Substance Info: (and synonyms)
Black Walnut tree

Background Info:

Family: Juglandaceae, contains 2 important genera:
Hickory/Pecan (Carya) and Walnut (Juglans)
See also: Walnut, for information on the nut.

Synonyms: Carya, Jupiter's Nuts, Carya persica (Greek), Carya basilike (Greek).

Californian walnut - Juglans californica
English walnut - Juglans regia.
Black walnut - Juglans nigra.
White walnut or Butternut - Juglans cinerea

Walnut is the common name given to twenty species of deciduous trees in the genus Juglans, of which six species are native to the United States. Walnut is native to California, but about 15 related species occur in North and South America as well as in central and southern parts of Europe and Asia. The largest walnut plantations are in California. J. californica is planted as a shade tree and is often used for rootstock for other walnuts. Its edible nuts are a delicacy, and oils are used by painters and in soap making. The hard wood is important in cabinet making.

The Black walnut is native to the eastern United States, native to Virginia, growing from Maine west to southern Michigan and south to Texas and Georgia. It is important for its timber, used in fine furniture, rather than for its nut, the flesh of which is tasty but surrounded by a hard, thick shell (outer husk) that makes the nut difficult to utilize.

The common or English walnut, which is native to areas stretching from the Balkans to China, extending from Greece and Asia Minor, over Lebanon and Persia, all along to the Himalayas, but now widely grown in many other temperate areas, is one of the most important nut crops grown.

The tree grows to a height of 40 or 60 feet, with a large spreading, rounded top, and thick, massive stem. It occurs in woods and on mountain slopes. Some Walnut trees are 300 years old. Black walnut is the tallest of the walnuts, with the potential to reach 100 feet. The Walnut tree has compound leaves which are spaced alternately along the branches, which consists of small yellowish green leaflets. Walnuts are monoecious, with male flowers borne in long, unbranched, drooping catkins and female flowers borne singly or in short spikes. The walnut fruit is a nut, borne singly or in pairs, and enclosed in a solid, non-splitting green husk. The edible, oil-rich nut kernal is enclosed in a thick, hard, ridged, black shell. Black walnut heartwood is heavy, hard, strong, and durable, with a chocolate-brown color prized by furniture manufacturers and many other industries. The Black walnut is much oilier and richer tasting than that of the English walnut found in grocery stores.

The flowers of separate sexes are borne upon the same tree and appear in early spring before the leaves. The Walnut tree flowers and produces pollen, after 20 to 30 years of growth, in late spring to early summer, extending from April to May in the Northern Hemisphere. The pollen of all these trees is large and does not travel far. However, in areas where the trees are cultivated commercially, heavy exposure to the pollen can cause allergy symptoms. Walnut pollen is generally considered to be moderately allergenic. The western species of walnut (in California) is thought to be a more important cause of allergic sensitization than the Black walnut. The walnut pollens are often the cause of inhalant allergies, and the nuts may cause food allergy.

Some plants planted near or under the Black walnut tree tend to yellow, wilt, and die. This occurs because the walnut tree produces a non-toxic, colorless, chemical called hydrojuglone. Hydrojuglone is found in leaves, stems, fruit hulls, inner bark and roots. When exposed to air or soil compounds, hydrojuglone is oxidized into the juglone, which is highly toxic. The secretion of biochemical materials into the environment to inhibit germination or growth of surrounding vegetation is called allelopathy, which enhances the plants survival and reproduction. Several related trees such as English walnut, hickories and pecan also produce juglone, but in smaller amounts compared to black walnut.


Adverse Reactions:


[ 1 / 2 ]

Asthma has not been ascribed to the pollen, but has been to walnut wood dust, which is also a cause of contact dermatitis. (Bush 1983 ref.2421 8) (Estlander 2001 ref.4409 7) (In: Weber 2012 ref.28248 7)

Weber RW. Allergen of the month-english walnut. Ann Allergy Asthma Immunol 2012 Sep;109(3):A13

[ 2 / 2 ]

There is a high cross-reactivity suggested between members of the genus Juglans. (Yman 1982 ref.1241 0)

Yman L. Botanical relations and immunological cross-reactions in pollen allergy. 2nd ed. Pharmacia Diagnostics AB. Uppsala. Sweden 1982: ISBN 91-970475-09

Occupational reactions

[ 1 ]

Occupational allergic contact dermatitis. Exposure to wood dusts may cause various skin and mucosal symptoms. Allergic dermatoses, caused by wood dusts, diagnosed at the Finnish Institute of Occupational Health during 1976-1999 are reported here. 16 had allergic contact dermatitis and, 2 had contact urticaria. 9 men (3 cabinet makers, 3 joiners, 1 carpenter, 1 knifemaker and 1 machinist) were mainly exposed to tropical hardwoods. 1 man had dermatitis caused by western red cedar. 5 patients, 3 men and 2 women, were exposed to Finnish pine or spruce dusts, and 1 man to aspen. 7 also had rhinitis, 4 asthma or dyspnoea and 3 conjunctivitis. On patch testing, 10 men reacted to 9 different wood dusts, including teak (5), palisander (3), jacaranda (2), mahogany (2), walnut (2) and obeche (1). Reactions to wood allergens, including lapachol (2), deoxylapachol (1), (R)-3,4-dimethoxydahlbergione (2), 2,6-dimethoxy-1,4-benzoquinone (1), mansonone A (2) and salicyl alcohol (1), were noted in 4 cases. All but 1 of 5 patients exposed to pine or spruce dusts reacted to the sawdusts, all 5 to colophonium, 3 to abietic acid, 2 to tall oil resin, 3 to wood tar mix and 4 to other wood gum resins. Of the 2 CU patients, 1 was prick and RAST positive to obeche, 1 reacted with urticarial dermatitis to punah wood dust on chamber exposure. Occupational allergic dermatoses are mainly caused by the dusts of hardwoods, mostly due to Type IV allergy, but may also be caused by softwood dusts. Patch tests can be done with wood dusts, but should be confirmed by patch testing with wood allergens if possible. (Estlander 2001 ref.4409 8)

Estlander T, Jolanki R, Alanko K, Kanerva L. Occupational allergic contact dermatitis caused by wood dusts. Contact Dermatitis 2001;44(4):213-217

Information supplied from an abridged section of:
Allergy Advisor - Zing Solutions

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