Pecan, Hickory

Further Reading

Pecan nut f201

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Code: t22
Latin name: Carya pecan
Source material: Pollen
Family: Juglandaceae
Common names: Pecan tree, Hickory tree

Synonym: C. pecan

Allergen Exposure

Geographical distribution
The Juglandaceae family contains 2 important genera: Hickory/Pecan (Carya) and Walnut (Juglans).

The Pecan is a deciduous tree, usually 23 to 34 m tall but sometimes growing to over 50 m. The tree has a rather narrow crown and usually occurs in forests. Some Pecan trees are over 150 years old. They have compound leaves with lance-shaped leaflets. The bark is a pale gray or whitish brown, scaly, and deeply furrowed, and the twigs are hairy. The trees lose their leaves each year and bear sweet, edible nuts, deep brown in colour, that range from 2.5 to 5 cm in length.

The Pecan is native to North America. The range of Pecan covers the warmer temperate zone and subtropical areas. It is very common in the South and Southeast of the USA, but is also planted far beyond this range. In the USA, Texas is the largest producer of Pecan nuts, and is second only to Georgia in the production of hybrid (orchard-grown) varieties.

The Pecan has separate male and female flowers on the same tree (monoecious). Tiny pistillate flowers hang down in tassels. The tree flowers in spring, shedding enormous quantities of pollen. The pollen season extends from April to June in the Northern Hemisphere.  Pollination occurs by wind. Although the pollen is very allergenic, it 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.

The Pecan tree is an important crop tree cultivated for its timber and edible nuts. Pecan orchards in the southern US produce more than 250 million pounds of Pecan nuts in an average year. Four-fifths of the Pecan harvest is sold as shelled nuts.

Unexpected exposure
Although the wood is brittle, it is valuable for flooring, furniture and panelling. Oil from the nuts is an ingredient in processed foods, is used in the manufacture of cosmetics and soap, and is a drying agent in paints.

No allergens from this plant have yet been characterised.

Potential cross-reactivity

Cross-reactivity could be expected between species of the genus Carya and, to a moderate degree, to those of the genus Juglans, e.g., Walnut tree (1).

Clinical Experience

IgE-mediated reactions
Pecan tree pollen is considered to be highly allergenic, resulting in allergic rhinitis, conjunctivitis and asthma (2).

In an Israeli study, 705 allergic patients living in 3 cities and 19 rural settlements were tested for sensitivity to Date palm pollen and Pecan tree pollen. Sensitivity to the pollen extracts of Date palm and Pecan tree was much higher among residents of rural than of urban communities, with a clear relationship between the abundance of these trees in a region and the incidence of positive skin prick tests to their pollen. Sensitivity was frequent in those close to commercial Date or Pecan plantations. In general, sensitivity to Date pollen extracts was lower than to extracts of Pecan pollen. Pollen levels decreased with increasing distance from the trees, and were low at approximately 100 m from a source (3).

In an Israeli study of 395 participants comprising 78.2% of a rural community, 11.6% were skin-prick test positive to Pecan tree pollen, and they constituted 25.4% of the atopic population. Of those who were found atopic to 1 or more allergens, 50.3% had symptoms, whereas the parallel figure for those atopic to Pecan pollen was 76.1%; 58.7% of the Pecan-atopic participants had hayfever, 43.5% had asthma, and 31.5% had both hayfever and asthma. Of the Pecan atopics, 65.2% had clinical symptoms coinciding only with the Pecan pollen season, and an additional 10.9% had perennial symptoms. The measurement of Pecan tree pollen during this period, May, showed that the pollen grains comprised 70% of the total airborne grains (2).

Further studies in Monclova, Mexico, (4) and in a rural kibbutz community (Netzer Sereni) in Israel (5) have confirmed the allergenicity of Pecan tree pollen. Of 247 patients evaluated in Monclova, Mexico, 4.8% were skin test-positive for Pecan pollen (6). Pollen from this tree has also been documented in aerobiological surveys in Caxias do Sul in southern Brazil (7).

In a study in St. Louis, Missouri, USA, skin test results indicated that Box elder, Willow and Pecan elicited the highest number of allergic reactions (8). Three hundred and seventy-one allergy patients were tested serologically for hypersensitivity to prevalent tree pollens in the area surrounding New York. The highest prevalences of hypersensitivity were for Oak (34.3%), Birch (32.9%), Box elder (32.8%), and Pecan tree (27.1%) tree pollens (9). In a study that examined aeroallergen sensitisation rates in children of the military in Texas undergoing skin testing for rhinitis, of 209 patients, 27.8% were skin test-positive for Pecan tree pollen (10).

Other reactions
Some plants rooted near or under Hickory/Pecan trees tend to yellow, wilt, and die. This occurs because these trees produce a non-toxic, colourless chemical called hydrojuglone. Hydrojuglone is found in the leaves, stems, fruit hulls, inner bark and roots. When exposed to air or soil compounds, hydrojuglone is oxidized into juglone, which is highly toxic. However, Hickory, Pecan tree and English walnut produce juglone in small amounts, as compared to Black walnut.

Compiled by Dr Harris Steinman,


  1. Yman L. Botanical relations and immuno-logical cross-reactions in pollen allergy. 2nd ed. Pharmacia Diagnostics AB. Uppsala. Sweden. 1982: ISBN 91-970475-09
  2. Rachmiel M, Verleger H, Waisel Y, Keynan N, Kivity S, Katz Y. The importance of the Pecan tree pollen in allergic manifestations.
    Clin Exp Allergy 1996;26(3):323-9
  3. Waisel Y, Keynan N, Gil T, Tayar D, Bezerano A,
    Goldberg A, Geller-Bernstein C, Dolev Z,
    Tamir R, Levy I, et al. Allergic responses to date palm and Pecan pollen in Israel. [Hebrew] Harefuah 1994;126(6):305-10, 368
  4. Ramos Morin CJ, Canseco Gonzalez C. Hypersensitivity to airborne allergens common in the central region of Coahuila. [Spanish] Rev Alerg Mex 1994;41(3):84-7
  5. Rachmiel M, Waisel Y, Verliger H, Keynan N,
    Katz Y. Correlation between exposure to allergenic pollens and allergic manifestations. [Hebrew] Harefuah 1996;130(8):505-11, 584
  6. Ramos Morin CJ, Canseco Gonzalez C Hypersensitivity to common allergens in the central region of Coahuila [Spanish].
    Rev Alerg Mex 1993;40(6):150-4
  7. Vergamini S, Valencia-Barrera R, de Antoni Zoppas BC, Perez Morales C, Fernandez-Gonzalez D. Pollen from tree and shrub taxa in the atmosphere of Caxias do Sul (Rio Grande do Sul, Brazil).
    Aerobiologia 2006; 22(2):141-148
  8. Lewis WH, Imber WE Allergy epidemiology in the St. Louis, Missouri, area. III. Trees.
    Ann Allergy 1975;35(2):113-9
  9. Lin RY, Clauss AE, Bennett ES. Hypersensitivity to common tree pollens in New York City patients.
    Allergy Asthma Proc 2002;23(4):253-8
  10. Calabria CW, Dice J. Aeroallergen sensitization rates in military children with rhinitis symptoms. Ann Allergy Asthma Immunol 2007;99(2):161-9


As in all diagnostic testing, the diagnosis is made by the physican based on both test results and the patient history.