I'm not sure what to say when you say that the same soil was used for sowing seeds and the germination was OK and I assume the seedlings were as well.

And I say that b'c juglone, the toxic substance we're talking about, is present out to the drip line of the tree canopy and also is secreted by any underground roots and rootlets that go out beyond that drip line.

And juglone stays in the soil. It's also present in the leaves that drop to the ground albeit with less toxicity.

Is there no where else you can plants your plants? It's one thing to get some seeds to germinate but it's quite another thing to have deep roots from mature plants.

Carolyn

Three years ago I made a garden near a black walnt tree,right under its canopy. among other things I planted just a few tomatoes. They were doing fine and were about to flower. Then the first one died... then the second....They just wilted and died. Somebody said that probably I over fertilized them. But I did not think so. Then I heard about JUGLONE and black walnt. I went and bought couple of other tomato plants and planted Them On a mound and brought the soil from another plcae. Guess what eventually they died too.
I have had peppers, shiso, basils, 4-Oclack flowers right there but they survived. The following year I move to a different location and never had a single tomato plant to wilt and die. Tomatoes must be sensative to juglone than most plants.I have also noticed that pine, box elder, tulip tree and maple cannot make it near black walnut.
But there are a host of other things that are doing fine.
I would suggest that if you have no choice, plant your tomatoes in container(like 5-gal buckets) and fill them with soil from another location. Even then your tomatoes might have problem, b'c struff dripping from branches and leaves also contain some juglone.

I hate to say this, but yes I think you are doomed if you plant tomatoes in that spot. If there is no place else to plant them can you find a place for containers? I've grown some mighty fine maters in containers.

Did I understand you right-you used dirt from that area to start seeds? Dirt should not be used for seed starting, walnut tree or not. A sterile mix should always be used.

I hate to be negative in my response because I'm an ex jayhawker myself before moving to Louisiana.

Best of luck to you.