dangers of black walnut tree

Asked August 25, 2014, 4:04 PM EDT

tomatoes were covered with sticky sap from the leaves of a black walnut tree. Heavy rain washed the sticky stuff off, are the tomatoes going to be safe to eat when ripe. Also is this sap going to contaminate the soil for growing next year,they are in containers. Is there anywhere I can take the tomatoes to see if they are contaminated with this juglone (the sap from the tree), if you can not tell me if they are safe?

Minnehaha County South Dakota trees and shrubs horticulture

3 Responses

Juglone occurs naturally in all parts of the black walnut, including the walnuts themselves. Juglone toxicity refers to the compound killing plants, not humans. Your tomatoes should be safe to eat.

If your plants have not died, the soil in the containers should be fine. If you don't want to risk it, simply remove the soil and start over.

Here is an article about Black Walnut Toxicity:


I have read that the juglone from the black walnut can be fatal to dogs and horses. To horses even from the bark if used for bedding them and such. I guess I was concerned because dogs are suppose to have a much higher immune system then humans, and if it can be toxic to them, why wouldn't it be toxic to humans?

The sap on your tomatoes was most likely honeydew from aphids that are living on the black walnut tree leaves. The honeydew is harmless (just excreted excess sugar from the aphids) and does not contain juglone. I have two black walnut trees and both support large numbers of aphids (along with things that eat aphids - ladybugs, lacewings, etc.).