‘Magnolia’ from that present in other accessions of M. rotundifolia and controlled by the Run1 gene on chromosome 12. The ‘Magnolia’ resistance locus was termed as Run2.1. Powdery mildew resistance was also mapped in a fourth population (08391), which had 255 progeny and resistance from M. rotundifolia ‘Trayshed’. A locus accounting for 50% of the phenotypic variation mapped to chromosome 18 and was named Run2.2. This locus overlapped the region found in the ‘Magnolia’-based populations, but the allele sizes of the flanking markers were different. ‘Trayshed’ and ‘Magnolia’ shared at least one allele for 68% of the tested markers, but alleles of the other 32% of the markers were not shared indicating that the two M. rotundifolia selections were very different. The last population, 08306 with 42 progeny, derived its resistance from a selection Vitis romanetii C166-043. Genetic mapping discovered a major powdery mildew resistance locus termed Ren4 on chromosome 18, which explained 70% of the phenotypic variation in the same region of chromosome 18 found in the two M. rotundifolia resistant accessions. The mapping results indicate that powdery mildew resistance genes from different backgrounds reside on chromosome 18, and that genetic markers can be used as a powerful tool to pyramid these loci and other powdery mildew resistance loci into a single line. Electronic supplementary material The online version of this article (doi:10.1007/s00122-010-1511-6) contains supplementary material, which is available to authorized users.
Forty-one accessions of 12 Vitis L. and Muscadinia Small species were evaluated for resistance to grape phylloxera (Daktulosphaira vitifoliae Fitch) using an in vitro dual culture system. The performance of the species tested in this study was consistent with previously published studies with whole plants and helps confirm the utility of in vitro dual culture for the study of grape/phylloxera interactions. This in vitro system provides rapid results (8 wk) and the ability to observe the phylloxera/grape interaction without interference from other factors. This system also provides an evaluation that overemphasizes susceptibility, thus providing more confidence in the resistance responses of a given species or accession. Among the unusual responses were the susceptibility of V. riparia Michx. DVIT 1411; susceptibility within V. berlandieri Planch.; relatively wide ranging responses in V. rupestris Scheele; and the lack of feeding on the roots of V. californica Benth., in contrast to the severe foliar feeding damage that occurred on this species. Vitis californica 11 and V. girdiana Munson DVIT 1379 were unusual because phylloxera on them had the shortest generation times. Such accessions might be used to examine how grape hosts influence phylloxera behavior. Very strong resistance was found within V. aestivalis Michx. DVIT 7109 and 7110; V. berlandieri c9031; V. cinerea Engelm; V. riparia (excluding DVIT 1411); V. rupestris DVIT 1418 and 1419; and M. rotundifolia Small. These species and accessions seem to possess enough resistance to enable their use in breeding with minimal concern about phylloxera susceptibility.
Fifty-five grape rootstock selections produced by L. A. Lider, nine Vitis vinifera×Muscadinia rotundifofia (VR) hybrids produced by H. P. Olmo (both of the University of California, Davis), and three fanleaf degeneration-susceptible grape rootstocks were planted in 1979 in a site in the Napa Valley, California, known to be infested with grapevine fanleaf virus (GFLV) and viruliferous Xiphinema index. All of these rootstock selections were field-budded with V. vinifera cv. Cabernet Sauvignon. The site was chosen because of the relatively uniform distribution of virus and vector. Shoot tips from the scions were first assessed for the presence of GFLV in 1981 with enzyme-linked immunosorbent assay (ELISA)