VIEWPOINT
HORTSCIENCE 51(6):653–663. 2016. nut (seed) has hypogeal germination and forms a dominate taproot with abundant lat- Carya eral roots. Leaves are odd-pinnately com- Crop Vulnerability: pound with 11–17 leaflets, borne in alternate L.J. Grauke1 arrangement, with reduction in leaflet number, size, and pubescence as the tree matures. USDA–ARS Pecan Breeding and Genetics, 10200 FM 50, Somerville, TX Seedlings have long juvenility (5–12 years) 77879 and bear imperfect flowers at different loca- tions on the tree. Male and female flowers Bruce W. Wood mature at different times (dichogamy). Male USDA–ARS Southeastern Fruit and Tree Nut Research Laboratory, 21 flowers are borne on staminate catkins in Dunbar Road, Byron, GA 31008 fascicles of three, with catkin groups formed on opposite sides of each vegetative bud, Marvin K. Harris sometimes with a second pair above, borne Department of Entomology (Professor Emeritus), Texas A&M University, opposite each other, at 90 to the first. The College Station, TX 77843 catkins are borne at the base of the current season’s growth or from more distal buds of Additional index words. Carya illinoinensis, germplasm, diversity, breeding, repository, the previous season whose vegetative axis characterization aborts after pollen shed. This pattern of catkin formation is characteristic of all members of Abstract. Long-established native tree populations reflect local adaptations. Represen- section Apocarya, but is distinct from the other tation of diverse populations in accessible ex situ collections that link information on sections of the genus (Figs. 2–4). Pollen is phenotypic expression to information on spatial and temporal origination is the most produced in very large quantities and is carried efficient means of preserving and exploring genetic diversity, which is the foundation of by wind. Pistillate flowers are borne on few- breeding and crop improvement. Throughout North America, sympatric Carya species flowered (1–9) terminal spikes. Individual sharing the same ploidy level tend to hybridize, permitting gene flow that contributes to trees whose catkins shed pollen before pistil- regional diversity and adaptation. The topographic isolation of many fragmented late flower receptivity are termed ‘‘protan- populations, some of which are small, places native Carya populations of United States, drous’’ or ‘‘first male.’’ Individual trees whose Mexico, and Asia in a vulnerable position and justifies systematic collection and female flowers reach receptivity before pollen characterization. The characterization of indigenous Mexican pecan and other Carya shed are termed ‘‘protogynous’’ or ‘‘first populations will facilitate use for rootstocks and scion breeding and will contribute to female.’’ As a population, trees are hetero- pecan culture. The Asian species, as a group, are not only geographically isolated from dichogamous, with mixed periods of bloom North American species, but also occur in disjunct, fragmented populations isolated from enabling cross pollination and outcrossing other Asian species. Section Sinocarya includes the members of the genus most (Thompson and Romberg, 1985). This repro- vulnerable to genetic loss. With all species, recognition of utility based on characteriza- ductive system facilitates heterozygosity in tion of ex situ collections may contribute to the establishment of in situ reserves. Global individuals and populations, and is accompa- Carya genetic resources should be cooperatively collected, maintained, characterized, nied by inbreeding depression. The fruit of and developed. The integration of crop wild relatives into characterization and pecan is a nut enclosed in a dehiscent husk that breeding efforts represents a challenging opportunity for both domestic and in- splits along more or less winged sutures to ternational cooperation. Genomic tools used on the accessible collections of the National release the hard-shelled nuts. Nuts are tan to Collection of Genetic Resources for Pecans and Hickories (NCGR-Carya) offer great brown, mottled with black stripes at the apex potential to elucidate genetic adaptation in relation to geographic distribution. The and spots at the base. Shells are relatively thin greatest progress will be made by integrating the disciplines of genetics, botany, as compared with most hickories, and protect pathology, entomology, ecology, and horticulture into internationally cooperative efforts. the edible kernels, which are rich in mono- and International germplasm exchange is becoming increasingly complicated by a combina- polyunsaturated fats. Young pecan trees are tion of protectionist policies and legitimate phytosanitary concerns. Cooperative slow to bear fruit, but increase in fruit pro- international evaluation of in situ autochthonous germplasm provides a valuable duction with age and size. Mature trees tend to safeguard to unintended pathogen exchange associated with certain forms of germ- be large and long lived, producing crops that plasm distribution, while enabling beneficial communal exploration and directed alternate with regional synchrony in a roughly exchange. This is threatened by the ‘‘proprietary’’ focus on intellectual property. alternating manner with occasional masting The greatest risk to the productive development of the pecan industry might well be (mass seeding) (Chung et al., 1995). a myopic focus on pecan production through the lens of past practice. The greatest limitation to pecan culture in the western United States is reduced water quantity and Ecogeographical distribution quality; in the eastern United States the challenge is disease susceptibility; and Native stands of pecan are distributed insufficient cold hardiness in the northern United States. The greatest benefit for the primarily in well-drained soils of the Mis- entire industry might be achieved by tree size reduction through both improved sissippi River and its tributaries from north- rootstocks and scions, which will improve both nut production and tree management, ern Illinois and southeastern Iowa south to impacting all areas of culture. This achievement will likely necessitate incorporation of the Gulf Coast of Louisiana and west to the crop wild relatives in breeding, broad cooperation in the testing leading to selection, and Edwards Plateau of Texas in the United development of improved methods linking phenotypic expression to genomic charac- States (Fig. 1). Isolated populations occur as terization. The development of a database to appropriately house information avail- far east as southwestern Ohio and central able to a diverse research community will facilitate cooperative research. The Alabama, and as far west as Chihuahua, acquisition of funds to pursue development of those tools will require the support of Mexico. Pecan has the westernmost distribu- the pecan industry, which in the United States, is regionally fragmented and focused on tion of any of the Carya species. marketing rather than crop development. Hybridity in sympatric species of Carya sharing the same ploidy level is evident in the Introduction to the crop to North America (Fig. 1). It is the most abundance of known hybrids (Table 1) and economically valuable and agriculturally sig- has recently been examined using molecular Biological features nificant member of Carya. At maturity, pecan markers (Grauke and Mendoza-Herrera, Pecan [Carya illinoinensis (Wangenh.) K. is a large, long-lived, monoecious, polymor- 2012). Distribution maps are provided here Koch] is a tree species autochthonous (native) phic, anemophilous, and deciduous tree. The by section and ploidy to aid in visualizing
HORTSCIENCE VOL. 51(6) JUNE 2016 653 sold at reduced prices as ‘‘Seedling’’ nuts. Production is erratic, with years of low pro- duction (but high kernel quality) often followed by very heavy crops (but low kernel quality). Farm gate price per pound tends to alternate in opposition to yield, with heavy crop years generally bringing low prices. In the 4 years between 2011 and 2014, total U.S. pecan pro- duction averaged 125.2 million kg per year (276 million lbs) with a mean annual farm gate Fig. 3. Valvate terminal buds (L) and paired, value of 525 million dollars. Production from opposite 3-fascicled catkins (R) of Apocarya improved pecans was relatively flat during the (Carya illinoinensis). Catkins are borne at the 1990s, but has recently increased due to sub- base of the current season’s growth as well as stantial increases in crop value (Fig. 9). from buds lower on the shoot in which the Increased exports to Asian markets have vegetative apex aborts. contributed to the strong prices received and to increased planting in recent years, which should result in increased future production. Fig. 1. (A) Carya illinoinensis (Wangenh.) K. Koch U.S. exports to China increased dramatically indigenous pecan tree, Red River Valley, TX. beginning in 2007. Before that year, the (B) Native distribution of species, and area of peak export to China had been almost 6000 culture in North America. (C) ‘Pawnee’ nuts in t in 2006. Since the 2007 crop year, exports husk, in shell, longitudinal, and cross sections. to China have not dropped below 20,000 t (D) Diversity in shape and size of adult phase and in crop year 2012 exceeded 45,000 t leaves [‘Upton’ (BW 105-21); ‘Aggie’ (BW (Zedan, 2015a). National area of bearing and 104-30); ‘Hadu 3’ (BW 104-35)]. (E) Diversity nonbearing improved pecan orchards in- in pecan nut size and shape. creased from 124,900 to 131,560 ha (an increase of 6660 ha or 5%) between the 2007 and 2012 agricultural census (NASS, Fig. 4. Imbricate terminal buds (upper L) that swell 2012). Production from native/seedling pe- greatly at growth initiation (lower L), and cans has continued to decrease since the 3-fascicled catkins borne in axils of bud scales 1970s (Fig. 10). National land area of bearing (R) in Carya (Carya laciniosa). Catkins are and nonbearing native/seedling pecan trees borne at the base of the current season’s decreased from 110,550 to 88,382 ha (a re- growth. duction of 22,168 ha, or 20%) between the 2007 and 2012 agricultural census (NASS, Fig. 2. Naked terminal buds (L) and single, Carya dabieshanensis (Liu and Li, 1984)] 2012). If those figures are trustworthy, that is 3-fascicled catkins (R) of Sinocarya (Carya are collected from natural stands and mar- an alarming rate of loss in native area and cathayensis). keted primarily in China. should increase the motivation to characterize remnant stands of old native trees across the Primary crop products and their value range. potential gene flow among North American (farm gate) Carya species (Figs. 5–7). Known distribu- Commercial U.S. pecan production is di- Domestic and international crop tions of Asian Carya species are more frag- vided into two major categories: ‘‘Native/ production mented, with no species overlap (Fig. 8). Seedling’’ and ‘‘Improved.’’ ‘‘Native’’ pecans United States (regional geography). are ungrafted trees growing in natural, regen- Georgia produces the largest percent of the Plant breeding and its products erating stands that were not established by U.S. improved crop. Most production comes The primary commercial crop is pecan humans. Native pecans represent the original from mature orchards in the southern part of nuts, an increasingly global commodity. Pe- forest, may hold the key to understanding the state concentrated near the city of Albany. can is the only Carya species that has climatic and edaphic adaptation, and are the New acreage in middle Georgia, especially benefited from significant breeding and se- genetic foundations for crop improvement. on the Fort Valley Plateau, is being planted to lection. Most attention has been focused They also constitute large acreages producing more recently developed cultivars of pecans, on scion breeding and selection. Open- relatively low yields of often poor-quality while older orchards in southern Georgia pollinated seed stocks are used to grow nuts. ‘‘Seedling’’ pecans are ungrafted trees are being renovated to new cultivars (Wise, rootstocks, which vary by region. In addition, grown from nuts that have been introduced 2010). New Mexico is the second leading nuts of Carya ovata and Carya laciniosa are and are therefore evidence of some level of state in improved pecan production. Most harvested from wild trees and consumed selection. ‘‘Improved’’ pecans are selected New Mexico production comes from the locally. In China, nuts of local native hicko- cultivars grafted onto seedling rootstocks. Mesilla River Valley in Dona Ana County. ries [mixtures of Carya cathayensis and Highest yields are achieved in orchards of This production is primarily from ‘Western’ ‘Improved’ pecans, with around 2000 kg/ha/ and ‘Wichita’, with new plantings incorporat- year being realized as the statewide average ing ‘Pawnee’ and other more recent releases. in some western states where all production Oklahoma is the center of native pecan pro- Received for publication 1 Oct. 2015. Accepted for is solely from improved plantings. By contrast, duction, followed by Texas. During the early publication 12 Feb. 2016. states with large numbers of native pecans, decades of the 20th century, native produc- Mention of a trademark, proprietary product, or such as Texas, have low production per unit tion exceeded improved production. Following vendor does not constitute a guarantee or warranty area. This is due to lower yields produced by World War II, improved production increased of the product by the U.S. Department of Agricul- ture and does not imply its approval to the natives (<500 kg/ha/year), and because many sharply and overtook native production during exclusion of other products or vendors that also native pecans are not harvested annually (Ares the 1960s. Increased planting of soybeans in may be suitable. et al., 2006). Highest prices are obtained for Louisiana and Arkansas in the 1970s resulted 1Corresponding author. E-mail: lj.grauke@ars. well-managed, improved pecans, whereas nuts in the destruction of thousands of hectares of usda.gov. from unmanaged improved cultivars may be native pecans (Grauke et al., 1995), beginning
654 HORTSCIENCE VOL. 51(6) JUNE 2016 Table 1. Taxa recognized as species by the National Collection of Genetic Resources for Pecans and Gibbons, 2011; Nunez,~ 2014). Mexican com- Hickories, listed alphabetically within section. mercial pecan production is largely based on Section Sinocarya cultivars developed in the United States, 1. Carya cathayensis Sarg. Chinese hickoryz (probably includes Carya dabieshanensis) primarily ‘Western’ and ‘Wichita’, although 2. Carya hunanensis Cheng and R.H. Chang–Hunan hickoryz the recent release of ‘Nortena’~ (P erez et al., 3. Carya kweichowensis Kuang and Lu–Guizhou hickoryz 2015) represents selection from local germ- 4. Carya tonkinensis Lecomte–Viet Nam hickoryz plasm which is applauded. Recent crop in- z 5. Carya poilanei (A. Chev.) J. Leroy–Poilane’s hickory creases have been due to both increased Section Apocarya planting and improved cultural practices, 6. Carya aquatica (F. Michx.) Nutt.–Water hickory 7. Carya cordiformis (Wangenh.) K. Koch–Bitternut hickory including drip irrigation (Flores and Ford, 8. Carya illinoinensis (Wangenh.) K. Koch–Pecan 2009). Area harvested was estimated at about 9. Carya palmeri Manning–Mexican hickory 70,000 ha in 2014, with over 82,000 ha Section Carya planted. Chihuahua is the major pecan- 10. Carya floridana Sarg.–Scrub hickory producing state in Mexico, accounting for 11. Carya glabra (Mill.) Sweet–Pignut hickory [includes Carya ovalis (Wangenh.) Sarg.–Red hickory] 50% to 60% of national production, followed 12. Carya laciniosa (F. Michx.) Loudon–Shellbark hickory by Coahuila, Durango, Sonora, and Nuevo 13. Carya myristiciformis (F. Michx.) Nutt.–Nutmeg hickory Leon (Nunez,~ 2014; Puente, 2004). The 14. Carya ovata (Mill.) K. Koch–Shagbark hickory [includes Carya carolinae septentrionalis (Ashe) Engl. & United States is the main export market for Graebn.–Southern shagbark hickory] 15. Carya pallida (Ashe) Engl. & Graebn.–Sand hickory Mexican pecans (Flores and Ford, 2009). 16. Carya texana Buckley–Black hickory The world’s third largest pecan producer 17. Carya tomentosa (Poir.) Nutt.–Mockernut hickory is South Africa producing about 2.5% of the Interspecific hybrids world pecan crop. Pecan production began in 1. Carya ·brownii (C. cordiformis · C. illinoinensis) Sarg. the Nelspruit area of Mpumalanga. Current 2. Carya ·collina (C. texana · C. tomentosa) Laughlin plantings are concentrated to the west, in the 3. Carya ·demareei (C. cordiformis · C. ovalis) Palmer Northern Cape Province in the Vaalharts 4. Carya ·dunbarii (C. laciniosa · C. ovata) Sarg. · · Irrigation district. The climatic conditions 5. Carya laneyi (C. cordiformis C. ovata) Sarg. of that area are very similar to the U.S. 6. Carya ·lecontei (C. aquatica · C. illinoinensis) Little 7. Carya ·ludoviciana (C. aquatica · C. texana) (Ashe) Littley western pecan region. The dry climate is 8. Carya ·nussbaumeri (C. illinoinensis · C. laciniosa) Sarg. not conducive to the pecan scab disease 9. Carya ·schneckii (C. illinoinensis · C. tomentosa) Sarg.y (Fusicladium effusum Winter) allowing for 10. C. illinoinensis · C. ovata (see Manning, 1962) reduced production costs. Available land, 11. C. illinoinensis · C. myristiciformis (see Grauke and Mendoza-Herrera, 2012). water, and a strong international market for zAsian species. Section Sinocarya Cheng and R.H. Chang (in Chang and Lu, 1979) has been proposed for pecans has resulted in accelerated planting all Asian spp. based on naked buds. Those listed in Sinocarya also lack lacunae in nut shells, which Carya of pecans in South Africa, averaging about poilanei has, as do all members of Apocarya. 4000 ha of new plantations per year. Cultivars yParentage disputed. being planted there are primarily ‘Wichita’, ‘Choctaw’, and ‘Navaho’, all products of the U.S. Department of Agriculture (USDA) Agricultural Research Service (ARS) Pecan Breeding Program (Grauke and Thompson, 1997). Production in the southern hemisphere has the advantage of being in the off-season of the northern hemisphere, placing their current season crop in competition with the early U.S. pecan harvest for the lucrative Thanksgiving/Christmas holiday gift market. However, about 90% of the South African crop is sold in-shell to China (Zedan, 2015b). The pecan industry in Australia was de- veloped largely since the 1960s, when a por- tion of the Stahmann family migrated to Australia from the Las Cruces area of New Mexico, where the family pioneered New Mexico pecan production in the 1930s. Most pecans in Australia are produced in New South Wales, with 80% of production being from the Stahmann Orchards at Moree, New South Wales. Total production area in Aus- tralia is currently 1400 ha, with annual pro- duction of about 3500 t in-shell or about 1.4% of the world production. Australian produc- tion has the advantage of being in the off- season of the northern hemisphere, placing Fig. 5. Distribution of diploid species of section Apocarya. the current season Australian crop in partial competition with the early pecan harvest in the United States for the lucrative holiday a consistent downslope in the production trend International. Mexico is second to the gift market. However, 66% of Australian for natives (Fig. 10). Since native pecans yield United States in the world of pecan pro- production is consumed locally and the less per acre, and nuts are sold at lower prices, duction and has recently harvested over remainder is primarily targeted at Asian, there is an increasing incentive to convert 100,000 t, accounting for over 40% of total rather than U.S. markets. The primary native stands to improved orchards. world production (Table 2) (Branson and cultivars grown in Australia are standards
HORTSCIENCE VOL. 51(6) JUNE 2016 655 It was estimated (B. Garris, personal communication) that by 2025, there would be worldwide production of over 441,000 t (973,000,000) lbs in-shell (an increase of 182% over current production), with the primary pecan-producing nations being United States > Mexico > South Africa > Australia.
Urgency and Extent of Crop Vulnerabilities and Threats to Food Security
Genetic uniformity in the ‘‘standing crops’’ and varietal life spans Improved pecan culture is a young in- dustry, developed from a diverse native forest of long-lived trees. The ‘‘standing crop’’ in- cludes both indigenous ‘‘native’’ trees and the established orchards of the ‘‘improved’’ pecan industry. The first grafted pecan orchard was propagated by the slave gardener Antoine at Oak Alley Plantation, LA, in 1846 (Taylor, 1905). Individual pecan trees are long lived and commonly survive over 200 years. Given individual tree longevity, unselected native Fig. 6. Distribution of diploid species of section Carya. trees still bearing nuts are older than the U.S. commercial pecan industry. The genetic di- versity of the native pecan forest has always been assumed to be robust, but systematic methods of characterizing that diversity have not been applied. Increasing evidence indi- cates that the geographic distribution of ge- netic diversity reflects long-term adaptation (Bock et al., 2016; Grauke et al., 2011; Sagaram et al., 2011; Sparks, 2005). Char- acterization of extant diversity in ex situ collections should contribute to breeding and selection. The historic trend in pecan culture has been managed to increase uniformity. Prop- agation by grafting or budding allows devel- opment of large acreages of a few common genotypes. As a cultivar gains regional ac- ceptance in the market, acreage increases. Four cultivars (Stuart, Western, Desirable, and Wichita) accounted for over 57% of total improved pecan acreage in 1990 (Thompson, 1990). Once selected and propagated by grafting, a cultivar becomes essentially ‘‘im- mortal’’, as it is re-propagated. ‘Desirable’, which originated in about 1933, is still among the most commonly planted cultivars in Georgia (Wells, 2014), despite substantial susceptibility to pecan scab disease that Fig. 7. Distribution of tetraploid species of section Carya. necessitates multiple prophylactic fungicide sprays (Wells, 2014). ‘Western’, introduced of the U.S. western pecan industry, such as Gale, 2015), and specifically the traditional in 1916, is still the most commonly planted ‘Wichita’ and ‘Western’. consumption of its native hickory nut, C. cultivar in western orchards. ‘Pawnee’, intro- Pecan nuts are also produced in Argen- cathayensis (Grauke et al., 1991). U.S. duced in 1984, was the first cultivar released tina, Egypt, Brazil, Uruguay, Chile, Peru, pecan grower and sheller groups have for early season nut maturity (Thompson The Peoples Republic of China, India, and expressed concern over the threat posed by and Hunter, 1985). That trait has contributed Turkey; with small plantings also in the the development of a pecan industry in to increased profits in the lucrative U.S. Dominican Republic, Zimbabwe, Israel, China. However, a vibrant Chinese pecan holiday markets surrounding Thanksgiving Georgia, Italy, Syria, and Iran. industry based on existing U.S. cultivars and Christmas. ‘Pawnee’ is among the most Much of the global attention on pecan is is ‘‘not likely to be realized in light of popular cultivars being propagated in Geor- due to increasing consumption in China. resource- and climate-related constraints, the gia (Wells, 2014), as well as in the north and China is working to develop its own pecan difficulty of expanding production on the basis west (Thompson and Conner, 2012). production industry, building on the historic of fragmented, small-scale production devel- Tree longevity, coupled with the time and respect and value given to the perceived oped over the last two decades’’ (Yang and expense to establish a new orchard, contrib- health value of nuts in general (Yang and Gale, 2015). utes to the challenge of incorporating new
656 HORTSCIENCE VOL. 51(6) JUNE 2016 stands from wild populations is suspected to have peaked by the middle of the 20th century, with existing native groves consist- ing of aging trees that are often too large for effective spray management or mechanical harvesting systems. Grazing cattle under trees harvested for nuts raises food regulatory issues that further threaten native culture, increasing the incentive to convert acreage to the less genetically diverse, but more lucrative grafted plantations. Designation of in situ populations that preserve regional genetic diversity while continuing the resil- ient and historically important cattle/pecan management system may be justified. Characterization of the geographic distri- bution of genetic diversity in pecan consis- tently confirms that the forest holds more diversity than we know how to use (Grauke et al., 2011; Ruter€ et al., 1999, 2000). Evaluation of diversity in existing ex situ collections of pecan should include more comprehensive screening for resistance to major pests (Harris, 1975). Research on existing collections shows variation in leaf morphology (Grauke et al., 2003; Sagaram Fig. 8. Distribution of species in section Sinocarya (all diploid). et al., 2011) and disease resistance (C. Bock, personal communication) related to geographic origin. Increased resolution with improved genomic tools should aid interpretation of the geographic distribution of genetic diver- sity. This may motivate return to targeted native areas for further collection. Conser- vation of in situ populations is invaluable to allow future generations to continue the exploration of this valuable natural resource. Of the other U.S. Carya species, the most threatened are Carya myristiciformis (Fig. 6) which was described as ‘‘nowhere abundant’’ by Sargent (1918), and is represented by fragmented, disjunct populations that spread from the Atlantic coast to central Mexico, and Carya floridana (Fig. 7) which is en- demic to central Florida and has the most restricted distribution of any North American hickory. Incentive to incorporate valuable traits is necessary to justify the time needed for breeding with hickories. Clinton Graves (Mississippi State University, retired) saw the potential of increased disease resistance with pecan/hickory hybrids (Graves et al., Fig. 9. Fifty years of U.S. ‘Improved’ pecan yields (million lbs, 10-year running average) in relation to 1982). He made several controlled crosses value (million $, 10-year running average). Data from NASS (2012). between pecan and nutmeg hickory that are represented in repository collections, but they offer neither morphological nor molecular cultivars. Unmanaged orchards that might be research being on the development and man- evidence of hybridity. Evidence of hybridity considered obsolete can be brought back into agement of grafted cultivars. Reduction in with other species in wild populations of production when the incentive of increased the area of native pecan stands is variable nutmeg hickory has been observed following price returns, as has been the case since 2003 across the range, but has been most extensive limited examination (Grauke and Mendoza- (Fig. 9). That argues in favor of only offering in Arkansas (Grauke et al., 1995). The Herrera, 2012) and should be systematically new cultivars with significantly improved harvested resource we call ‘‘native’’ pecans evaluated. Carya floridana has the smallest traits of bearing consistency, nut quality, is the result of thinning and minimally mature tree size among the hickories. Col- and critical local adaptations for disease and managing what was originally the ‘‘wild’’ lections from across the species’ range were insect resistance. forest (Harris, 1980). The development of made in 2009 and are well represented in managed native groves in Texas, Oklahoma, repository collections by self-rooted seed- Threats of genetic erosion in situ Louisiana, and Arkansas has historically lings. Ploidy is being examined using flow U.S. Carya species. The genus Carya been inextricably linked to cattle grazing, as cytometry (Whittemore, Xia, and Grauke, consists of 17 species worldwide (Table 1). trees were thinned to allow pasture for unpublished data) and crosses are being made The focus of horticultural attention has his- grazing, reducing tree crowding, and increas- in the breeding program in an effort to devise torically been on pecan, with the emphasis of ing nut production. Development of native strategies to capture the beneficial trait of
HORTSCIENCE VOL. 51(6) JUNE 2016 657 (2012) reported patterns in maternally in- herited plastid profiles that consistently dis- tinguished specimens of C. dabieshanensis from C. cathayensis in limited numbers of observations, but such distinctions also sep- arate geographic populations of pecan that are not recognized by even varietal, much less species status (Grauke et al., 2011). Carya hunanensis, Carya tonkinensis, Carya kweichowensis, and Carya poilanei are not represented in any known systematic ex situ collections. C. hunanensis was tested and found advantageous as a seedling root- stock for C. cathayensis (Huang J.Q. personal communication). C. tonkinensis is occasion- ally used as a rootstock in Yunnan, but use of the other western Chinese species C. kwei- chowensis is unknown. Given the rather limited distributions of the Asian Carya species, their characterization and appropri- ate conservation is encouraged even in the absence of known strategies of utilization. Debate continues over whether the tree considered by Manning (1963) to be Carya Fig. 10. Fifty years of U.S. ‘Native’ pecan yields (million lbs, 10-year running average) in relation to value sinensis Dode, is best classified as the (million $, 10-year running average). Data from NASS (2012). monotypic representative of section Rham- phocarya or the monotypic species Anna- Table 2. World pecan production. mocarya sinensis (Dode) Leroy (Lu et al., Country Total bearing hectares Nonbearing hectares Metric tons % of total production 1999). Regardless of its taxonomic classi- United States 188,622 30,607 126,174 52.0 fication, the tree should be incorporated Mexico 70,000 20,000 105,000 43.3 into studies of genetic diversity and its South Africa 6,000 21,000 6,000 2.5 existing populations recognized and pre- Australia 1,400 3,274 1.4 Others 2,000 0.8 served by both ex situ and in situ collec- Total 242,448 tions. Chinese populations have recently Land area based on information from NASS (2012). been studied using molecular markers from U.S. production = (Improved + Native/Seedling) pecans, 2012–14 (NASS, 2012). plastid (matK, rbcL-atpB, rpoC1, rps16, trnH-psbA, and trnL-F) and nuclear ge- nome (ITS and phyA) (Zhang et al., reduced tree size, either in rootstocks or scions, more about all species of Asian Carya in- 2013). No ex situ collections of the species both of which will be long-term efforts. Exist- cluding the geographic extent of their distri- have been observed, even at the Nanjing ing in situ populations of C. floridana are well bution and their abundance and diversity over Botanical Gardens. Indigenous stands of conserved by numerous public and private that range. The focus of Chinese efforts has the species at Cuc Phuong Forest Reserve conservation efforts. been on C. cathayensis, the foundation of in Vietnam were recognized as an impor- Mexican Carya species. Carya in Mexico a respected native nut industry centered at tant in situ reserve (Grauke et al., 1991) and includes Carya palmeri, C. ovata, and C. Linan, Zhejiang, People’s Republic of China. should be incorporated into systematic myristiciformis, in addition to pecan (C. The species has been extensively sequenced characterizations. illinoinensis). The first three are unrepre- and characterized using a variety of molecu- Living specimens of C. poilanei have not sented in systematic ex situ collections, but lar methods (Huang et al., 2013; Li et al., been seen since collections were made from may be represented in some de facto in situ 2014; Wang et al., 2011). Apomixis has been the type tree in Vietnam by Poilane in 1937 reserves. The three former species are all reported for the species (Zhang et al., 2012), (Manning, 1963), although collection efforts found in mixed native populations in the an observation consistent with the lack of have been made in the area where the type mountainous Parque Ecologico Chipinque, polymorphism in seedling accessions of was originally found (Grauke et al., 1991; D. south of Monterey, Mexico, where there is the species observed over several years in Stone, personal communication) as well as evidence of hybrids that include pecan (Grauke the NCGR-Carya collections (Grauke and in Laos (C. Philaphandeth, personal com- and Mendoza-Herrera, 2012). Manning (1962) Mendoza-Herrera, 2012). Although C. cathayen- munication). Because that species had traits reported the first interspecific hybrids between sis may be well-represented in Chinese collec- that linked Asian species to extinct Euro- pecan and C. ovata from San Luis Potosi, tions, characterizations of its diversity across pean species (Mai, 1981), it would be valu- Mexico. Observations in Mexico of early nut its limited range have only recently been able to locate surviving members, if they maturation in association with low chilling reported (Guo et al., 2015). Whether C. exist. show that pecan accessions from Mexican dabieshanensis deserves species rank or is Asian species are very poorly represented populations may contribute valuable traits for a subspecies of C. cathayensis remains to be in U.S. repository collections. We hope to breeding. Mexican pecan seedlings show out- determined. The nuts of the two taxa are work cooperatively with Asian scientists in standing vigor, manifest unusual patterns of often marketed together. Whether the col- developing appropriate molecular methods plastid diversity, and have patterns of root and lections of Guo et al. (2015) included pop- for characterizing diversity. We encourage leaf morphology likely indicating population ulations of C. dabieshanensis in their genetic establishment of representative Asian ex situ level adaptations of value in development of evaluations of Anhui Carya populations is collections where those materials are acces- both rootstocks and scions. unknown. Recent research (Li et al., 2014) sible for study. It is important to establish Asian Carya species. All Asian hickories comparing neutral genetic diversity between C. a baseline profile for each species for phylo- occur in fragmented, topographically isolated cathayensis and C. dabieshanensis imply in- geographic interpretations as well as to rec- populations with limited (known) distribu- creased interest in distinguishing and using ognize hybridization that may occur within tions (Fig. 8). It would be valuable to know that germplasm. Grauke and Mendoza-Herrera Asian species when pecan is introduced.
658 HORTSCIENCE VOL. 51(6) JUNE 2016 Current and emerging biotic, abiotic, molecular profiles consistent with being di- Abiotic (environmental extremes, climate threats and needs rect progeny of ‘Schley’ (Grauke, unpub- change). The long-lived nature of pecan Biotic (diseases, pests). Xylella fastidiosa lished data). orchards and cultivars makes them vulnera- is emerging as an important factor in the Since a draft of this report was presented ble to substantial changes in climate and global stewardship and dissemination of and discussed at the Crop Germplasm Com- associated weather events and patterns. Pe- pecan germplasm. The pathogen is a Gram- mittee meeting in Frisco, TX, early July can production can be injured by late spring negative, straight, rod-shaped, aerobic, afla- 2015, PBLS was confirmed in Arizona and and early autumn freezes, excessive cloud gellate bacterium. It grows optimally from New Mexico by Drs. Natalie Goldberg and cover, excessive rain during pollination, and 26 to 28 C and is described as ‘‘nutritionally Richard Heerema (Vitanza, 2015). Efforts to drought. Different Carya species are well fastidious,’’ requiring specialized media for characterize the extent of disease distribution adapted to different environmental niches, growth in the laboratory. Sanderlin and and to coordinate protocols for its pheno- potentially possessing genes that could be Heyderich-Alger (2000) reported that X. typic and biochemical identification are introduced into pecan to produce cultivars fastidiosa causes leaf scorch in pecan, which underway (J. Randall and K. Ong, personal adapted to new abiotic environments. There has become known as Pecan Bacterial Leaf communication). is a need to develop pecan rootstocks adapted Scorch (PBLS). A disease screen of commer- International distribution of graftwood to specific soil environments, such as ex- cial cultivars for susceptibility to PBLS has been curtailed by this disease. Phytosani- tremes in pH, salts, and essential nutrient (Sanderlin, 2005) reported variable expres- tary inspection protocols call for seasonal elements. sion of leaf scorch, with all susceptible to inspections, with inventories found to be symp- infection and symptom development. Graft tomatic being tested with enzyme-linked im- Status of Plant Genetic Resources in the transmission of the pathogen was demon- munosorbent assays (ELISAs). The efficacy of NPGS Available for Reducing Genetic strated. Sanderlin and Melanson (2010) dem- ELISA to detect PBLS may depend on refine- Vulnerabilities onstrated insect transmission of X. fastidiosa ment of sampling and testing protocols. Steps by pecan spittlebug, Johnson-grass sharp- necessary to protect the domestic pecan indus- Germplasm collections and in situ shooter, and glassy-winged sharpshooter. try from distribution of infected wood are being reserves Melanson et al. (2012) reported that sub- explored. Holdings. The USDA ARS Pecan Breed- species multiplex infects pecan. Currently, Regulatory officers dealing with phytosa- ing and Genetics Program maintains two five subspecies of the pathogen are proposed nitary issues require lists of insects and worksites where ex situ collections of the based on molecular sequence similarities: diseases that attack pecan. These lists are in NCGR-Carya are held. Both are in Texas: the subsp. fastidiosa includes strains that cause need of update as new pests such as X. Brownwood worksite in Brown County, and disease in alfalfa, almond, grapevine, and fastidiosa have been recognized and old pests the College Station worksite in Burleson maple; subspecies multiplex includes strains have been renamed. Updated lists are pro- County. At each location, there are three causing disease in pecan, almond, elm, vided, along with the full Crop Vulnerability general categories of holdings: pecan cultivar peach, pigeon grape, plum, sycamore, and Report, at the website of the USDA–ARS collections (accessions grafted onto pecan other hardwoods; and subspecies pauca in- Pecan Breeding and Genetics Program seedling rootstocks), pecan provenance col- cludes strains causing disease in citrus. Sub- (http://cgru.usda.gov/carya). Over time, ob- lections (primarily seedlings growing on species sandyi, was proposed for strains solete names used for a pest may obscure our their own roots, grown from seed collected infecting oleander, whereas subspecies tashke recognition of an organism. In the list of from geographic populations), and species was proposed for strains infecting chitalpa diseases, synonyms are included to assist collections (collected directly from wild trees in New Mexico (Randall et al., 2009). Once those unfamiliar with updated nomenclature. and held as seedlings on their own roots, infected with X. fastidiosa, the disease is The disease list began with Gottwald et al. grafted to pecan rootstocks, or in some cases chronic and yields are reduced (Sanderlin (2001), and then relied on the database of received as named cultivars from various and Heyderich-Alger, 2003). Disease trans- Farr and Rossman (2015) for confirmation. sources and grafted to pecan seedling mission via grafting can be reduced by hot References to disease citations are included rootstocks). water treatment of scions before grafting in the table based on the latter database, with Brownwood: The Brownwood worksite is (Sanderlin and Melanson, 2008). Melanson occasional additions. The list of insects that the original home of the USDA ARS Pecan and Sanderlin (2015) recommended that hot affect pecan began with an unpublished list Breeding and Genetics Program, with a his- water treatment be adopted as standard graciously provided by Bill Ree (TAMU tory tracing back to 1930. The worksite phytosanitary treatment in regions that im- Entomologist). Stan Wellso (USDA ARS includes a total of 96.7 ha, of which 62.7 ha port pecan scion wood. Recent research by Entomologist, retired) added many bupres- are owned by the City of Brownwood and Sanderlin (2015) reported that seedlings tids known to attack pecan and hickory, and made available to USDA ARS under a mem- grown from seven open-pollinated seed common names have been added when ap- orandum of understanding. Almost 2 ha and stocks differed in susceptibility to infection propriate. That list is also available at the all headquarters buildings located at 701 after mechanical inoculation with X. fastid- above mentioned website. Woodson Road were donated by the city to iosa. ‘Cape Fear’, known to be extremely International distribution of germplasm ARS in 1973 (and accepted by USDA in susceptible as a scion, produced seedling requires free exchange of information to help 1976). An additional 32 ha of land adjacent to rootstocks that were among the most sus- guard against unintentional introduction of the station was purchased by ARS in 1978 ceptible, with ‘VC1-68’ and ‘Apache’ in the pests. A list of insects affecting Carya in as a permanent site for the National Clonal same category. ‘Curtis’, ‘Elliott’, and ‘Riv- China was provided to Marvin Harris by Germplasm Repository which was officially erside’ seedling rootstocks were less sus- Zhi-hong Xu (Zhejiang A&F University, designated in 1984. Louis Romberg, the first ceptible. Seedling rootstocks from ‘Moore’ Linan, People’s Republic of China) and is being ARS breeder, began grafting selected culti- and ‘Stuart’ were intermediate. Since X. translated and compared with the U.S. list vars at Brownwood in the early 1930s for use fastidiosa can be transmitted at high fre- (M. Harris, personal communication). A re- as parents in breeding. Those collections quency from infected rootstocks to newly port of diseases affecting pecan in South were the reason the site was designated in grafted trees, selection of resistant root- Africa (Marais, 2015) includes some not 1984 as the NCGR-Carya (Postman et al., stocks would be beneficial to pecan nurser- reported in the United States [e.g., Neofusi- 2006). The National Plant Germplasm Sys- ies and pecan producers. The foundation coccum parvum (Pennycook & Samuels) tem mandated that plants within the system clone of the USDA ARS Pecan Breeding Crous, Slippers & A.J.L. Phillips, Neocosmo- be ‘‘backed up’’ by additional inventories to Program has been ‘Schley’ (Grauke et al., spora vasinfecta E.F. Sm]. The active in- avoid catastrophic loss of accessions. Collec- 2015). The two most susceptible seed stocks volvement of interdisciplinary specialists is tions have been transferred from land owned in recent tests (Sanderlin, 2015) are ‘Cape necessary to maintain accuracy and currency by the City of Brownwood onto ARS owned Fear’ and ‘VC1-68’, which both have in these lists. land, and materials transferred from each
HORTSCIENCE VOL. 51(6) JUNE 2016 659 worksite to the other. Grafted accessions of program website at www.CGRU.usda.gov/ 222 SNP loci. Those are being evaluated for the NCGR-Carya at Brownwood are found as Carya. Both are continuously updated. polymorphism using an eight-member diver- isolated, old trees all across the Old Main Passport information. Summaries of pe- sity panel using Kompetitive Allele Specific Orchard, but are concentrated in the Romberg can cultivar origination records published by PCR. Block and C-Pans of the City Orchards. Since Thompson and Young (1985) form the foun- Two accessions of pecan (‘Pawnee’ and 1991, grafted accessions have been added to dation of cultivar passport information and 87MX3-2.11) were selected to investigate the the NCGR BWV orchard, now occupying 5.7 are available for most commercial cultivars feasibility of generating a pecan reference of 22 ha available on ARS land contiguous to on the program website (http://cgru.usda. sequence. They were selected due to the the original orchards. The oldest inventories gov/Carya/pecans/cvintro.htm). Accessions extensive use of ‘Pawnee’ in current breeding of grafted accessions at the Brownwood have been georeferenced to provide estimates progenies, and the low level of polymor- orchard were used to develop molecular of latitude and longitude based on details phism observed in the 87MX3-2.11 acces- methods for cultivar identity (Marquard available in acquisition information. Mo- sion, based on microsatellite analysis. et al., 1995) as well as for subsequent micro- lecular profiling offers confirmation or cor- Isolated DNA was used by HudsonAlpha satellite profiles, and are invaluable for the rection at all nomenclatural levels and is Genome Sequencing Center to produce a integrity of cultivar verification. ongoing. draft survey sequence for pecan (Jenkins There are 354 grafted accessions in the Genotypic characterization data. About et al., 2015). The draft assembly allows SNP Brownwood orchards, including 96 that are 170 pecan cultivars were profiled for malate discovery, provides a scaffold to which other not present in College Station. Ninety-five dehydrogenase, phosphoglucose isomerase, Carya genomic sequence may be aligned and percent of the grafted accessions at Brown- phosphoglucomutase, leucine aminopep- has formed the foundation for the develop- wood are pecan, with 5% being other species tidase, and diaphorase (Marquard et al., ment of a community strategy for further of Carya. A small provenance collection is 1995). About 800 pecan seedlings from development of genomic tools for pecan. A maintained at Brownwood, comprised of provenance collections at College Station PowerStation Gx2 Starter computing platform only 120 pecan seedlings growing on their were evaluated for the same isozymes (PSSC Labs, Lake Forest, CA) running a Linux own roots, representing 63 mother trees from (Grauke et al., 1995), and over 200 of those (Red Hat Enterprise, Raleigh, NC) operating 49 populations. The planting consists of self- inventories still exist in provenance planta- system and CLC Genomics Workstation soft- rooted seedling trees in a low-maintenance tions in College Station and Brownwood. ware (CLC Bio, Waltham, MA) was obtained planting of 2.9 ha (of 9.75 ha available). Microsatellite profiles at 14 nuclear loci to house and work with that data in 2013. College Station: The College Station and 3 plastid loci have been developed for Preliminary efforts at RAD-Seq by USDA worksite was obtained by leasing 145.7 ha about 180 inventory specific pecan cultivars, ARS in conjunction with Dr. P. Klein (Texas of land from Texas A&M University. The 260 pecan seedlings in provenance collec- A&M University) have yielded sequence in- 99-year lease was initiated in 1987 and will tions at College Station, TX, and Byron, GA, formation that has been aligned to the survey expire in 2086. Initially, primary use of the and 180 hickory samples from repository scaffolds, and provided a large numbers of site was as nurseries for the Pecan Breeding collections (not all maintained as living SNPs and microsatellite loci for development, Program and as test orchards in the National accessions). given sufficient resources. Mattison et al. Pecan Advanced Clone Testing System. Col- Several cases of ‘‘identity confusion’’ (2013) measured transcript levels of three lege Station collections include 349 grafted have been resolved using microsatellite nut allergens from ‘Desirable’ and ‘Sumner’ accessions, 91 of which are not present in markers (Grauke et al., 2015). Profiles of over the course of nut development using Brownwood. The majority of Carya species most commercially important U.S. pecan reverse transcription quantitative polymer- collections are maintained at the College cultivars have been developed using micro- ase chain reaction. Those transcripts have Station worksite. The majority of provenance satellite markers. Existing SSR profiles pro- been shared with Dr. Jennifer Randall at orchards are maintained at the College Sta- vide molecular verification of identity within New Mexico State University, who is using tion worksite (over 2000 self-rooted trees our collections. Those profiles have proven the sequence information to annotate the growing on about 12 ha, representing native useful in identifying unknown accessions draft survey sequence (Barnes et al., 2016). pecan populations from Mexico to Missouri, sent for determination, since parentage can Phenotypic evaluation data. Extensive with detailed collections from Arkansas and often be inferred. Currently, no commercial nut quality information (nut length, width, coastal Louisiana). A small seedling popula- laboratories are using the pecan microsatel- height, weight, volume, and percent kernel), tion from a noteworthy putatively native lites, possibly because of the challenges of typically from several seasons, is collected stand in Alabama is also maintained. consistently scoring fragment-based markers. for fruiting accessions of NCGR-Carya col- Associated collections: The Southeastern Standard methods are needed to allow com- lections. Those collections are represented by Fruit and Tree Nut Laboratory in Byron, GA, parison of results obtained from different standardized digital photographs of nuts and maintains significant collections of pecan cul- analysis platforms. kernels. A small number of those images are tivars, self-rooted hickory seedlings (Wood and Using inventories of accessions main- available on the unit website. Extent of Grauke, 2011), and an excellent pecan prove- tained at the Southeastern USDA–ARS Fruit dichogamy is known for most named pecan nance collection (Bock et al., 2016; Grauke and Tree Nut Laboratory in Byron, GA, cultivars and photographs of catkin and et al., 1989; Ruter€ et al., 1999, 2000; Wood patterns of genetic diversity have been shown pistillate flowers are available for many of et al., 1998). The pecan provenance orchard at to vary with geographic distribution over the the College Station accessions. Leaf and nut Byron, GA, was established before the estab- range of pecan, with markers on maternally scab ratings based on the Hunter-Roberts lishment of provenance orchards at the Col- inherited plastids being more closely related scale of worst expression have been accumu- lege Station worksite, from the same seed to geographic distribution than nuclear lated over multiple seasons, but disease is sources, and constitutes an exceptionally valu- markers (Grauke et al., 2011). often not expressed under Texas’ climate able resource for the pecan industry. Pheno- A Carya diversity panel with 50 entries conditions. typic evaluations of leaf area and nutrient was analyzed for single nucleotide polymor- Analysis of broad pecan provenance col- content have been cooperatively reported phism (SNPs) on the Juglans SNP chip (You lections has shown that leaf morphology (Grauke et al., 2003). Selected accessions were et al., 2012) by colleagues working with varies in geographic patterns that might have evaluated for nut quality and genotyped using walnut at the NCGR-Davis and University adaptive significance (Grauke et al., 2003; microsatellites (Grauke et al., 2011). of California, Davis. With the help of Dr. Sagaram et al., 2007, 2011). Current research David Kuhn (USDA–ARS Miami), the re- in provenance orchards (J. Bernal, unpub- Associated information sults of the Carya panel have been interpreted lished data) confirms that leaf morphology, Genebank and/or crop-specific web site(s). and used to generate an informative phylo- including leaf pubescence, varies between Accessions are listed on the Germplasm genetic tree. The most informative SNPs provenances and may impact insect popula- Resources Information Network and the were identified and primers developed for tions on trees grown in a common garden.
660 HORTSCIENCE VOL. 51(6) JUNE 2016 Open-pollinated seed stocks from reposi- 2015). Areas needing future attention include standardized segregating population repli- tory collections were used in screening ef- isolated, relatively uncharacterized popula- cated across regions. The standardized seg- forts to find resistance to nematodes. Sources tions of indigenous Carya species in Mexico regating population would require regional of nematode resistance were not found. Seed- and Asia. The motivation to accomplish those and multi-institutional cooperation, would lings showed patterns of growth phenology inventories is increased by the distribution of provide means to analyze environmental and vigor that varied between provenances of exotic germplasm into those areas, and the effects on genetic expression, and would origin. Seedlings from southern sources be- increased recognition of the frequency of provide a platform for development of im- gan growth first, continued growth longest, interspecific hybridizations that can impact proved phenotyping methods. and achieved greatest diameters and heights indigenous populations. The rapid destruc- Existing ex situ collections of the NCGR- (Grauke and Starr, 2014). tion of local native pecan populations argues Carya should be cooperatively evaluated in favor of continuing broad characterizations using available genomic methods. Coopera- Prospects and Future Developments of the distribution of genetic and geographic tive strategic planning of appropriate con- diversity to better understand adaptation and trolled crosses for use as shared mapping The U.S. pecan industry is regionally facilitate development of regional cultivars and populations, followed by aggressive phe- fragmented, with management and produc- rootstocks. notypic screening and genomic evalua- tion constraints based on climatic and National Plant Germplasm System col- tion, should produce demonstrably refined edaphic variation across diverse growing re- lections are mandated to distribute germ- methods of progeny selection while further gions, coupled with regional competition for plasm to both domestic and international developing the ‘‘road map’’ of progressively limited markets. It is also characterized by cooperators. The recognition of a pathogen improving genomic information. The fruit of division between pecan producers and the such as X. fastidiosa, which could be distrib- those efforts will be accessible to future nut shelling industry that has traditionally uted inadvertently along with germplasm, students of the genus only if the information controlled marketing and is increasingly requires the development and refinement of linking verified living inventories of acces- important in the protection of food safety. phytosanitary certification procedures rele- sions, the passport information describing International interest in the crop is increas- vant to both domestic and international their origination, and phenotypic character- ing, as evidenced both by increased exports cooperators. izations already accumulated can be associ- of the U.S. crop and increased pecan pro- Cooperative germplasm evaluation and ated with genomic characterizations. Pecan duction in other countries. In both the United exchange will benefit by the rapid develop- cultivar development is necessarily multidis- States and other countries, commercial pro- ment of improved analytical methods and ciplinary, increasingly international in scope, duction relies on a small subset of selected phytosanitary protection. Increasing access and will be facilitated by a central database genotypes. The USDA ARS Pecan Breeding to the broad diversity of the genus should serving the accumulated information. Al- and Genetics Program is responsible for open new opportunities for crop improve- though much of that information can be breeding and development of the pecan, as ment through the application of genomic appropriately maintained within the GRIN- well as for the collection, characterization, methods applied to appropriate populations Global database, genomic and bioinformatic evaluation, maintenance, and international that have been well characterized phenotyp- utilities will necessitate cooperative develop- distribution of Carya germplasm. The U.S. ically (Ouborg et al., 2010). The motivation ment with other scientists and industry users pecan producing industry is uniting toward for maintaining diversity within the NPGS that is beyond the scope of existing program the goal of improving domestic and interna- has always been clear: human need (utility) is budgets. We have access to a collection that tional marketing, but is apprehensive about the primary criterion for genetic conserva- represents a global forest. As stewards of that the distribution of improved pecan cultivars tion. However, since the primary users resource, we must now use it. to competitor nations. The U.S. pecan re- of genetic collections are crop breeders search community has been focused on local (Namkoong, 1991; Palmer, 1989), it is neces- Literature Cited crop production issues, and grant application sary to establish effective cooperation across Ares, A., W. Reid, and D. Brauer. 2006. Production attempts seeking broad cooperation on long- regional breeding programs. The 29 pecan and economics of native pecan silvopastures in term strategic research have been unsuccess- cultivars that have been released over the central United States. Agrofor. Syst. 66:205–215. ful (Stevenson, 2012). 85-year history of the USDA ARS Pecan Barnes, S.D., H. Zhong, L.J. Grauke, C.P. Mattison, The driving force for increasing knowl- Breeding Program have all been distributed R.A. Stamler, J. Schmutz, J. Jenkins, R. edge about the genus Carya has been the without charge to the U.S. pecan nursery Heerma, M.Z. Song, and J.A. Randall. 2016. economic value of the pecan nut production industry for propagation. They represent di- Computational annotation of the pecan ge- nome. Plant and Animal Genome Conference industry, which motivated the development verse parentage that inadvertently incorpo- XXIV. Poster 1135.
HORTSCIENCE VOL. 51(6) JUNE 2016 661 laboratory. USDA–ARS. 23 June 2015.
662 HORTSCIENCE VOL. 51(6) JUNE 2016 Vitanza, S. 2015. Issues in agriculture. Newsletter Wood, B.W. and L.J. Grauke. 2011. The rare-earth Feb. 2015.
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