Juglans CGC Report, June 1996

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.

No. of accessions at NCGR, Davis

Year
Location of collections
Participants
Species collected
No. received
No. alive (no. of seedlings)

1983

Japan

Westwood

J. ailantifolia

11

10 (106)

1984

N Mex, Ariz

Parfitt

J. major

J. microcarpa


20

3

17 (127)

3 (9)

1987

Mexico

Parfitt

J. olanchana

J. mollis

J. pyriformis


2

6

1

0

1 (4)

0

1989

Ecuador

Dixon

J. neotropica

2

2 (7)

1988

Pakistan

Thompson

J. regia

45

33 (120)

1990

China

McGranahan

Leslie

Barnett


J. regia

55

43 (158)

1990

USA

Millikan

J. cinerea

14*

11*

1994

Kyrgyzstan

McGranahan

Leslie


J. regia

74**

6(64)**

1995

China

McGranahan

Leslie


J. regia

16

? (too early for certainty)

*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.