I. Introduction
The genus Juglans contains approximately 23 species, which
include natives to North, Central, and South America, Eastern
Europe, and Asia. The two species of greatest direct economic
significance are the Persian (English) walnut (Juglans regia)
, grown for its nut crop, and eastern black walnut (J. nigra),
grown primarily for its timber. Several other species and hybrids
in Juglans, notably J. hindsii (Northern California
black walnut) and J. hindsii x J. regia (paradox
hybrid), are of great indirect commercial importance as rootstocks
for cultivars of J. regia.
During 1993/94-1995/96, annual world walnut production totaled
approximately 560,000 metric tons (all figures in-shell basis).
Of this, the United States produced approximately 200,000 to
235,000 metric tons annually, which was valued at about 250 million
dollars each year. Approximately 60% of the US crop is sold as
shelled nut meats. Data from the last five years indicate that
34-35% of the US crop is typically exported. China is the other
major contributor to the world walnut crop and produces about
200,000 metric tons annually, of which about 18% is typically
exported.
Most (99%) of US walnuts are produced in California, which in
1994 had 171,000 bearing acres of the crop . Although there is
an interest in growing walnuts in other parts of the US, commercially
satisfactory varieties adapted to the different growing environments
are not available.
II. Present Germplasm Activities
A. Collection and maintenance, National Clonal Germplasm
Repository, Davis
Walnuts have been assigned to the National Germplasm Repository
(NCGR) in Davis, California (George White, Curator). Approximately
12 acres of the repository are devoted to walnuts, and the collection
contains 268 accessions of Juglans spp. (1170 individual
seedling trees). Fourteen Juglans spp. are represented.
Related accessions include 12 of several Pterocarya spp.
and one of Platycarya sp. Other smaller collections of
Juglans spp. have been maintained elsewhere in the US,
e.g. J. cinerea at the repository in Corvallis.
The Davis NCGR walnut collection was "topped" with
a commercial tree hedger during the winter of 1996 as recommended
by a Juglans CGC subcommittee. The operation reduced tree
height and should improve pest management and light distribution
within and between trees. A microjet irrigation system was installed
and is now used throughout the walnut collection.
B. Exploration and acquisition
Exploration missions have been completed since 1986 (Table 1).
The trips have resulted in acquisition of 126 new accessions
of Juglans spp. at the Davis NCGR (Table 1).
Table 1. Summary of germplasm exploration activities.
Year | Location of collections | Participants | Species collected | ||
---|---|---|---|---|---|
1983 | Japan |
Westwood | J. ailantifolia | ||
1984 | N Mex, Ariz | Parfitt | J. major J. microcarpa |
|
|
1987 | Mexico |
Parfitt | J. olanchana J. mollis J. pyriformis |
|
|
1989 | Ecuador |
Dixon | J. neotropica | ||
1988 | Pakistan |
Thompson | J. regia | ||
1990 | China |
McGranahan Leslie Barnett |
J. regia | ||
USA | Millikan | J. cinerea | |||
1994 | Kyrgyzstan | McGranahan Leslie | J. regia | ||
1995 | China |
McGranahan Leslie | J. regia |
*Located at Corvallis NCGR.
**Accessions fumigated in Kyrgyzstan did not survive.
C. Evaluation.
1. Descriptor List
The International Plant Genetic Resources Institute (IPGRI)
published Descriptors for walnut (Juglans spp.),
(1994, McGranahan et. al.). Under standard IPGRI format, the
publication establishes descriptors to be used for passport, management,
environment and site, characterization, and evaluation.
2. Description of NCGR accessions
For five consecutive years (1988-92), data on the phenology,
flower, and yield characteristics was obtained and entered into
GRIN from 524 trees of Juglans spp. at the NCGR, Davis.
Therefore, of the total 1170 trees of Juglans spp at the
repository, about 45% have been evaluated. Most of the trees
of J. cathayensis, J. californica, J. ailantifolia,
J. microcarpa, J. hindsii have been evaluated.
Low percentages of the trees have been evaluated for other Juglans
spp., including J. major, J. regia, J. nigra,
and J. mandshurica.
3. Isozyme / blackline grant
In a one-year project (1992-93), eight isozyme systems
were used to characterize 396 accessions of Juglans spp.
Eleven species and five hybrids of Juglans were represented,
but accessions of J. regia comprised 88% of the selections
tested. Thirty one accessions were identified to maximize isozyme
diversity in a subset of selections that will be tested for hypersensitivity
to CLRV.
4. J. regia evaluation
Evaluations have continued for the second year (1995-96) on phenology,
bearing habit, flowering and fruiting abundance, and 17 descriptors
of nut and kernal quality. The goal is to record at least two
years of complete data on each J. regia individual.
D. Enhancement
1. Breeding programs
a. US: Walnut Improvement Program, UC Davis
The most comprehensive enhancement activities for walnuts occur
through the Walnut Improvement Program, a cooperative effort between
the University of California at Davis, the USDA, and the Walnut
Marketing Board. The program is led by Dr. Gale McGranahan, Department
of Pomology, University of California (UC) Davis and includes
independent and cooperative work of several collaborators. The
program emphasizes resistance to blackline, precocity, early harvest,
late leafing, high yields and excellent quality. Cooperative
and independent work emphasizes selection and development of genetic
resistance to Phytophthora root and crown rots, parasitic nematodes,
and crown gall disease.
b. France: INRA
Walnut improvement at INRA emphasizes late leafing, blight resistance, and lateral fruitfulness.
c. China: Ministry of Forestry
Walnut improvement in China emphasizes development of varieties
with adaptation to China's growing conditions.
d. Goals of breeding programs
i. Hypersensitivity to Cherry leafroll virus (CLRV) in Persian cultivars
This is a priority at the UC Davis breeding program.
ii. Introgression of traits for precocity, short season, and blight resistance from germplasm to improved lines for Persian cultivars
These are priorities at each of the major walnut breeding
programs.
iii. Resistance to soilborne pests in rootstocks
This is a priority at the UC Davis breeding program.
III. Status of crop vulnerability
A. In the US
J. regia is an introduced species in the United States.
Except for NCGR collections, the gene pool in the US is largely
limited to US cultivars and their relatives, which represent very
little of the species' variation. Most domestic commercial walnut
varieties are derived from the same gene pool of a few progenitors.
Over 10% of potential walnut production is lost to pests and
diseases annually. For many of the major diseases, chemical forms
of control are either unavailable or ineffective. Walnut blight,
Phytophthora root and crown rots, and blackline disease (caused
by cherry leafroll virus, CLRV) continue as major sources of
loss in the major walnut growing region of California.
B. In other areas of the world
Important sources of Juglans germplasm are being lost due to extensive logging in Kyrgyzstan. The stability of native species of Juglans in Central and South America is uncertain.
IV. Germplasm Needs
A. NCGR collection maintenance
1. Continuity of leadership
Before curator George White assumed leadership of the NCGR at
Davis, there was consensus that discontinuous management jeopardized
the existing NCGR Juglans collections and interfered with
successful addition of new accessions to the collection. Juglans
CGC members have expressed concern that gains made by White for
the NCGR could be lost if he retires and the collection reverts
to discontinuous leadership.
2. Tropical species of Juglans
Accessions of Juglans spp. collected from subtropical
climates in Central and South America have failed at Davis due
to chilling and freezing injury. The germplasm is potentially
very valuable, and the Juglans CGC believes that subtropical
location should be obtained for preservation of cold-sensitive
accessions of Juglans spp.
B. Exploration
Exploration locations have been prioritized according
to genetic diversity that is potentially available and according
to the stability of the germplasm sources.
1. Kyrgyzstan and central Asian republics
These areas are identified as priorities for exploration. Large-scale commercial logging of native walnut forests is occurring in Kyrgyzstan and central Asian republics. These areas are important sources of genetic diversity for J. regia. There is concern that political and economic instability in these areas will result in continued and accelerated loss of Juglans genetic diversity.
2. Central and South America
Approximately 11 species of black walnut are native to areas
of Central and South America. A repository location with a subtropical
climate is needed before additional exploration and collection
Central and South American Juglans spp. is justified.
Nevertheless, it is considered a high priority to develop a representative
collection of these species in the US.
C. Evaluation
1. Description of NCGR Juglans collection
There is need for continuing description of the phenology, flower,
and yield characteristics in existing accessions. In proportion
to their representation in the NCGR collection, low percentages
of the trees have been evaluated for horticultural traits among
accessions of J. major, J. regia , J. nigra,
and J. mandshurica. There is a need for intensified evaluations
of resistance to soilborne pests (plant parasitic nematodes, Phytophthora
spp., and Armillaria spp.) among accessions of many species
represented in the collection.
2. Species hybrids for rootstocks
Data available on performance of hybrids as rootstocks is limited.
NC Paradox hybrid (a hybrid from J. hindsii x J. regia)
is widely used California due to its superior vigor and documented
resistance and tolerance to several soilborne pests. However,
negligible information is available on horticultural traits or
genetic resistance to soilborne pests among other hybrids of Juglans
spp. that are represented in the NCGR collection. Evaluation
of these traits may be critical as the walnut industry is forced
to rely less on methyl bromide fumigation as a form of pest control.
D. Enhancement
1. Continued Introgression for improved English walnut cultivars
To develop English cultivars with improved precocity,
lateral bearing, and short-season crop development, the Walnut
Improvement Program must continue its main approach, which includes
hybridization between English cultivars and individuals with desired
traits followed by backcrossing. Continued introgression is also
required for development of English cultivars with tolerance or
resistance to walnut blight and hypersensitivity to CLRV.
2. Rootstock improvement
Intensified interdisciplinary efforts are needed for continued
development of improved rootstocks. Improvements that are especially
needed in walnut rootstocks include tolerance to CLRV, and resistance
or tolerance to Phytophthora spp., Armillaria spp.,
and parasitic nematodes. Improvements in responses to pests must
be accompanied by horticultural acceptability.
V. Recommendations
A. Continuity in repository management
Juglans CGC recommends that continuity in leadership
be emphasized at NCGR, Davis. The rationale is to optimize activities
that contribute to quality, quantity, and availability of the
Juglans collection at NCGR, Davis. To some extent, the
collection suffered from transition phases before curator White
assumed responsibilities.
B. Additional location for subtropical collections of Juglans
spp.
We recommend that a subtropical location be designated
for maintenance of cold sensitive Juglans acquisitions.
Apparently, the designation is a prerequisite to preservation
of acquisitions from several Latin American species of Juglans.
C. Intensified evaluation
Progress of walnut improvement programs depends heavily on evaluations
that identify desirable traits. Much of the Juglans collection
at Davis remains to be evaluated for horticultural traits and
resistance to pests. Evaluations are needed for improvement of
both scions and rootstocks. The need for alternatives to methyl
bromide emphasizes the urgency for intensified evaluation of resistance
to soilborne pests, including parasitic nematodes, Phytophthora
spp., and Armillaria spp.
D. Exploration
Loss in natural populations of Juglans spp. due to logging
intensify the need for exploration. The Juglans CGC recommends
that future explorations emphasize areas where populations of
Juglans spp. are in jeopardy.