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Reid, W. 1990. Eastern black walnut: Potential for commercial nut producing cultivars. p. 327-331. In: J. Janick and J.E. Simon (eds.), Advances in new crops. Timber Press, Portland, OR.

Eastern Black Walnut: Potential for Commercial Nut Producing Cultivars

William Reid

  1. INTRODUCTION
  2. GENETIC TRAITS IMPORTANT FOR IMPROVED NUT PRODUCTION
    1. Lateral Bud Fruitfulness
    2. Late Leafing
    3. Anthracnose Resistance
    4. Precocity and Productivity
    5. Nut Quality
  3. CURRENTLY AVAILABLE CULTIVARS
  4. FUTURE PROSPECTS FOR CROP DEVELOPMENT
  5. REFERENCES
  6. Table 1
  7. Fig. 1

INTRODUCTION

The Eastern black walnut, Juglans nigra L., is valued for both the fine cabinet wood it produces and the high quality edible nuts it bears. Juglans nigra is found growing throughout the Eastern U.S. (Fig. 1). The range of this species extends from Kansas eastward to the Atlantic seaboard, from Minnesota to Texas in the West, and from New England to Northern Florida in the East (Fowells 1965). Within their native distribution, black walnuts grow in a wide range in climatic conditions (Funk 1979). Black walnuts are found growing in USDA plant hardiness zones 4 through 8 and are adapted to growing seasons that range from 140 to 280 days. Rainfall within the native range varies from 178 cm/year in the Appalachian mountains to 64 cm/year in northern Nebraska. Trees can be found growing in many different sites but they grow best in deep, well-drained alluvial soils.

The fruit produced by the black walnut is a drupe-like nut surrounded by a fleshy, indehiscent exocarp. The nut has a rough, furrowed, hard shell that protects the edible seed. Fruits are usually produced in clusters of 2 to 3 and are borne on the terminals of the current season's growth. The seed is sweet, oily and high in protein.

Over 10,000 metric tons of black walnuts are harvested annually in the United States. This production is based entirely on hand harvested nuts from seedling trees growing in native stands throughout the Midwest and Northeast. Harvested nuts are brought to local buying stations where they are hulled and bagged for shipment to a commercial sheller. Until recently only one commercial sheller, Hammons Products Co., Stockton, Missouri, bought and processed black walnuts. In 1988 a second firm, Indiana Walnut Products Co., West Lafayette, began buying and shelling nuts. Sixty percent of commercially processed black walnut kernels are packaged for the retail trade, 30% are used in the manufacture of ice cream while the remaining 10% are used in commercial baking and candy making (Hammons 1973). Well-managed, seedling black walnuts produce nuts averaging 20% kernel (Zarger 1946) but in commercial shelling operations only 6 to 10% usable nut meat is recovered. In contrast, over 40% nut meat is recovered in the commercial shelling of Persian walnuts. The low kernel yields for black walnut has forced shellers to develop products manufactured from their main waste product, walnut shells. Ground black walnut shells are used for metal cleaning and polishing, oil well drilling, and as an ingredient in paint, explosives, and make-up (Cavende 1973). Utilization of the shell has been the primary reason a black walnut industry based on seedling nuts has remained profitable.

'Thomas', the first black walnut selected for nut production, has been propagated since 1881 (Heiges 1896). Since that time over 750 cultivars have been selected and named. For the most part these cultivars were selected by amateur nut growers from the native population solely on the basis of nut quality characteristics. Although black walnut is not a new crop, the growing of walnut trees solely for nut production remains the passion of backyard enthusiasts. The commercial production of black walnuts from orchards of superior nut producing cultivars is nonexistent.

Black walnut is an under-exploited crop. All genetic improvement programs for black walnut have concentrated on increasing its value for limber (Beineke 1983) and not for nut production. The potential for generic improvement in nut yield, percent kernel and kernel quality is great. Even under the low level of selection pressure applied by amateur nut growers, cultivars have been identified that exhibit one or more of the important generic traits needed for further crop improvement.

GENETIC TRAITS IMPORTANT FOR IMPROVED NUT PRODUCTION

Black walnut has great genetic potential for improved nut yield and quality The extensive naive range of this species has ensured a large gene pool from which advanced selections can be made. Generic traits that would lead to advances in a tree improvement program include: lateral bud fruitfulness, late leafing, resistance to the anthracnose fungi, Gnomonia leptostyla, (Fr.) Ces. & Not., precocity and productivity and improved nut quality.

Lateral Bud Fruitfulness

The selection of clones that produce pistillate flowers on lateral buds has resulted in significant yield increases in Persian walnut, J. regia (Ramos et al. 1984). Lateral bearing Persian walnut cultivars tend to be more precocious and are better suited to high yielding, high density plantings. The lateral bearing characteristic is also found in black walnut (Sparks 1982). Black walnut trees exhibiting this characteristic have a profusion of short, spur type branches distributed along main scaffold limbs. With this growth pattern, both leaves and nuts are borne throughout the tree canopy resulting in increased yield potential. The heritability of the lateral bearing characteristic is unknown for black walnut but Hansche et al. (1972) found this trait to have a moderate level of heritability in Persian walnut.

Late Leafing

Black walnut is quite susceptible to freezing temperatures once growth begins in the spring. Entire shoots are killed by temperatures below -2°C. Pistillate flowers are borne on the terminals of these frost tender shoots and a late spring frost can destroy all potential for nut production. Late leafing clones can avoid yield losses due to frost. Date of leafing in black walnut has a high degree of heritability (Kucera et al. 1974) but leafing dates for currently propagated cultivars are unknown. In addition to frost avoidance, late leafing black walnut clones may avoid infections by the walnut blight bacterium, Xanthomonas campestris pv juglandis in the same manner as late leafing Persian walnut cultivars (Forde 1975). Further, immature or expanding black walnut leaves have shown the greatest resistance to infection by the anthracnose fungi (Cline and Neely 1984), thus the late leafing characteristic may also contribute to anthracnose infection avoidance.

Anthracnose Resistance

The anthracnose fungus attacks leaves, new shoots and fruit of the black walnut. Initial foliar infections begin shortly after full leaf expansion during extended periods of leaf wetness (Kessler 1984). The disease progresses rapidly in mid-summer and results in premature defoliation by late August This premature leaf drop is one of the major causes of poor kernel fill (Reid 1986) and alternate bearing (Sparks 1979) in black walnut. Complete resistance to the anthracnose fungi has not been found although a wide range in susceptibilities exists (Beineke and Masters 1973, Berry 1960). The 'Thomas' and 'Ohio' black walnut cultivars have been noted for their anthracnose resistance, although both cultivars will contract the disease under conditions of high disease pressure (Berry 1960). Heritability for anthracnose resistance is high (Beineke and Masters 1973) indicating genetic gains in resistance to this devastating disease can be made.

Precocity and Productivity

High development costs for establishing a walnut orchard demand the planting of cultivars that bear large crops at an early age. Cultivars displaying lateral bud fruitfulness generally come into production well in advance of non-lateral bearing cultivars. The precocity and productivity of black walnut cultivars have not been studied but should be prime considerations in selecting new cultivars. If lateral bearing black walnuts are similar to lateral bearing Persian walnuts, first commercial yields should be expected in the fifth year after establishment. Mature orchards should be able to produce more than 2 t/ha of hulled walnuts.

Nut Quality

Nut quality characteristics have been the primary focus of the evaluation of black walnut cultivars. Over 400 cultivars have been named and their nuts evaluated for nut weight and percent kernel (Berhow 1962; Zarger 1945). Nut samples are evaluated annually by many State nut growers associations. Results from 28 years of nut evaluations in Kansas are summarized for selected cultivars in Table 1.

Shell thickness and structure are the most important determinant of percent kernel and nut crackability. The highest quality walnuts have a thin outer shell with no internal convolutions protruding into the nut meat. The inner shell partition between kernel halves should be very thin to allow easy removal of kernel pieces. Most thin shelled black walnut cultivars yield over 30% kernel. The heritability of a thin shell has not been studied for black walnut but in Persian walnut, heritability for shell thickness is high (Hansche et al. 1972).

Kernel quality and plumpness is strongly influenced by tree care and harvesting practices (Chase 1941), but with trees receiving optimum care, wide differences in kernel quality still exist between cultivars. High quality walnuts have light colored kernels with an absence of kernel veins. Dark colored or strongly veined kernels are associated with rancidity by consumers. Kernel color and venation are under a moderate level of genetic control in Persian walnut (Hansche et al. 1972) and a similar level of heritability should be expected for black walnut.

CURRENTLY AVAILABLE CULTIVARS

Several currently available cultivars deserve further evaluation for their potential as commercial orchard trees. Cultivars that bear nuts on laterals branches and produce nuts with more than 30% kernel are the primary candidates for commercial plantings. These cultivars include, 'Cranz', 'Football 11', and 'Sparks 127' (Table 1). 'Clermont', 'Kwik-Krop', and 'Sparrow' have been noted for their consistent productivity and should be evaluated further. Black walnut cultivars are not widely available from commercial nurseries and initial trials will most probably be developed by field grafting seedling rootstocks.

FUTURE PROSPECTS FOR CROP DEVELOPMENT

Large areas of crop land in the Midwest are suited to the production of black walnut (Noweg and Kurtz 1985). Lands no longer suitable for profitable corn or soybean production often provide adequate sites for black walnut orchards. The technology for the cultivation and mechanical harvesting of this crop currently exists because techniques and equipment can be easily adapted from the Persian walnut industry A well developed market and marketing system based on seedling black walnuts is already in place and could rapidly change to accommodate thin shelled, high quality cultivars.

Commercial black walnut orchards based on thin shelled cultivars have not been developed due to the lack of cultivar performance data and undemonstrated crop profitability. Large trial plantings of currently available cultivars that become financially successful will be necessary to stimulate a black walnut orchard industry. The incorporation of additional positive traits into walnut cultivars will only be made after an established black walnut industry demands further crop improvement.

REFERENCES

Table 1. A sample of black walnut cultivars exhibiting genetic traits important for future crop improvement and their nut quality characteristics.

Cultivar State of originz Percent
kernely		 No. nuts/kgy Notable genetic traits
'Bowser' Ohio 30.7 65.6 High kernel quality
'Clermont' Ohio 35.5 59.0 Productive
'Cranz' Pennsylvania 31.4 71.9 Lateral bearing
'Emma K' Illinois 35.3 62.0 Thin shell
'Football 11' Missouri 30.0 43.1 Lateral bearing
'Kwik-Krop' Kansas 31.5 58.1 Precocious, productive
'Ohio' Ohio 26.7 64.7 Anthracnose resistance
'Sauber' Ohio 34.1 65.6 Thin shell, high kernel quality
'Sparks 127' Iowa 31.5 64.0 Lateral bearing
'Sparrow' Illinois 28.7 58.3 Productive
'Thomas' Pennsylvania 24.0 51.9 Anthracnose resistance
zAll cultivars listed were discovered as chance seedlings of unknown or questionable parentage
yMeans of data collected in the years 1959-1987 by the Kansas Nut Growers Association.

Fig. 1. The natural range of Juglans nigra L. in North America (Source: Fowells 1965).
Last update August 28, 1997 by aw