The Persian or English walnut (Juglans regia) is widely cultivated, with commercial production in France, Italy, Turkey, China, and the United States. Practically all of the U.S. production of Persian walnuts is in the central valley of California, which now has about 169,000 bearing acres with an average yield of around one and one-third short tons per acre. Many orchards produce over two tons, and three tons per acre are common in many modern plantings. Walnuts have two major outlets: the exported in-shell market (about 35% of production) and the domestic shelled market (about 68% of production). A cooperative handles about half the crop, while several independent handlers sell the remainder. Walnuts are sensitive to both low and high temperatures. Temperatures in excess of 90 °F will begin to sunburn nuts. Freezing temperatures will damage tender growth in the spring and fall. Dormant trees can tolerate 15 °F without injury if soils are moist. Dry winter soils and cold temperatures cause winter kill. A minimum of 800 hours of winter chilling are required to avoid delayed bud break and poor crops. Walnuts do best on deep, medium textured, well drained soil. Under these conditions, both rootstocks, the Northern California Black Walnut (J. hindsii) and Paradox (J. regia x J. hindsii), do well. Under less favorable soil conditions, Paradox is the preferred rootstock. A mature walnut orchard requires 4 to 4.5 acre-feet of water per acre per year if the trees are to produce the maximum number of high quality nuts possible. Hartley, preferred for its in-shell quality, is the leading cultivar, with about 30% of the acreage. In recent years, the Chandler variety has accounted for most new plantings. It is known for high kernel quality and yields. Yield factors include: bearing habit, bearing area, flower differentiation, fruit set, nut size, kernel percentage, and kernel quality. Major insect pests of walnut include codling moth, navel orangeworm, and walnut husk fly. The major diseases are walnut blight, deep bark canker, Phytophthora, and blackline. Major research efforts include the walnut breeding program, which includes blackline and Phytophthora susceptibility of new cultivars and root-stocks, codling moth and walnut husk fly control, epidemiology and control of walnut blight, pruning and planting strategies, and clonal propagation.
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