Question
I have a 20 foot tall cottonwood tree.
It is leaking sap at the bottom of the tree on the main trunk.  Is this normal?

Answer
This sounds like borer damage. Borers are insects that enter a wound and bore under the bark and into the wood of the trees. The wet areas is a sign that the insect is still in the trunk. Wood-boring insects are among the most destructive pests of ornamental trees and shrubs. Borers are the larvae, or immature stage, of certain moths and beetles. They tunnel and feed under the bark in living wood, destroying water- and sap-conducting tissues. This causes girdling, branch dieback, structural weakness, and decline and eventual death of susceptible plants. Infestation sites also provide entry points for plant pathogens.

Borers rarely infest healthy plants growing in their natural environments. However, when trees or shrubs are transplanted into the landscape, stresses such as drought, soil compaction, sun scald, or injuries can weaken them and make them more susceptible to attack.

The control of borers is to spray the area with an insecticide called Merit or Onyx. This will kill the insect under the bark.  Check with your local garden type store for these insecticides. Some folks have used a non chemical method that can work. After trees and shrubs are infested with borers, non-chemical controls are limited. Consider inserting a flexible, small gauge wire into borer entry holes to puncture and kill the tunneling insect.

The tree are under stress so I would recommend that they be fertilized with 1-10-10 fertilizer at the rate of 1 lb per inch of trunk diameter scattered around the tree and watered in good. Apply just before a rain storm and you will not need to water.