Slime Flux is often caused by bacterial wet wood. There’s no cure but taking some simple steps should prevent it getting worse.
Have you seen the pictured symptom before ? The sap that flows out from wounds and cracks on the trunks of various trees is known as slime flux. Elms, oaks, and poplars often display this symptom, along with various other ornamental trees.
Slime flux is the external sign of a condition called bacterial wet wood, which is caused by bacteria growing inside the tree. Bacterial wet wood is characterised by water-soaked xylem, a high pH, and the presence of various bacterial species.
There are two types of bacterial wet wood: non-pathological, which is found in the central core of dead xylem and does not cause any symptoms, and pathological, which occurs in the outer sapwood and can cause slime flux, leaf wilting and chlorosis. The bacteria produce gases as they multiply within the tissue, causing pressure to build up inside the tree. As the pressure increases, sap is forced out of openings in the bark. This sap is clear to tan at first, then darkens upon exposure to the air. The flux on the bark will then be colonised by many different airborne bacteria, fungi, and yeasts that give it a slimy texture and a fetid odor. Insects like bees and wasps are often attracted to the sugary exudate.
Bacterial wet wood can cause wilting and dieback as toxins and alcohols kill plant cells. Grass may also be killed where the flux runs down off the trunk. Bacterial wet wood is probably triggered by heat, drought, and other stress factors. There is no cure for bacterial wet wood, but taking care not to wound the tree and reducing stress factors by appropriate soil management (de-compaction, fertilisation, mulching, irrigation) should help keep the tree healthy.