What is the white powdery substance on the black walnut husk? Is it poisonous?

by Dee Ann
(Lawrence,KS, USA)

I pick up my walnuts, but haven't had time to husk them.
Many of them develop a white powdery substance on the husk.
What is that and is it harmful - or can I go ahead husk and shell the nuts and use them raw and cooked (baked)?

Thank you! I can't find this information anywhere on the web.

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Sep 27, 2021
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by: Dan

More than likely, this is being caused by the fungus Phyllactinia guttata. Typically, this fungus causes a white powdery substance on the hull and leaves of the tree. It can spread to cover the entire leaf.

I would inspect the tree to see if the leaves are also affected. Doing so would help you verify that this is more than likely what is going on.

Most of the time, this doesn't get bad enough to warrant control.

I would wash the walnuts before breaking them open to harvest the fruit, which should be unaffected.

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