The United States produces approximately 380,000 metric tons of Persian (English) walnuts annually, valued at over $550 million. US production has increased steadily over the last several decades and is located almost entirely in California. Over 40% of the US crop is exported. China is the other major commercial producer and exporter with a crop size of about 450,000 metric tons annually and increasing production.
Nut production of the native eastern black walnut is principally from natural stands in the eastern US and averages 17 metric tons annually. This tree is also highly prized for its timber and annual harvest exceeds 12 million cubic feet. The total standing volume is estimated to exceed 3.4 billion cubic feet with a value in excess of $500 billion. Annual exports of walnut wood products are estimated at $325 million.
The major problems facing the walnut industry are crown gall, nematodes, Phytophthora spp., walnut blight, cherry leafroll virus, codling moth, and potentially insufficient chilling under anticipated climate change. Over 50% of the walnut industry in California is based on two cultivars, Chandler and Hartley. Dependence on two clonally propagated cultivars results in a high degree of genetic vulnerability and there is a relatively narrow germplasm base in reserve to combat these problems.
Thousand cankers disease, a fungus (Geosmithia) vectored by the walnut twig beetle, presents a new and potentially serious threat to the California nut industry, black walnut forests of the eastern US, and current germplasm collections. The few remaining stands of native butternut are severely threatened by both butternut canker and hybridization with introduced heart nut. Most other Juglans species are forest trees valued for their wood and nuts, often with limited natural ranges, native to regions experiencing population pressure, and threatened by logging and grazing activities.
Germplasm is maintained by the NCGR at Davis CA, the University of Missouri (MU), and the USFS Hardwood Tree Improvement and Regeneration Center (HTIRC) at Purdue. Breeding programs are primarily in the public sector. The University of California-Davis breeding program has emphasized improvements in Persian walnut yield and disease resistance. Use of diverse species for rootstock improvement is an increasing component. Black walnut improvement programs in the eastern United States are directed toward timber (HTIRC) and nut (MU) production.
International collecting activities have emphasized broadening the narrow germplasm base found in existing cultivars and identifying sources of disease resistance. Most Latin American walnut species have been sparsely collected and poorly characterized. The rapid decline of butternut warrants accelerated efforts to identify and collect disease resistant genotypes.
The genus Juglans includes about 21 species of trees and large shrubs whose natural distributions range, in the Old World, from southeastern Europe to eastern Asia and Japan, and, in the New World, from the eastern half of the US, California, and the southwestern states south through Mexico and Central America to South America and the West Indies. The most economically important species is J. regia, the English or Persian walnut, cultivated for its edible nuts; second in economic importance is J. nigra, the eastern black walnut, grown primarily for its timber. Several other species and hybrids, notably J. hindsii (northern California black walnut) and Paradox (hybrids of J. hindsii and J. regia, sometimes with contributions from other species), have considerable commercial importance as rootstocks for cultivars of J. regia.
In 2007, annual world Persian (English) walnut production totaled approximately 1,600,000 metric tons (all figures in-shell basis). The United States produces approximately 380,000 metric tons, annually with a value of about 550 million dollars. Approximately two thirds of the US crop is sold as shelled nut meats. About 50% of the US crop is typically exported and in 2002 the US supplied 55% of total walnuts exported worldwide. China is the other major contributor to the world walnut crop and produces over 600,000 metric tons annually and supplied18% of the world’s exports in 2002.
Most (99%) of the Persian walnuts produced in the US are grown in California, which in 2009 had 223,000 bearing acres of the crop. Although there is an interest in growing walnuts in other parts of the US, acceptable cultivars adapted to the different growing environments are not available.
J. nigra (eastern black walnut) yields in excess of 11,000 tons of in-shell nuts annually but demand for black walnut kernels continues to exceed supply. Most of these nuts are collected from wild trees in Missouri, Illinois, Indiana and Iowa. Eastern black walnut is also one of the most highly valued hardwood species. The USDA Forest Service Forest Inventory Analysis (FIA) indicates that more than 15.4 million acres of timberland in 30 states contain black walnut. The vast majority of this resource is in natural stands, with a small percentage grown in plantations. In the North Central Region an estimated 7 million cubic feet of black walnut growing stock and 5.3 million cubic feet of black walnut non-growing stock are harvested annually. Because of its high commercial value and the long period of time required to produce saw-timber grade trees, the demand for this species has exceeded supply for several decades.
The primary commercial importance of the Northern California black walnut (J. hindsii) is as a rootstock for commercial Persian walnut (J. regia) orchards or as parent of the widely used hybrid rootstock ‘Paradox’ (J. hindsii x J. regia). This species is also a producer of high quality burl wood.