My mulberry tree was leaking sap earlier and it left ugly black stains down the trunk. The tree has deeply furrowed bark. Can I remove this?
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That leaking sap & stained bark appears to be caused by bacterial wetwood, also known as slime flux. This is a common bacterial infection that creates slightly caustic sap & raises pressure within the wood. The pressure forces the sap out at weak points, staining the bark. The sap smells sort of fermented, right? That's the result of the bacteria having chowed down on the sugars in the sap. You may also notice bugs showing up for a party.
You shouldn't try to remove the bark. You can remove extra sap by infrequent gentle washing with plain water (rain is free, don't buy a hose for this.) Removing the discolored bark or trying to plug/patch will just cause additional spots where sap will leak instead, and may interrupt any compartmenting the tree's done on its own. The best management of the situation is to keep the tree free from additional stress, and living with what it looks (and smells) like.
The good news about slime flux is it can actually prevent other sorts of rot! It also generally does not kill trees. (I've had one infected in this way for 10 years, and it's healthier & larger than ever aside from the flux.)
First, I can't stress this enough, before considering to remove the sap from the trunk, it would be best to take the time to identify the reason for the leaking sap ie Your tree may very well have bigger problems than just sap stains on the trunk...
Second, I would be very wary of removing sap from a tree trunk, doing so can lead to inflicting even more damage on the tree.
Third, keep in mind it will be almost impossible to remove all the sap from the trunk .
Keeping the chisel as near parallel to the trunk as you can, very! carefully slice, pry off the sap. Whatever you do, do not let the chisel cut into the bark, or allow the sap to pull away the bark from the trunk. If the sap is really hard, use a hammer to very! carefully insert the chisel into the sap.
Once you've "very! carefully" finished removing as much of the sap as you can from the trunk with the chisel, take a clean (with rubbing alcohol or similar) woodworkers card scraper or decorators/painters hand scraper and very! carefully (this time keeping the tool perpendicular to the trunk) scrape off as much of the remaining sap as you can without damaging the bark.
I recommend stopping there, don't try to remove anymore sap.
If you felt compelled to try and remove even more sap, the following may work (disclaimer, I've never tried it).
Mix up some eco-friendly "degreasing" cleaning solution (read the label to ensure it is safe to use around plants, etc), then apply directly to the sap via a (new) stiff bristle cleaning brush. Allow the solution to "work into" the sap as per the instructions on the label. Then thoroughly rinse off with cold water.