Bacteria may infect this sap causing it to darken and stain the bark, eventually taking on a foamy appearance and unpleasant odor.

Once inside the xylem, the internal pressure of the tree is raised, from the normal range of 5 to 10 psi (0.3 to 0.7 bar) up to 60 psi (4 bar), due to bacteria fermenting and emitting a gas mixture of methane, nitrogen, carbon dioxide, and oxygen.

This accumulation of liquid and gas causes that part of the tree to have a damp, dark brown appearance known as wetwood.

[1] Eventually, the pressure will cause the sap and gasses to burst through the xylem and out of cracks in the trunk and ooze down the side of the tree.

[3] Xanthomonas spp., Agrobacterium spp., Acinetobacter spp., Corynebacterium spp., Bacteroides spp., Clostridium spp., Edwardsiella spp., Klebsiella spp., Lactobacillus spp., Methanobacterium spp.,[4] Brevundimonas bullata, Paracoccus spp.