Sounds like it could be the same thing I had....dog vomit slime mold...see the posts below!

I think it's bacterial wetwood (also called slime-flux disease by some), a common disease that affects many trees. In many areas, the disease is most prevalent in elm, cottonwood, aspen and willow.

Several bacteria are associated with wetwood. Symptoms of the disease include am often yellow-brown discoloration of the wood, generally confined to the central core of the tree. This affected wood is wetter than surrounding wood and is under high internal gas pressure. The gas pressure and high moisture content cause an oozing of slime from wood and branch crotches (or in your case, at the basal flare?). The ooze often is foul-smelling, slimy, and colonized by yeast organisms when exposed to air. The white foam you describe is commonly is seen in affected areas, especially on certain species of Salix (willow). A buildup of gas pressure produced from bacterial activity and consisting mainly of methane and nitrogen forces the liquid from the core of the tree.

Bacteria associated with wetwood are common in soil and water and probably enter trees through root wounds (string trimmer or lawn mower damage on your tree?) As far as I know, there are no effective methods exist to treat/eliminate wetwood disease. Preventing damage and stress to a tree's roots and stem is the best way to avoid a wetwood issues. Drought conditions increase wetwood problems, so if you intend to keep the tree, it is important that the tree receives adequate water, especially during summer months.

Al

Al very likely correct in his diagnosis and perscription!