and in some areas has become naturalized. Cultivated J. regia was introduced into western and northern Europe very early, in Roman times or earlier, and to the Americas in the 17th century, by English colonists. Important nut-growing regions include California in the United States; France, Serbia, Greece, Bulgaria, Romania and Hungary in Europe; China in Asia; Baja California and Coahuila in Mexico; and Chile in Latin America. Lately, cultivation has spread to other regions, such as New Zealand and the southeast of Australia. It is cultivated extensively from 30° to 50° of latitude in the Northern Hemisphere and from 30° to 40° in the Southern Hemisphere. Its high-quality fruits are eaten both fresh or pressed for their richly flavored oil; numerous cultivars have been selected for larger nuts with thinner shells. Ecology It tends to grow taller and narrower in dense forest competition. It is a light-demanding species, requiring full sun to grow well. Juglans regia is infested by Rhagoletis juglandis, commonly known as the walnut husk fly, which lays its eggs in the husks of walnut fruit. Other plants often will not grow under walnut trees because the fallen leaves and husks contain juglone, a chemical which acts as a natural herbicide. Horses that eat walnut leaves may develop laminitis, a hoof ailment. Cultivation Walnut trees grow best in rich, deep soil with full sun and long summers, such as the California central valley. Juglans hindsii and J. hindsii × J. regia are often used as grafting stock for J. regia. Mature trees may reach in height and width, and live more than 200 years, developing massive trunks more than thick. Cultivars Walnut cultivars include: Pests Particular cultivars of J. regia may be more infested by R. juglandis than others because of varying walnut husk softness or thickness. 'Eureka', 'Klondike', 'Payne', 'Franquette' and 'Ehrhardt' cultivars are among the most susceptible to infestation. Production In 2019, world production of shelled walnuts was 4.5 million tonnes, led by China with 56% of the total harvested (table). Secondary producers were the United States and Iran. Toxicity Allergy Walnuts and other tree nuts are food allergen sources having potential to cause life-threatening, IgE-mediated allergic reactions in some individuals. Uses Nutrition English walnut kernels are 4% water, 65% fat, 15% protein, and 14% carbohydrates (table). In a 100 gram reference amount providing 654 calories, the kernels supply several dietary minerals in rich content (20%