"Guyz-N-Flyz" wrote:

I think my uncle received a load of black walnut leaf mulch; consequently,
his asparagus, Brussels sprouts, cucumbers, tomatoes and other veggies are
dying.
Is there anything that will neutralize the affects of juglone toxicity,
once he removes the offending leaf mulch?

In my experience, black walnut leaves are quite ephemeral. That is,
they don't even last long enough to be raked, much less piled up
and/or transported for mulch. Your uncle's problem may be something
entirely different.