White hickory

Further Reading

Pecan t22

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Code: t41
Latin name: Carya tomentosa (syn. C. alba)
Source material: Pollen
Family: Juglandaceae
Common names: White Hickory, Mockernut hickory, Mockernut, Whiteheart hickory, Hognut, and Bullnut

Synonym: C. alba

Allergen Exposure

Geographical distribution
White hickory, closely related to the Pecan (Hickory) tree (Carya pecan), is a member of the Juglandaceae (Walnut) family and is the most abundant of the Hickories. The Carya or Hickory genus comprises 17 species, of which 15 are found in the United States, 1 in Mexico and 1 in southern China. In the United States, Hickories are found principally in the eastern states and not in the Pacific states (1).
The Hickories are similar to the Walnut in that they are tall spreading trees with smooth gray bark and large deciduous pinnate leaves; but they generally have fewer leaflets than the Walnut does. The bark becomes rough or scaly on mature trees.

White hickory is native to the USA. It is long-lived, sometimes reaching the age of 500 years (2). It is monoecious; male and female flowers are produced on the same tree, appearing after the unfolding of the leaves. The staminate flowers are borne in long pendent catkins that, unlike those of the Walnut, are branched. The catkins are about 10 to 13 cm long and may be produced on branches from axils of leaves of the previous season or from the inner scales of the terminal buds at the base of the current growth. The female flowers appear in short spikes on peduncles terminating in shoots of the current year. Flowers bloom from April to May, depending on latitude and weather. Usually the male flowers emerge before the female flowers. Hickories produce very large amounts of pollen in the late spring, often in amounts equal to those from Walnuts, and the pollen is dispersed by the wind.

Fruits, solitary or paired, are globose, about 2.5 to 9.0 cm long, and with a short neck-like base; they ripen in late summer. The fruit has a thick, 4-ribbed husk 3 to 4 mm thick that usually splits from the middle to the base. The nut is distinctly 4-angled, with a reddish-brown, very hard shell 5 to 6 mm thick, containing a small edible kernel. The seed is dispersed in late summer to autumn.

The wood is used for fuel and for products where strength, hardness, and flexibility are needed.

No allergens have been characterised.

Potential cross-reactivity

Cross-reactivity could be expected between species of the genus Juglans and of the genus Carya, e.g., White hickory and Pecan tree.

Clinical Experience

IgE-mediated reactions
Anecdotal evidence suggests that asthma and hayfever are possible following exposure to pollen from this tree; however, no specific studies on this have been reported to date. The most important Hickory causing hayfever and asthma is Pecan (C. pecan) (2). Hayfever from Pecan is often described, especially in areas where it is cultivated (3). Because of the close family and genus relationship between White hickory and Pecan, results of studies on the latter may be applicable to the former. See Pecan tree t22.

The pollen of the other Hickories is also known to cause hayfever where the trees are abundant (2,4). Other species of Carya may be involved, e.g., the Shagbark (or Shellbark) hickory (C. ovata), Nutmeg hickory (C. myristicaeformis Nutt) and Pignut hickory (C. glabra) (2-3). In hayfever studies, the various species are generally not distinguished from each other because their pollens are believed to inter-react more or less perfectly (2).

In a study of 371 allergy patients tested serologically for hypersensitivity to prevalent tree pollens in the area surrounding New York over the years 1993-2000, the highest prevalence of hypersensitivity was for Oak (34.3%), Birch (32.9%), and Maple (32.8%) tree pollens. Lower prevalences were observed for Beech (29.6%), Hickory (27.1%), Ash (26%), Elm (24.6%), and Poplar (20.6%) trees (5).

In an earlier study in St. Louis, Missouri, USA, a population was skin-tested with pollen from 12 wind-pollinated tree species. Pollen extracts of Box elder, Willow and Hickory elicited the strongest allergic reactions; Oak, Birch, Sycamore, Black walnut and Poplar more-moderate reactions; while allergens from Cottonwood, Maple, Elm and White ash were less reactive (6).

Sensitisation to the close genus member C. pecan has been demonstrated in Israel (7) and Mexico (8-9).

Compiled by Dr Harris Steinman, harris@zingsolutions.com.


  1. Wodehouse RP. Hayfever Plants. 2nd revised edition. Hafner Publishing Co., NY, USA. 1971
  2. Wikipedia contributors, ”Mockernut Hickory”, Wikipedia, The Free Encyclopedia, http://en.wikipedia.org/w/index.php?title=Mockernut_Hickory&oldid=210074320 (accessed July 3, 2008)
  3. Weber RW. Pecan. Ann Allergy Asthma Immunol 2004;93(3):A-6
  4. Brown GT. Seasonal hayfever with special reference to the Middle Altantic states.
    J Med Soc N J. 1932;29:483-90
  5. Lin RY, Clauss AE, Bennett ES. Hypersensitivity to common tree pollens in New York City patients.
    Allergy Asthma Proc 2002;23(4):253-8
  6. Lewis WH, Imber WE Allergy epidemiology in the St. Louis, Missouri, area. III. Trees.
    Ann Allergy 1975;35(2):113-9
  7. 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
  8. 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
  9. 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

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