Hello,

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The picture appears to be anthracnose, a fungal disease common during wet springs. 

Clean ing up fallen leaves, twigs and fruit, and keeping the tree watered when weather is hot and dry, will help the tree. 

To prevent anthracnose, the tree must be sprayed with a fungicide in early spring as leaves are unfolding, and every 2-3 weeks while spring weather is wet. Once leaves are fully expanded and mature, they are much less prone to anthracnose and no spraying is necessary

Here are some articles for reference-

https://www2.ipm.ucanr.edu/agriculture/walnut/anthracnose/

http://hyg.ipm.illinois.edu/pastpest/200718b.html

You may wish to consult a certified arborist, one who has taken training and passed certification testing. You can find them here by zip code-

www.treesaregood.org

Or, you may submit branch and leaf samples, with pictures of the whole tree, and pictures of leaves to MSU Plant Diagnostic lab. Instructions for sample taking and fee schedule are here-

https://pestid.msu.edu

They are majestic trees! Thank you.

One other issue is causing walnut leaves to fall early right now in lower Michigan. Butternut curculio insects cause leaves to fall- see pictures of where to look for damage here, and please compare to yours.

https://www.canr.msu.edu/news/walnut_leaves_falling_in_quantity