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
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.
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.
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
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.
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.
Theor. Appl. Genet. 116:305-311.
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 ×
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
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
V. candicans ×
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
9365-85 (UCD GRN-4)
(V. rufotomentosa ×
(Dog Ridge × Riparia
V. champinii c9038
(probably V. candicans ×
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
V. champinii c9021
(probably V. candicans ×
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/
HarmA & C egg
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
× 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.