Perspective on the black walnut toxicity problem–apparent allergies to man and horse.

The article discusses the historical understanding and ongoing research concerning the toxicity of the black walnut tree, specifically the compound juglone that is present in the tree and is believed to harm or stimulate surrounding vegetation.

Historical Context of Black Walnut Toxicity

• The toxic properties of walnut trees, specifically English or Persian walnut, were noted as early as 37 A.D. by Plinius Secundas.
• A chemical substance known as juglone, identified in various family members of walnut tree like black walnut, butternut, and the hickories including pecan, was believed to cause damage to surrounding plants.

Modern Research and Controversy

• Around 1940, researcher Massey demonstrated that tomato plants can be adversely affected by the presence of black walnut roots in the surrounding soil.
• Researcher Brooks carried out a more extensive study over 12 years investigating the impact of black walnuts on about 300 mature trees across six states. Despite the primarily negative effects, some plant species appeared to be stimulated by the black walnut.
• This finding sparked a controversy within the Northern Nut Growers Association, with some claiming that the toxicity of black walnut was a myth, and others arguing against this claim.
• The USDA also weighed in on this matter, stating that tomato plants grown near walnut trees showed ordinary growth and fruiting patterns, hence suggesting the lack of juglone toxicity.

Conflicting Reports and Further Investigation

• Diverse studies published in the NNGA Proceedings have not managed to provide a definitive conclusion to this debate.
• In 1940, a paper described extensive damages to tomato and alfalfa crops in the field due to proximity with black walnut trees, though the same damage could not be replicated in greenhouse conditions.
• Furthermore, observations in California walnut orchards did not record damage to other plants beyond what could be accounted for by competition for resources such as water.