Thank you Amy for your question to "Ask Extension".

The clear “bleeding” sap you saw after a spring pruning cut is usually not harmful to the tree and generally doesn’t require treatment. Sap drip after pruning can happen, and it won’t damage the tree or its general health.

Often, when sap keeps a cut surface moist, various organisms can colonize the sap flow (algae/yeasts/lichens/molds), which can look orange/gray. In many cases this is not a concern and doesn’t need to be scrubbed off.

What I would do:

• Don’t “clean” it by scraping or gouging the wood/bark. That can enlarge the wound and slow closure.
• Do not apply wound sealer/paint. These products generally aren’t recommended.
• Just let the cut dry and seal naturally.

Walnut pruning wounds can be entry points for wood-rotting fungi and other canker diseases, especially with large cuts or if stubs were left. Keep an eye on the area over the next several weeks/months. Concerning signs can include:

• Bark becoming sunken or cracked around the cut
• Spreading discolored area beyond the cut surface
• Dieback of the twig/branch beyond the cut
• Lots of tiny black “pimples” (fruiting bodies) in the dead bark/wood

If you see those symptoms, the best “fix” is usually pruning back to healthy wood—but only if you can do so without creating an even larger wound.

Quick pruning check (to prevent future issues)
     Make thinning cuts back to the branch collar/lateral—don’t leave stubs. 
     Avoid making very large cuts; walnut wounds larger than ~4 inches close slowly and can be an entry point for fungi.
     For walnuts, pruning is generally best in the dormant season for routine structural work and to remove problems like crossing/diseased wood.  

I hope that you find this information useful and as always Happy Gardening.

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