13. Schneiderhan, F. J. The black walnut (Juglans nigra L.) as a cause of death to apple trees. Phytopathology 17: 529-540. 1927.
Possible Black Walnut Toxicity on Tomato and Cabbage
By Otto A. Reinking
New York State Agricultural Experiment Station
The toxicity or antagonism of black walnut roots and those of certain other plants has been a controversial question. L. H. MacDaniels and W. C. Muenscher in a report on page 172 of the Thirty-first Annual Meeting of the Nut Growers' Association held in 1940 cited evidence pro and con relative to the toxic effect of black walnut on various crops. They concluded that because of conflicting evidence, the problem of walnut toxicity was still unsolved and needed further investigation. In 1942, Babette I. Brown reported on page 97 of the Thirty-third Annual Report of the Northern Nut Growers' Association, on the injurious influence of bark of black walnut roots on seedlings of tomato and alfalfa. It was concluded, from carefully conducted tests, that walnut roots produce a substance that may be injurious to certain other plants. Experimentation showed that the walnut root bark produces a substance that is injurious to alfalfa and tomato seedlings.
During the past years, a number of instances of stunting and wilting of tomato plants in the vicinity of black walnut trees has been observed. In 1942, a very definite case of wilting and stunting was noted in cabbage plants growing in the vicinity of a black walnut tree.
Severely wilted tomato plants were observed on July 30, 1943, in a field of tomatoes near Egypt, New York. This case was typical of others observed in tomato fields in recent years. The wilting and stunting were all located in one corner of the field, on both sides of which large black walnut trees were growing, and extended out in the field for a distance somewhat greater than the height of the trees. The rest of the field planted with the same stock of tomatoes was entirely healthy. The field had been planted to beans in 1942 and prior to that had been in grass for at least 7 years. The vascular bundles of affected plants were browned as in Verticillium or Fusarium wilt and in some bacterial diseases. No cankers or discolorations were observed on the external parts of the plants. In order to determine whether or not the wilting was caused by a fungus or bacterium, plants were collected for microscopic examination and for culturing to show possible presence of pathogens. The microscopic examinations showed the absence of fungi or bacteria in the vascular system or other plant tissues. The browning in the vascular bundles appeared to be confined to the phloem tissue. All attempts to culture a pathogenic fungus or bacterium from affected tissue was negative. Portions of diseased plants with discolored vascular bundles were placed in a damp chamber and no fungus or bacterial growth developed from the vascular system. From these field and laboratory studies, it was concluded that the wilting and stunting were not produced by a plant pathogen. Since the affected plants in the field were all confined to the area adjacent to black walnut trees, and the fact that it had been shown that the bark of this tree does produce a substance that is toxic to certain plants, it was concluded by circumstantial evidence alone that the wilting possibly was due to black walnut toxicity or antagonism of some sort.
In August of 1942, studies were made on wilted and stunted cabbage plants growing in a semicircle on one side of a field adjacent to a walnut tree (Fig. 1). The field was located near Hall, New York, in a region known to be infested with cabbage yellows. From a distance, the affected plants appeared to have yellows, but upon close study, it was found that they were merely wilted and stunted and did not show the other typical symptoms of the yellows disease. The root systems of wilted plants did not show the presence of club root or black rot infection. The plants in the field were all of one variety and came from the same seed bed. Microscopic studies and attempts to culture a fungus from the vascular bundles of affected plants showed the absence of any fungus that might have caused, the disease. Since the affected plants showed no symptoms of known cabbage diseases and as they were growing in a semicircle adjacent to a walnut tree, it was concluded that the presence of the root system of this tree might have been the cause of the trouble.