Sap is oozing out of an old pruning cut on my tree. Is this a serious problem?

Question: 

Sap is oozing out of an old pruning cut on my elm. Is this a serious problem?

Answer: 

The sap oozing from the elm wound is probably due to bacterial wetwood or slime flux.  Bacterial wetwood or slime flux is a common on elm, cottonwood, and mulberry.  It also occurs on maple, birch, ash, linden, redbud, and other deciduous trees.  Symptoms include the bleeding or oozing of sap from tree wounds.  The sap is oftentimes colonized by fungi, yeasts, and other bacteria, resulting in a frothy, slimy, foul-smelling liquid. 

The bacteria associated with wetwood are common in nature.  They enter the tree through wounds in the trunk, branches, or roots.  High pressure builds inside affected trees from bacterial activity.  Eventually the gasses and fluids work their way out through cracks or wounds on the tree.  Wetwood is more common in years when trees are suffering from drought stress. 

Wetwood or slime flux may weaken affected trees somewhat, but usually doesn’t kill them.  Preventing wounds and avoiding stress are the best ways to deter wetwood problems.  Once a tree is oozing sap, there is no way to eliminate the disease.  Proper pruning, watering when needed, and avoiding soil compaction over tree roots are ways to minimize wetwood or slime flux problems

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